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1999 S420; 2007 S600
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416 Posts
Discussion Starter #1 (Edited)
Hiya folks, I've had a problem with no heat coming from the vents in my 500. The 420's heat is fine so I just swapped the duovalve between the two. Makes no difference.

It's worth noting that on a hot day, with the heat on "HI" on both sides, the air coming from the foot well vents and from the center console (for the rear seats) seemed to be as cold as the air when I chilled it down to "LO". Even though the compressor obviously wasn't on.

The heater core could be clogged. COULD be. But this is a '99 with 70k miles. There's no corrosion or sediment evident anywhere in the cooling system, and I've replaced the water pump, and done various other things that drained the coolant. If it was a '94 or '96 and had 150k-200k miles I'd think it's plausible but as it is, I just do not believe that there's a blockage in the heater core. I'll end up ripping out the dashboard, putting in a new heater core, and find out that I still have no heat.

Here's my question: What ELSE could cause no heat whatsoever? I'm assuming there's a cabin thermometer; what if that's faulty?

I pulled trouble codes from the climate control a couple of days ago and came up with Eb1 231 (ECT sensor) and Eb1 234 (Sun Sensor). I have a new coolant temp sensor on order, should arrive in a couple of days. I have not had any of the codes recur in the last few days. Could an engine coolant temperature sensor on the fritz cause this? I'd assume the sun sensor cannot.

Could this be a problem with the vanes in the duct work?

I can believe there's an electrical problem, but I'm just reticent to pursue replacing the heater core until I've eliminated all less invasive possibilities.

Is there any definitive way to determine whether the heater core is clogged without removing the dashboard?

Thanks all! :thumbsup:
 

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2003 S55 AMG Kompressor
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361 Posts
You can try to operate the valves directly, it should be straight forward to check if the heater matrix is blocked. Rather unlikely however
 

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1995 mercedes S420
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228 Posts
Hiya folks, I've had a problem with no heat coming from the vents in my 500. The 420's heat is fine so I just swapped the duovalve between the two. Makes no difference.

It's worth noting that on a hot day, with the heat on "HI" on both sides, the air coming from the foot well vents and from the center console (for the rear seats) seemed to be as cold as the air when I chilled it down to "LO". Even though the compressor obviously wasn't on.

Here's my question: the heater core could be clogged. COULD be. But this is a '99 with 70k miles. There's no corrosion or sediment evident anywhere in the cooling system, and I've replaced the water pump, and done various other things that drained the coolant. If it was a '94 or '96 and had 150k-200k miles I'd think it's plausible but as it is, I just do not believe that there's a blockage in the heater core. I'll end up ripping out the dashboard, putting in a new heater core, and find out that I still have no heat.

What ELSE could cause no heat whatsoever? I'm assuming there's a cabin thermometer; what if that's faulty?

I pulled trouble codes from the climate control a couple of days ago and came up with Eb1 231 (ECT sensor) and Eb1 234 (Sun Sensor). I have a new coolant temp sensor on order, should arrive in a couple of days. I have not had any of the codes recur in the last few days. Could an engine coolant temperature sensor on the fritz cause this? I'd assume the sun sensor cannot.

Could this be a problem with the vanes in the duct work?

I can believe there's an electrical problem, but I'm just reticent to pursue replacing the heater core until I've eliminated all less invasive possibilities.

Is there any definitive way to determine whether the heater core is clogged without removing the dashboard?

Thanks all! :thumbsup:
few months ago i thought i had the same problem cuz i changed the mono valve "duovalve" twice so i thought its blocked somewhere, but the problem was the 2 duovalve were bad,lol. so re-install the duovalve back on ur S420 and see if its still working. to find out if u have a clogged or not try to blow air to the hoses that go to the heater core . thats wt i did but i found out the air was comin out from the other hose. that will help to push anything clogging the system out.
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter #4
You can try to operate the valves directly, it should be straight forward to check if the heater matrix is blocked. Rather unlikely however
This is one of the diagnostic modes in the climate control head unit?
 

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600SEL
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1,083 Posts
If you disconnect connector from the duo valve it should open and give you a full heat mode. This test will eliminate all electrical circuits and push button unit.
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter #6
If you disconnect connector from the duo valve it should open and give you a full heat mode. This test will eliminate all electrical circuits and push button unit.
Interesting! I'll give that a shot this evening.
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter #7
Team:

I just received a new engine coolant temperature sensor and popped it in. Started the car, warmed it a bit... and boom, the heat is back. Perfect!

That's certainly another culprit to check, in addition to the duovalve!

Thanks all!
 

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Premium Member
1997 S600 (sold)
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4,660 Posts
So educate me. Are there two coolant temp sensors? One feeds the engine management and dash gauge, and the other feeds the climate control? Is that right? So you weren't seeing any issues with coolant temp via the gauge?

Brett
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter #9
Brett - I wondered the same. I do not know if there are two coolant temperature sensors. I did not see any problems on the gauge; actually, I had been using the gauge to observe the previous overheating (or, not overheating, but hot-running) condition before I found that the fan clutch was shot and replaced it. The gauge was reading accurate temperatures which I verified using an infrared thermometer.

I had an error code in the climate control head unit for the ECT Sensor (Eb1231). I cleared that code and it didn't come back after a few drive cycles, but the heat still didn't work.

As I mentioned in the original post the known-good duovalve from the 420 made no difference in the 500.

However, I popped in the new coolant temp sensor today and cranked up the heat and that was that... warm air in the footwells and up towards the windshield!

The one I replaced is at the forward edge of the intake manifold. If there's a "sensor" and a separate "sender", I don't know where the "sender" is located. The old sensor was black on its nose, the submerged part. I didn't test it for resistance before I ditched it. In retrospect it probably would have been informative to do so.
 

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Hello there guys my wife just got a 1995 s420 it drives great. I mean great. brakes need to be checked out there is air in the lines or something, they are relly soft, and the fluid smells like mushrooms. But She just got this car Friday 11/9/2012. Now to the point.

There is no heat. I have put the car in rest, and I can hear the driver side monovalve spinning, but no heat. The therm or the main therm. the one that tells what the engine temp is, is right on moves to up to running temp. In the time it should. After that is when I did the rest check. It does blow a little hot air after I turn the rest mode off, but very briefly. The monovalve sound like it is slurping. That would be the best way to describe it. The AC works fine. She bought this car at a used car lot. A place where she has bought cars from forever. The man that owns the lot is sitting on another s420. He said it had a few problems, an oil leak he cant figure out and said he thinks it has a water pump leak. he said something about it having two water pumps. Is that correct? Two water pumos. From what I have told you about the one she bought do you think having him switch out the monovalve from the one he is sitting on, presuming the heat on the other one works, would correct the problem or would you just trade for the other one, and replace the water pump?
 

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1997 S500 Coupe
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136 Posts
If I understood correctly, he has 2 S420's on the lot and you got one that runs perfect but has a heater problem, while the other one has a water pump problem?

Keep the heater problem one, the duo valve is usually the culprit. You can take the part apart, test it with some 12 volt electricity wires and see if it opens up and closes. The part is cheaper than a water pump and is less work to replace, I did it myself and this is the first car I've ever worked on.

What you can do is check if the duo valve heats up on both cores (look at the duo valve and you will see it has two chambers, both should be hot.). If one of them isn't hot, then your duo valve has a problem. Ofcourse, the best way to test is it to take it out and check in my opinion. From there, you clean it up. If cleaning it works, great! If not, decide if you want to buy a new one or used one. I will warn you, I had 3 used duo valves fail on me within a month time. Only after those 3 messed up valves. I ended up finding my original one and taking all the duo valves apart and replacing the rubbers and working parts all into one to get a working one, since I was tired of spending all the money for new used ones. (Getting a new one right off the bat would've saved me the trouble).

If the duo valve works (via electricity inspection, it should make a clicking sound on both sides), then you might have a problem like the original poster did, with a messed up sensor.

Hope I helped, good luck!
 

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1997 CL500
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1,224 Posts
Center Air-Vents Buttons Issue

On my 1997 S-Class i have the following issue. The A/C works very well, and so does the heating, BUT when i press the buttons located between the front center vents ( red and Blue) the following happens :
1. When i press the blue button i get a nice strong stream of cold air from the center vents.
2. When i press the red button, the cold air stops and i only feel a weak stream of warm air, although i can hear the system making noise as if a strong stream would come out.
My question is if this is normal, or if something is wrong or stuck, and what can i do to fix this.
 

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If I understood correctly, he has 2 S420's on the lot and you got one that runs perfect but has a heater problem, while the other one has a water pump problem?

Keep the heater problem one, the duo valve is usually the culprit. You can take the part apart, test it with some 12 volt electricity wires and see if it opens up and closes. The part is cheaper than a water pump and is less work to replace, I did it myself and this is the first car I've ever worked on.

What you can do is check if the duo valve heats up on both cores (look at the duo valve and you will see it has two chambers, both should be hot.). If one of them isn't hot, then your duo valve has a problem. Ofcourse, the best way to test is it to take it out and check in my opinion. From there, you clean it up. If cleaning it works, great! If not, decide if you want to buy a new one or used one. I will warn you, I had 3 used duo valves fail on me within a month time. Only after those 3 messed up valves. I ended up finding my original one and taking all the duo valves apart and replacing the rubbers and working parts all into one to get a working one, since I was tired of spending all the money for new used ones. (Getting a new one right off the bat would've saved me the trouble).

If the duo valve works (via electricity inspection, it should make a clicking sound on both sides), then you might have a problem like the original poster did, with a messed up sensor.

Hope I helped, good luck!
Both Sides heat up
 

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1997 CL500
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1,224 Posts
On my 1997 S-Class i have the following issue. The A/C works very well, and so does the heating, BUT when i press the buttons located between the front center vents ( red and Blue) the following happens :
1. When i press the blue button i get a nice strong stream of cold air from the center vents.
2. When i press the red button, the cold air stops and i only feel a weak stream of warm air, although i can hear the system making noise as if a strong stream would come out.
My question is if this is normal, or if something is wrong or stuck, and what can i do to fix this.
Anyone ?
 

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1997 S500 Coupe & 1994 S500 Coupe
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849 Posts
At this point i dont know. This is chono123 by the way, made a new account.

The only thing i can think of now is the sensors/thermostats behind the climate control , there's 2 at 32 bucks each, but it doesnt seem to be the issue since cores are getting hot.

Did you consider the different valves that open behind the dash to redirect air as broken, not switching to hot air direction? Blower regulator maybe?
 

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'93 BLACK 400 SEL
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11 Posts
I have a 400SEL and the heat is very weak on full blast....however the windshield defroster gives me adequate heat.....where do you recommend I start to look for the culprit??
 

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1997 S500 Coupe & 1994 S500 Coupe
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849 Posts
I have a 400SEL and the heat is very weak on full blast....however the windshield defroster gives me adequate heat.....where do you recommend I start to look for the culprit??
Duo valve isn't opening up fully. Clean it out if you're DIY. Mine had same issue, eventually heat will give out all together.
 

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1997 S500 Coupe & 1994 S500 Coupe
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