[conecaster]

Dear MB forum,

I have a 95 e320 wagon, the climate system is not working right and I was hoping some one here might have some expertise in getting to the bottom of it.

The problem is regulation, if you set the temp dial in the white zone at 75 and you drive it the car will heat and be fairly normal, but if you stop and the car idles the vents go cold.

It doesn't want to moderate up in the white zone in AC mode as either, the AC is basically full cold.

Here's what I've done,

Replaced Heater control valve and aux pump

replaced over head climate sensor in the light panel

replaced suction pump for the over head sensor

Here is what I have tested

The Y/G wire on the control valve to the + battery terminal with a test light between the two points.

put full AC on to super cool the interior, then set dial at 75 degrees to see if the light shows regulation of the valve- if light is on valve is closed- no blinking light until the dial is rolled hot near the full on position at about 90 on the dial. Then it blinked a few times and went back to full on. The light switches off in the full heat position.

Used a lighter on the over head sensor to test the Auto setting for adjustment in the white zone, the fan blower speed increased.

Next test was to pull the center control panel and access the heater core resistor. The resistor at ambient temp 66 degrees slowly built up to 11.06 then after a few second my screen went blank after jumping to 12 and it was showing OL as if there was no resistance. I installed about a 15k resistor on the circuit to replace the sensor. Ordered a new sensor.

I started the car hoping to have better control valve regulation with a colder heater core sensor resistance. The car held a vent temp of 70 in mid temp dial setting of 75 degrees while cold at idle, engine gauge temp of 40. Once the engine got hot enough the engine thermostat opened and the vent temp fell again. I presume the closed thermo puts more circulation pressure on the heater core.

So as the engine warmed up the idle vent temps dropped with a 15k resistor installed to bye pass the heater core sensor.

I do not know where to go from here,

the usual fixes for these kind of problems have had no effect,

the climate control relay box behind the battery was replaced about 4 years ago as was a brand new control unit with new dial and buttons in the center console.

if you have any experience with these climate control system I would much appreciate your advice.

Paul

 

[sbaert]

Your symptoms sounds water pump related causing reduced water flow at idle.

MB water pumps have a tendency for the impeller to slip & wear, especially with age or if the wrong coolant type is used.

Only use genuine MB or Zerex G5 coolant mixed 50/50 with distilled water. Nothing else.

IF the wrong coolant was used, your coolant passages including the heater core might be restricted.

 

[conecaster]

Your symptoms sounds water pump related causing reduced water flow at idle.

MB water pumps have a tendency for the impeller to slip & wear, especially with age or if the wrong coolant type is used.

Only use genuine MB or Zerex G5 coolant mixed 50/50 with distilled water. Nothing else.

IF the wrong coolant was used, your coolant passages including the heater core might be restricted.

I installed a new high quality water pump a year ago, a week ago I Back flushed the Heater core and it was freely flowing, It has new coolant in it, I have considered the thermo I installed is malfunctioning but can not see how. Nothing came out of the core when I flushed it, I thought some crumbled plastic from a deteriorated Y connector going into it had gotten in there. No luck.

I think the issue is in the control of the valve, based on the lack of a blinking test light even with an over sized resistor installed to emulate colder heater core temp.

 

[ds190]

You have been very thorough. I don't have any input, but here is a link to the FSM section on ACC:

83 Climate Control System

Please post how you solve the problem when that happens, hopefully soon.

 

[jftu105]

I installed a new high quality water pump a year ago, a week ago I Back flushed the Heater core and it was freely flowing, It has new coolant in it, I have considered the thermo I installed is malfunctioning but can not see how. Nothing came out of the core when I flushed it, I thought some crumbled plastic from a deteriorated Y connector going into it had gotten in there. No luck.

I think the issue is in the control of the valve, based on the lack of a blinking test light even with an over sized resistor installed to emulate colder heater core temp.

If there is a problem with the water pump or thermostat, your symptoms would be related to engine overheating, not cabin air not hot enough. Even you used wrong coolant, it would affect long term life of the water pump and radiator, not reduced coolant flow. Just my thinking, you want to first verify if you do have very hot air coming out by pressing the defrost button when the engine has warmed up. If you do have very hot air coming out, then you have no problem with engine cooling system.

Therefore, to me, it is more related to the control of the climate system. I will try set the temperature dial to 80 or 85 or even higher, to see if you got hot air. If you don't, the controller or the control valve obviously is not doing its job. The climate control mixes the cold and hot air to give you the cabin temperature. There is also an air sampler on the passenger side inside the dash. When you turn the key, you can hear it spinning when you press your ear to it.

If the cold temperature control works fine, i.e., if you set lower temperature, AC will work harder to make it colder, then I think that the climate control is fine. It is a matter of why hot air won't be delivered, which points to the cabin air control valve.

Just some thoughts. Hopeful, they help.

jftu105

 

[conecaster]

If there is a problem with the water pump or thermostat, your symptoms would be related to engine overheating, not cabin air not hot enough. Even you used wrong coolant, it would affect long term life of the water pump and radiator, not reduced coolant flow. Just my thinking, you want to first verify if you do have very hot air coming out by pressing the defrost button when the engine has warmed up. If you do have very hot air coming out, then you have no problem with engine cooling system.

Therefore, to me, it is more related to the control of the climate system. I will try set the temperature dial to 80 or 85 or even higher, to see if you got hot air. If you don't, the controller or the control valve obviously is not doing its job. The climate control mixes the cold and hot air to give you the cabin temperature. There is also an air sampler on the passenger side inside the dash. When you turn the key, you can hear it spinning when you press your ear to it.

If the cold temperature control works fine, i.e., if you set lower temperature, AC will work harder to make it colder, then I think that the climate control is fine. It is a matter of why hot air won't be delivered, which points to the cabin air control valve.

Just some thoughts. Hopeful, they help.

jftu105

Thank you for the opinion about the water pump and thermo, I suspected as much.

Yes, the heater does heat it just goes cold at idle. As soon as RPM go up the heat comes back be it the dial has to be set higher than it should. The light test shows that on full heat the valve opens and stays that way.

I also think there is a electrical issue with the climate controls but as I listed above I replaced the over head sensor with no effect. I also insured it has air flow through it by installing a new suction pump and inspected the piping to it making sure there is air flow. The sensor is responding to heat and in AC mode the heat increases the blower speed.


From my reading of the system the Interior over head sensor and the Heater core sensor combine to regulate the control valve.

The fact I got very little blinking on my light test after super cooling the interior tells me the regulation is off.

 

[edcarls]

I didn't see any comment regarding the electric water pump that circulates the coolant while the car is stationary or idling. IRRC,(on my 92 W124) there is an electric pump below the expansion tank that pumps hot coolant to the heater core.

I drove to Canada several times during winter and the car maintained its cabin temperature while idling that we use the car as heat sanctuary while we watched the kids skate on ice.

 

[neanderthal]

Have you looked at the monovalve/ duovalve/ whatever it's called?

 

[Floobydust]

Been there, done that . . . .

Paul -

I have been through exactly what you are experiencing for which I went down almost exactly the same diagnostic fault tree. For me, the problem started after I brought the car our of winter storage.

Your key finding is, "put full AC on to super cool the interior, then set dial at 75 degrees to see if the light shows regulation of the valve- if light is on valve is closed- no blinking light until the dial is rolled hot near the full on position at about 90 on the dial. Then it blinked a few times and went back to full on. The light switches off in the full heat position". This says the issue is not related to the thermostat, water pump, circulation through the core, or other water circuit related items. The CCU is commanding the hot coolant flow to shut off. This only happens for two reasons: a CCU fault or a faulty signal from one of the CCU sensors. Note that a faulty signal can arise because water and/or air is not moving as designed as well as an outright sensor fault.

At start up when the interior temperature (measured by the 10K ohm NTC thermistor behind the suction fan) is far from the set point on the CCU dial, the system applies near full heat or full A/C along with high fan to narrow the gap. As the interior temperature starts to approach the set point, the system starts to modulate the heat (cooling) and lower the fan speed to bring vent air temp close to the set point temp and provide a low volume comfy circulating air. As the car gains or loses heat, additional A/C or heat is applied to drive things back toward the set point.

What I found in my investigations is that heater core temperature and the measurement thereof are critical to proper functioning of the system. Because the system tries to maintain the vent temperature so close to the set point temperature, small errors can cause strange behavior. I even found an MB service bulletin that called for replacing all of the sensors in cases like this.

In particular I found, an error that causes the CCU to think the air coming out of the heater core is warmer than it really is will cause the behavior you are observing. Essentially what happens is that the false warm signal will cause the CCU to reduce the water flow to try and meet the set point temperature. But because the core is really colder than the set point (due to the error), the system ends up just shutting off the water flow trying to make it even colder.

The bad news is that I was never able to affirmatively prove a fault cause, but the good news is that I found a work-around that cured the problem. What I did was solder a 270 ohm resistor in series with the heater core temp sensing thermistor. This has the net effect of lying to the CCU and telling it that the core air temp is a few degrees colder than it really is. This prevents the CCU from getting into that deadband loop I described above. My system has worked perfectly ever since. Note that you don't want to over do it on this resistor. If the value is too large, the system will experience large temperature / fan speed excursions. 100 to 270 ohms is probably the right range. The other good thing about this is that will keep your feet a bit toastier as well.

But before you do that, check the following, as they may also cause the CCU to think the heater core is warmer that it is:

- Check to make sure you have power to the coolant pump that is part of the mono/duo valve assembly (I suspect you have, but just to be sure).

- Check the operation of the vacuum pods that control the airflow to the heater outlet vents. If they do not open or open just a bit, the lack of airflow will make the core sensor read artificially high.

- Make sure the heater outlets are open and unobstructed, even the ones that come out under the seats in the back (if your car has them).

- Check that the air intakes and/or pollen filter (if equipped) are free and clear. Low air flow from the fan = core too warm, especially at idle.

- Make sure the fan motor is turning freely and at the correct speed. Low air flow from the fan = core too warm, especially at idle.

- Use a pulse counter to read the CCU error codes (search the forum for the procedure on this), if any. In particular, the system is good as detecting open/shorted sensors. Check out any faults identified from there.

- Check/replace the coolant temperature sensor. The coolant temp is used as part of the calculation of how much on time the heater valve needs for a given delta T (ambient versus set point).

- Check/replace exterior air temperature sensor. The value from this sensor is also used as part of the calculation of how much on time the heater valve needs for a given delta. In particular, if the CCU thinks that outside air temp is much warmer than it really is relative to the set point temperature, it will close the heater valve and may even turn up the A/C duty cycle. The CCU can then get stuck in the same dead band situation as above

Note that you can check the values of all the sensors from on of the 14 pin plugs on the back of the CCU. Let the car sit overnight so that the car and air temps are the same.

If all this checks out, consider the resistor mod. I did after checking all of the above out!

Hope this helps and please post what you ultimately determine to be the cause and/ro what you do as corrective action.

- FD

 

[conecaster]

Paul -

I have been through exactly what you are experiencing for which I went down almost exactly the same diagnostic fault tree. For me, the problem started after I brought the car our of winter storage.

Your key finding is, "put full AC on to super cool the interior, then set dial at 75 degrees to see if the light shows regulation of the valve- if light is on valve is closed- no blinking light until the dial is rolled hot near the full on position at about 90 on the dial. Then it blinked a few times and went back to full on. The light switches off in the full heat position". This says the issue is not related to the thermostat, water pump, circulation through the core, or other water circuit related items. The CCU is commanding the hot coolant flow to shut off. This only happens for two reasons: a CCU fault or a faulty signal from one of the CCU sensors. Note that a faulty signal can arise because water and/or air is not moving as designed as well as an outright sensor fault.

At start up when the interior temperature (measured by the 10K ohm NTC thermistor behind the suction fan) is far from the set point on the CCU dial, the system applies near full heat or full A/C along with high fan to narrow the gap. As the interior temperature starts to approach the set point, the system starts to modulate the heat (cooling) and lower the fan speed to bring vent air temp close to the set point temp and provide a low volume comfy circulating air. As the car gains or loses heat, additional A/C or heat is applied to drive things back toward the set point.

What I found in my investigations is that heater core temperature and the measurement thereof are critical to proper functioning of the system. Because the system tries to maintain the vent temperature so close to the set point temperature, small errors can cause strange behavior. I even found an MB service bulletin that called for replacing all of the sensors in cases like this.

In particular I found, an error that causes the CCU to think the air coming out of the heater core is warmer than it really is will cause the behavior you are observing. Essentially what happens is that the false warm signal will cause the CCU to reduce the water flow to try and meet the set point temperature. But because the core is really colder than the set point (due to the error), the system ends up just shutting off the water flow trying to make it even colder.

The bad news is that I was never able to affirmatively prove a fault cause, but the good news is that I found a work-around that cured the problem. What I did was solder a 270 ohm resistor in series with the heater core temp sensing thermistor. This has the net effect of lying to the CCU and telling it that the core air temp is a few degrees colder than it really is. This prevents the CCU from getting into that deadband loop I described above. My system has worked perfectly ever since. Note that you don't want to over do it on this resistor. If the value is too large, the system will experience large temperature / fan speed excursions. 100 to 270 ohms is probably the right range. The other good thing about this is that will keep your feet a bit toastier as well.

But before you do that, check the following, as they may also cause the CCU to think the heater core is warmer that it is:

- Check to make sure you have power to the coolant pump that is part of the mono/duo valve assembly (I suspect you have, but just to be sure).

- Check the operation of the vacuum pods that control the airflow to the heater outlet vents. If they do not open or open just a bit, the lack of airflow will make the core sensor read artificially high.

- Make sure the heater outlets are open and unobstructed, even the ones that come out under the seats in the back (if your car has them).

- Check that the air intakes and/or pollen filter (if equipped) are free and clear. Low air flow from the fan = core too warm, especially at idle.

- Make sure the fan motor is turning freely and at the correct speed. Low air flow from the fan = core too warm, especially at idle.

- Use a pulse counter to read the CCU error codes (search the forum for the procedure on this), if any. In particular, the system is good as detecting open/shorted sensors. Check out any faults identified from there.

- Check/replace the coolant temperature sensor. The coolant temp is used as part of the calculation of how much on time the heater valve needs for a given delta T (ambient versus set point).

- Check/replace exterior air temperature sensor. The value from this sensor is also used as part of the calculation of how much on time the heater valve needs for a given delta. In particular, if the CCU thinks that outside air temp is much warmer than it really is relative to the set point temperature, it will close the heater valve and may even turn up the A/C duty cycle. The CCU can then get stuck in the same dead band situation as above

Note that you can check the values of all the sensors from on of the 14 pin plugs on the back of the CCU. Let the car sit overnight so that the car and air temps are the same.

If all this checks out, consider the resistor mod. I did after checking all of the above out!

Hope this helps and please post what you ultimately determine to be the cause and/ro what you do as corrective action.

- FD

This was the kind of information I was hoping someone might offer.

So you think there is a code default in the CCu?

I installed a 50k resistor in it this morning between the sensor leads. I set up the light, started the car and let it idle on a dial setting of 75.

I watched the light. It was off, the car warmed up and for the first time it started blinking as normal. I think in my previous test I did not have a solid connection on the resistor.

But I drove the car like this hoping the CCU would think the core was warm and adjust for interior temp. That did not work so well, the vents still go cold at idle after driving it. But I can see now from the light test of the control valve that the CCU is regulating the water flow. But there is no sensor in it and I have to set the dial hotter to get a moderate temp, also the center vent is not closing completely and depending on dial setting will let cold air blend in.

When you say CCU, are you referring to the climate control unit behind the battery or the dial unit in the dash.

My plan is to replace the sensor and see what happens, then perhaps I will install the 270k resistor on the Sensor wire.

As to the other responses I appreciate the input but this car has a combined Aux pump and Control valve in one part, It goes in front of the battery. I replaced that unit first, The dash unit with buttons and dial was replaced with a factory new unit 4 years ago. The CCU unit behind the battery was replaced as well back then.

All of these more obvious problems where looked at and I even replaced both the over head sensor and suction pump for it.

Thank you,

P

 

[sbaert]

CCU = push button climate control unit in centre console.

Have the dash vent vacuum pods ever been replaced??? If not, perhaps there is a vacuum leak somewhere prevent the vent and/or blend doors from staying fullly open at idle.

 

[jftu105]

Paul -

I have been through exactly what you are experiencing for which I went down almost exactly the same diagnostic fault tree. For me, the problem started after I brought the car our of winter storage.

Your key finding is, "put full AC on to super cool the interior, then set dial at 75 degrees to see if the light shows regulation of the valve- if light is on valve is closed- no blinking light until the dial is rolled hot near the full on position at about 90 on the dial. Then it blinked a few times and went back to full on. The light switches off in the full heat position". This says the issue is not related to the thermostat, water pump, circulation through the core, or other water circuit related items. The CCU is commanding the hot coolant flow to shut off. This only happens for two reasons: a CCU fault or a faulty signal from one of the CCU sensors. Note that a faulty signal can arise because water and/or air is not moving as designed as well as an outright sensor fault.

At start up when the interior temperature (measured by the 10K ohm NTC thermistor behind the suction fan) is far from the set point on the CCU dial, the system applies near full heat or full A/C along with high fan to narrow the gap. As the interior temperature starts to approach the set point, the system starts to modulate the heat (cooling) and lower the fan speed to bring vent air temp close to the set point temp and provide a low volume comfy circulating air. As the car gains or loses heat, additional A/C or heat is applied to drive things back toward the set point.

What I found in my investigations is that heater core temperature and the measurement thereof are critical to proper functioning of the system. Because the system tries to maintain the vent temperature so close to the set point temperature, small errors can cause strange behavior. I even found an MB service bulletin that called for replacing all of the sensors in cases like this.

In particular I found, an error that causes the CCU to think the air coming out of the heater core is warmer than it really is will cause the behavior you are observing. Essentially what happens is that the false warm signal will cause the CCU to reduce the water flow to try and meet the set point temperature. But because the core is really colder than the set point (due to the error), the system ends up just shutting off the water flow trying to make it even colder.

The bad news is that I was never able to affirmatively prove a fault cause, but the good news is that I found a work-around that cured the problem. What I did was solder a 270 ohm resistor in series with the heater core temp sensing thermistor. This has the net effect of lying to the CCU and telling it that the core air temp is a few degrees colder than it really is. This prevents the CCU from getting into that deadband loop I described above. My system has worked perfectly ever since. Note that you don't want to over do it on this resistor. If the value is too large, the system will experience large temperature / fan speed excursions. 100 to 270 ohms is probably the right range. The other good thing about this is that will keep your feet a bit toastier as well.

But before you do that, check the following, as they may also cause the CCU to think the heater core is warmer that it is:

- Check to make sure you have power to the coolant pump that is part of the mono/duo valve assembly (I suspect you have, but just to be sure).

- Check the operation of the vacuum pods that control the airflow to the heater outlet vents. If they do not open or open just a bit, the lack of airflow will make the core sensor read artificially high.

- Make sure the heater outlets are open and unobstructed, even the ones that come out under the seats in the back (if your car has them).

- Check that the air intakes and/or pollen filter (if equipped) are free and clear. Low air flow from the fan = core too warm, especially at idle.

- Make sure the fan motor is turning freely and at the correct speed. Low air flow from the fan = core too warm, especially at idle.

- Use a pulse counter to read the CCU error codes (search the forum for the procedure on this), if any. In particular, the system is good as detecting open/shorted sensors. Check out any faults identified from there.

- Check/replace the coolant temperature sensor. The coolant temp is used as part of the calculation of how much on time the heater valve needs for a given delta T (ambient versus set point).

- Check/replace exterior air temperature sensor. The value from this sensor is also used as part of the calculation of how much on time the heater valve needs for a given delta. In particular, if the CCU thinks that outside air temp is much warmer than it really is relative to the set point temperature, it will close the heater valve and may even turn up the A/C duty cycle. The CCU can then get stuck in the same dead band situation as above

Note that you can check the values of all the sensors from on of the 14 pin plugs on the back of the CCU. Let the car sit overnight so that the car and air temps are the same.

If all this checks out, consider the resistor mod. I did after checking all of the above out!

Hope this helps and please post what you ultimately determine to be the cause and/ro what you do as corrective action.

- FD

This is the kind of replies which make this forum great, not those jumping the gun types of replies by blaming everything on water pump, OVP, coolant, etc.

Bravo!

In my earlier replies to this problem, I basically pointed out the same thing. Hot air is there and engine is not overheating; therefore, cooling system is not at fault. It has to be something related to sensor or the control unit.

jftu105

 

[conecaster]

They have not been replaced, but they all still work and the regulation of the valve was obviously faulty.

Some day I'll pull the dash and redo the essentials in there, EVAP, PODS, HEATER CORE

Until then I think FB has my problem diagnosed which is sensors and resistance values closing the Heater control valve and screwing up my climate system.

Thank You,

Paul

 

[conecaster]

If there is a problem with the water pump or thermostat, your symptoms would be related to engine overheating, not cabin air not hot enough. Even you used wrong coolant, it would affect long term life of the water pump and radiator, not reduced coolant flow. Just my thinking, you want to first verify if you do have very hot air coming out by pressing the defrost button when the engine has warmed up. If you do have very hot air coming out, then you have no problem with engine cooling system.

Therefore, to me, it is more related to the control of the climate system. I will try set the temperature dial to 80 or 85 or even higher, to see if you got hot air. If you don't, the controller or the control valve obviously is not doing its job. The climate control mixes the cold and hot air to give you the cabin temperature. There is also an air sampler on the passenger side inside the dash. When you turn the key, you can hear it spinning when you press your ear to it.

If the cold temperature control works fine, i.e., if you set lower temperature, AC will work harder to make it colder, then I think that the climate control is fine. It is a matter of why hot air won't be delivered, which points to the cabin air control valve.

Just some thoughts. Hopeful, they help.

jftu105

What do you mean by cabin air control valve?

I think The sampler as you call it must be the suction pump for the over head sensor, which is under the right outer vent, The other components I'm aware of are the Heater core sensor and the dash board control unit, the device with the dial.

Am I missing something here?
P

 

[Floobydust]

Sorry for the delay . . I was playing midnight plumber for a friend whose hot water heater blew up.

This was the kind of information I was hoping someone might offer.

So you think there is a code default in the CCU?

Yes, the CCU can store codes and these can be helpful in tracking these things down. I have attached the MB HVAC troubleshooting guide that gives the codes as well as the expected thermistor resistance values versus temperature. I have also attached the schematic for the system and the data sheet for the thermistors used for the various sensors (but not the coolant temp sensor). The schematic shows which pins on the CCU connectors connect to which sensor.

I installed a 50k resistor in it this morning between the sensor leads. I set up the light, started the car and let it idle on a dial setting of 75.

50K is a little high. This may have set the sensor "interruption" fault code for that sensor. Try this test with a 15K or 18K resistor instead.

I watched the light. It was off, the car warmed up and for the first time it started blinking as normal. I think in my previous test I did not have a solid connection on the resistor.

But I drove the car like this hoping the CCU would think the core was warm and adjust for interior temp. That did not work so well, the vents still go cold at idle after driving it. But I can see now from the light test of the control valve that the CCU is regulating the water flow. But there is no sensor in it and I have to set the dial hotter to get a moderate temp, also the center vent is not closing completely and depending on dial setting will let cold air blend in.

Yes, substituting a fixed values, especially one that is out of normal range can sometimes cause odd behavior.

When you say CCU, are you referring to the climate control unit behind the battery or the dial unit in the dash.

The CCU is the unit in the dash with the buttons. The Klima relay is behind the battery and is an A/C only thing.

My plan is to replace the sensor and see what happens, then perhaps I will install the 270k resistor on the Sensor wire.

That's 270 ohms, not K ohms.

As to the other responses I appreciate the input but this car has a combined Aux pump and Control valve in one part, It goes in front of the battery. I replaced that unit first, The dash unit with buttons and dial was replaced with a factory new unit 4 years ago. The CCU unit behind the battery was replaced as well back then.

Yes, it is true that the mono valve and pump are one part, but the electrical drive to the pump is separate and controlled by the CCU. It is important to verify that you have ~12 volts across the motor terminals when the system is calling for heat because, as you noted, one side of the motor is connected to +12 but the other is switched to ground via an NPN darlington transistor in the CCU. All the outputs, including this one, are over current protected in the CCU, so if the pump is drawing excessive current, the CCU will turn off that output even if it would otherwise be turned on (as a side note, there two different manufacturers for the CCU - one turns off all outputs for an over current issue while the other, much more common one, just turns off the affected output).

As the pump ages internal friction can increase and trigger the over current operation. This will also set error code 30. This is why I wanted to make sure you verified that there was 12 volts across the motor terminals.

All of these more obvious problems where looked at and I even replaced both the over head sensor and suction pump for it.

Thank you,

P

Please forgive any typos, I was up late last night.

I hope this helps.

- FD

 

[conecaster]

Thank you FB,

I checked out the Aux water pump circuit first thing, its running just fine. It was installed 2 years ago.

I found a resistance chart for the HC sensor and the measures are in K values, I first used 15k on the circuit to emulate the sensor, but it did not work very well. So I put 50k in thinking the circuit would read totally cold and provide heat to be on constantly but I'm disappointed that even with a 50k resistor replacing the sensor that the vent temps fell. this is similar to adding 270 ohms on top of your sensor but more extreme. My conclusion is a resistor is not a sensor, I have a HC sensor ordered but it will be the end of the week before I see it. The first thing I will do is install the sensor and see if it returns to normal. My ohm test on the sensor gave me the impression the sensor was dead. It read very slowly, peaked at 11 ohms on a 66 degree day or slightly low, then after a few seconds it jumped to 12 ohms before blanking out back to an OL screen meaning 0 Ohms. After that I will test all the sensors for values in ohms to get a picture of how well the sensors are working. If all of this fails to find and correct the problem I will add a 270 ohm resistor to the sensor to see if that fixes it. the only other thing I know to do is see if Mercedes can recalibrate the codes in the ccu.

here is the resistance chart for the sensor which explains why your 270 ohm resistor tricks the system, in case anyone reading this is not picking up that concept. The colder the heater core temp the higher the resistance value in the sensor. http://ww2.justanswer.com/uploads/EU/eurotec/2013-10-15_233511_testing_and_inspection.pdf

I will figure out which wires are the sensor wires from the PDF you sent and test all the sensors before replacing them.

FB, you are the CC king

Paul

 

[jftu105]

What do you mean by cabin air control valve?

I think The sampler as you call it must be the suction pump for the over head sensor, which is under the right outer vent, The other components I'm aware of are the Heater core sensor and the dash board control unit, the device with the dial.

Am I missing something here?
P

There is a flap to control the air flow out of the top vents and of the lower vents. Normally, you hit the pushbutton to select if you want most to the top or evenly distributed. In my car, it could happen that I got nothing to the top and suddenly, I could get it working properly after I hit full blower speed or automatic mode. Those flaps are controlled by vacuum. I don't know exactly how it works but thought it might be relevant. Mercedes designs it so that it gets more cool air to the top and more warm air to the bottom to keep the driver comfortable and alert.

I think that with the help from FD, you are almost there.

jftu105

 

[conecaster]

There is a flap to control the air flow out of the top vents and of the lower vents. Normally, you hit the pushbutton to select if you want most to the top or evenly distributed. In my car, it could happen that I got nothing to the top and suddenly, I could get it working properly after I hit full blower speed or automatic mode. Those flaps are controlled by vacuum. I don't know exactly how it works but thought it might be relevant. Mercedes designs it so that it gets more cool air to the top and more warm air to the bottom to keep the driver comfortable and alert.

I think that with the help from FD, you are almost there.

jftu105

Your describing the solenoid controller that actuates the vent pods through a vacuum router, the solenoids open and close valves that apply vacuum through tubes, it is located on the right side of the center column above the lower liner under the dash. But as I have said, all vents in this car are functioning properly.

P

 

[conecaster]

Here is an interesting thing that happened,

The vents go cold at idle. I run it through the car wash with the vents cold. they stay cold until the drier blowers come on to blow dry the car and blast the car with air, Then the vent temp rose to over 100 degrees.

FB, what does that tell you?
Paul

 

[sbaert]

Are you sure your blower fan is operational??? Could be as simple as a hairline crack in the blower strip fuse on the driver side strut tower.

Replace the strip fuse and ALL the fuses to establish a decent electrical baseline.