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Hey guys: been having problems with my a/c and aux fan not working for some time on my 1986 560 SEC.
- converted to r134 about 5 years ago and whole system was gone over. Problems with aux fan at that time. System keeps blowing aux fan preresistor relay and a/c cooling sensor at receiver/drier. Also, sensor on thermo housing that controls aux fan wasn't working.

I understand when the a/c system is turned on, aux fan should work on low speed. I could not find the pre-resistor which is suppose to be under the power steering. My electrician said he couldn't find it when he took off the fuse box and tried to trace the wired. ??? where is it and do I really need it?

I changed the pre-resistor relay and my a/c blower would turn off on high speed after a minute or two. I had electrician check the acc, blower motor regulator and blower motor (was replaced a few years ago) and aspirator motor and he said everything check out ok but the ACC unit still might be bad. (??). I also had him replace the two sensors on the Thermo (aux fan temp sensor and other temp sensor). When he tested the aux fan sensor, it wouldn't kick on until he tested the unit when the temp reached 105 degrees. It shut off again when the temp went down to 100. My understanding is the aux fan is suppose to stay on until the temp gets back down to 90 degrees. Electrician tested the wires and said they were good. WHAT CONTROLS DOES THIS SWITCH GO TO? Further, my a/c blower motor no longer shuts off after a few minutes after this. I'm guessing is was being shut off due to false temp sensor readings (auto shutoff at 115 degrees from thermo temp sensor readings? Or was blowing fuse: 20 amp modified fuse due to over voltage at hiigh speed? Coincidence it stopped shutting off after therm aux switch changed and jumped? Are the controls for the AUX fan in the ACC unit? For turning on aux fan when a/c is turned on? Where are controls for AUX fan temp sensor at thermo housing? I have electrical schematics but can't figure this out as I've looked at system and everything for aux fan at thermo housing switch seem to be ok, but there still seems to be a problem.

I was gong to replace the a/c coolant temp sensor at the receiver/drier but wanted to fix the cause of the problems first, as to what is blowing teh pre-resistor (slow aux fan speed) relay and killing the related sensor. Any ideas? Pre-resistor was my first idea but I can't find it.

Further, it seems as if the control unit that interpruts the signals and gives direction for the aux fan are not working properly. A bad ACC unit? Short somewhere?

My understanding of the AUX fan system is that it is suppose to come on when the when the a/c unit is engaged. Slow speed through the pre-resistor which has a reaya. Further, there is a pressure sensor on the receiver/drier. This was checked and this good, as compressor and fan clutch engage engage when a/c is turned on e Further, aux fan will come on high at 105 temp from sensor at thermo housing and, at 115, will shut off the a/c blower motor. Aux fan is suppose to stay on until temp goes back down to 90, then shut off. System isn't working as should per above and I'm confused at this point.

Electrician said the problem is either the bad a/c coolant temp switch *which could have caused the Pre-resistor relay to blow and bad ACC unit, even though it tested ok .
 

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The preresistor is located about one foot below your master cylinder, get a flash light to see it . It might be buried under leaves and what not. Vaccum out this area as the resistor gets hot .

Also your posts need a little tightening up . Take no offense but concise questions and descriptions get faster and better responses. Good luck the climate control systems of these cars can be vexing at times.
 

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My understanding of the AUX fan system is that it is suppose to come on when the when the a/c unit is engaged.
According to the schematic, the preresistor begins to power the fan only when the pressure exceeds 20 bar and the coolant temperature is below 105C. It continues to power it until the pressure drops below 15 bar.
Further, aux fan will come on high at 105 temp from sensor at thermo housing
Correct.
and, at 115, will shut off the a/c blower motor.
It's not clear from the schematic, but I suspect it also cuts out the compressor. Turning only the fan off won't do much to reduce coolant temperatures.
 

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Discussion Starter #5
Thanks for the diagram. It helped.

Thanks for the direction of finding the pre-resistor. It was exactly where you said and completely covered in debris. Cleared the area and there it was. Replaced it.

No offense taken. I'm in L.A. now and have gotten so many "different" takes on this system from "experts" in this area which has led to much confusion. I really need a good/honest mechanic in the west L.A. area or surrounding that knows these cars.

Yeap, if pressure sensor on receiver is not working or system is under pressurized, aux fan won't work. My system is pressurized fine as compressor is working. So, aux fan switch on thermo should be working (even replaced both sensors on thermo housing).

I'm going to try and get sensors replaced on receiver and see (I know, system needs to be vacuumed and recharged). Hopefully, this will fix aux fan problem when a/c turned on. I will advise.

Electrician told me ACC unit, blower motor and resistor and aspirator motor tested ok, yet my blower motor still cuts out on high speed after a while. Was told this is the resistor blowing the fuse when it gets too hot, so I guess I will replace this as well and hope it solve this problem as well.

Will advise. Thanks much for the replies.
 

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Yeap, if pressure sensor on receiver is not working or system is under pressurized, aux fan won't work. My system is pressurized fine as compressor is working. So, aux fan switch on thermo should be working (even replaced both sensors on thermo housing).

I'm going to try and get sensors replaced on receiver and see (I know, system needs to be vacuumed and recharged). Hopefully, this will fix aux fan problem when a/c turned on.
Let me reiterate: the aux fan does not necessarily run whenever the AC is on. It only comes on when the pressure rises above 20 bar (or the coolant temperature gets too high). This happens when the outside temperature is high enough that the condenser isn't shedding enough heat without the aux fan running. On a cooler day, or on the highway, the aux fan may never come on.
 

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Electrician told me ACC unit, blower motor and resistor and aspirator motor tested ok, yet my blower motor still cuts out on high speed after a while. Was told this is the resistor blowing the fuse when it gets too hot...
You're talking about the cabin air blower, now, right? Are you having to replace fuses? If not, then the fuse isn't blowing. Also, the heat generated by the resistor is lowest when the fan is on high, so this explanation makes no sense to me.

Check the fuse to ensure it is mechanically intact. The updated fuse cracks with age, works intermittently, and is difficult to detect visually. The earlier fuse is overtaxed and develops flaky connections with the fuse box. Either could cause the fan to cut out.
 

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Discussion Starter #8
Thanks, John. AUX fan would not come on at all when a/c was turned on. Not even on hot days and sitting in traffic when temp was going up. As I said prior, something kept blowing the pre-resistor relay, which is why I replaced the pre-resistor. Maybe an endeavor for not.

The original fuse for the blower motor was bypassed with 20 AMP fuse. Electrician was told to check this and all wiring and came back to me with report of no issues. What you say makes sense which is why I asked him to check in the first place but I guess I will take a look and no harm in changing this fuse. Your advice and logic are sound and cost of fix inexpensive.

Further, I was having a hard time as well understanding this theory of the resistor heating up and causing blower motor to fail . This mechanic has been around for a while and only works on old classic cars in SM and knows more than I, so I gave him credibility and benefit of the doubt.

Will replace the fuse and hope this solves the blower motor cutting out problem. I will advise.

The speed of the blower motor varies at times which led me to believe possible resistor issues as well which is why I contemplated replacing. Further, there seems to be variance in temp of cold air coming from a/c which is also a possible resistor issues (or ACC unit) which is why I had all tested. I just figured since the aux fan never came on when the a/c was on, this was also a possible cause and I wanted to see where things were at after this was fixed.

Thanks, John.
 

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I started thinking about the pressure in the a/c system again since you brought it up. I did convert the system to 134A a few years back. I think I read system had to be recharged with 15.3% less of system capacity to obtain the same pressure ratio of the old R12 in the system (since R134A weighs less). Is this right? How much is system capacity? If system pressure is overcharged, a/c coolant temp switch won't turn on aux fan right? I guess this needs to be tested. I just assumed system pressure was ok since the a/c pump and clutch kicks on when a/c is engaged. So the a/c coolant pressure switch opens at 15 and then closes again at 20 (shutting off the a/c pump)? Is this right? And, iF the system is either over pressurized or under pressurized, the aux fan won't work at either sensor (both on the thermo housing and receiver/drier?)?
 

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Yes.

Regarding pressure switch at receiver, my understanding is if system is under pressurized, AUX fan will not work on either switch. If over pressurized, AUX fan will still work but a/c compressor shuts off. So, AUX fan will only work in over pressurized system at the thermo housing aux fan switch. I have already confirmed this worked with electricians but it didn't stay on long. Kicked in around 105 and went off around 102.
 

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I just assumed system pressure was ok since the a/c pump and clutch kicks on when a/c is engaged. So the a/c coolant pressure switch opens at 15 and then closes again at 20 (shutting off the a/c pump)? Is this right?
Not exactly...

There are two pressure switches. If the system is undercharged the low pressure switch prevents the compressor from running at all. The system pressure must be above 2.6 bar or the compressor won't come on. It must remain above 2.0 bar or the compressor will shut back off. This is not your problem. The second, high pressure switch closes its contacts if the high side pressure reaches 20 bar. This happens as a function of the compressor and the amount of heat shed by the condenser. If the condenser cannot shed enough heat, either because air temperature is high or air flow is low, the pressure builds and the aux fan comes on low speed to help cool the condenser and reduce the pressure. The fan shuts off again if the pressure drops back to 15 bar. So, the aux fan doesn't come on at all until the system is running and the high side pressure gets high enough that the condenser needs extra airflow to maintain normal operation. This is normal, and likely to happen on hot days or when stuck in traffic.

There is also a dual contact coolant temperature switch. One set of contacts bypasses the aux fan resistor if coolant temperature exceeds 105*C, putting the fan on full and increasing airflow over the condenser and the radiator. This is still normal operation on very hot days or in traffic. If the coolant temperature continues to climb, and exceeds 115*C, the second set of contacts close, sending a signal to the climate control to turn the compressor off, reducing the load on the cooling system, in an attempt to prevent the system from boiling over. (This should never happen under normal operation.) This function has no effect on the aux fan; it is already running at full speed because the temperature is above 105*C, and continues to run until the engine cools down.
 

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Thanks, John for that detail. Very well explained. I apologize for being obtuse here but kindly indulge me:
There are 3 switches that have an effect on aux fan control:
1) Two switches on Receiver/Drier
a) a/c coolant pressure switch (Low pressure switch?)
b) a/c coolant temp switch (High pressure switch?)
2) Aux Fan Switch on Thermo Housing which is a 2 poled switch:
a) pole 1 turns on Aux fan when engine coolant gets to around 105 per wiring diagram
b) pole two turns off a/c compressor when engine temp gets to around 115

I do not know how the a/c coolant temp switch works on the receiver drier. All I know is that this switch controls the AUX fan and has it's own pre-resistor relay and there is a pre-resistor that functions to reduce the aux fan speed when a/c is engaged. Are you saying this switch functions on pressure readings as well from the a/c coolant system at the receiver/Drier? Ie, switch opens at 15 to turn on aux fan and closes at 20? I do know if I jump the wires that lead to this switch, they turn on aux fan (cause I did this). But how this switch works and when it works is still unclear to me. I was told by just about every mechanic it should come on when a/c is engaged. But perhaps this is not true based what you said? I thought the 15/20 pressure switch was the pressure switch at the receiver drier.

Regarding the a/c coolant pressure switch on the receiver/drier, when I jumped this, it turned on the a/c compressor clutch. I thought switch open at 15 pressure told me that when I tuned the a/c on, the system was NOT under pressurized. Are you saying this switch opens at 2.0? Further, because the system was not under pressurized, the aux fan switch on the Termo housing should work as it is dependent upon the a/c coolant pressure switch to work. My rudimentary understanding of the 20 pressure upper limit switch was that it just turned off the a/c compressor to save the system as you have described above, but that it doesn't effect the aux fan switch at the receiver/drier. If this is not the 15/20 pressure switch, what is the upper limit of this switch whereby it cuts off system? You stated if it goes under 2.0 after running , it will shut off the compressor, correct?

The aux fan switch on the Thermo housing has its own relay and , as you said, when the switch opens at 105, it will override the aux fan switch at the receiver/drier and turn AUX fan on high. Question, I was told that when this switch kicks in, AUX fan should stay on until temp gets back to around 90? Is this correct? As I said, mine turned on around 105 when tested and then went off again around 102.

Sorry I'm having such a hard time with this. So, either my a/c coolant temp switch at the receiver/drier is bad and/or my system isn't pressurized correctly? And, since I converted to 134A, what now should be the amount of coolant put into my system? I was told capacity is 45 and since 134A weighs less, to obtain the same pressure I needed to use about 15% less capacity in order to obtain the same pressure rations as the R12 did so the system will function correctly. Kindly advise.

Further, fluctuations in fan speed and temp of a/c possibly attributable to either blower motor resistor? acc unit ? malfunctioning mono control valve? By the time I'm done, I will have replaced everything in this system. LOL

Thanks much for you assistance. Truly appreciated.
 

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I can't proof read this any more. Hope it's clear and correct...
Thanks, John for that detail. Very well explained. I apologize for being obtuse here but kindly indulge me:
There are 3 switches that have an effect on aux fan control:
1) Two switches on Receiver/Drier
a) a/c coolant pressure switch (Low pressure switch?)
b) a/c coolant temp switch (High pressure switch?)
2) Aux Fan Switch on Thermo Housing which is a 2 poled switch:
a) pole 1 turns on Aux fan when engine coolant gets to around 105 per wiring diagram

b) pole two turns off a/c compressor when engine temp gets to around 115
Change bolded text to read:
a) A/C pressure switch (2 functions)
b) A/C aux fan pressure switch

2) Coolant Temperature Switch on Thermo Housing which is a 2 poled switch:
a) pole 1 turns Aux fan to high speed when engine coolant gets to around 105 per wiring diagram

There is no A/C coolant. The system is filled with refrigerant, Freon (or whatever the EPA makes us use today). There are no sensors measuring the temperature of the refrigerant, only pressure. When the Freon leaks out the A/C pressure switch opens and prevents the AC system from activating; the compressor won't run. The other function turns the compressor off if pressures reach unsafe levels (30 bar or 425 psi). So, this switch is off if the system pressure is too low, on when pressures are acceptable, and off if pressures reach dangerously high levels.

The the aux fan pressure switch closes when the refrigerant pressure from heat and the compressor reaches 20 bar (about 290 psi) and supplies a ground to the aux fan resistor relay, turning the relay on and the fan on to low speed. If the pressure drops to 15 bar (about 220 psi) the switch opens and the fan shuts off. So, you can see, the fan does not normally come on as soon as you turn on the A/C. It won't come on unless the high side pressure goes above 290 psi.

The coolant temperature switch on the thermostat housing has two functions and is only indirectly coupled with the AC and the aux fan. One set of contacts closes when the temperature reaches 105*C and turns the aux fan, via the relay, on high speed. This will happen even if the A/C is not on. The purpose is to lower the temperature of the (engine) coolant leaving the radiator. The second set of contacts closes at 115*C and signals the A/C controller to shut off the A/C to reduce the load on the cooling system.

I do not know how the a/c coolant temp switch works on the receiver drier. All I know is that this switch controls the AUX fan and has it's own pre-resistor relay and there is a pre-resistor that functions to reduce the aux fan speed when a/c is engaged. Are you saying this switch functions on pressure readings as well from the a/c coolant system at the receiver/Drier?
Only on pressure. It signals the relay to turn the fan on through the resistor, which gives low speed.

Ie, switch opens at 15 to turn on aux fan and closes at 20?
The other way around. The switch is open (off) until the pressure reaches 20 bar, at which point it closes (turns on) and stays closed until the pressure drops back to 15 bar.
I do know if I jump the wires that lead to this switch, they turn on aux fan (cause I did this).
Right, on low speed.
But how this switch works and when it works is still unclear to me.
Hopefully no longer.
I was told by just about every mechanic it should come on when a/c is engaged.
No. This is true of cars with no engine driven fan, but not ours. The engine driven fan can handle the system needs alone under many conditions.
But perhaps this is not true based what you said? I thought the 15/20 pressure switch was the pressure switch at the receiver drier.
Yes, one of two.

Regarding the a/c coolant pressure switch on the receiver/drier, when I jumped this, it turned on the a/c compressor clutch.
That sounds like the the low pressure switch. It prevents the compressor from running when the charge in the system is too low.
I thought switch open at 15 pressure told me that when I tuned the a/c on, the system was NOT under pressurized. Are you saying this switch opens at 2.0?
Yes, it opens if the pressure drops to 2.0 bar (about 29 psi).
Further, because the system was not under pressurized, the aux fan switch on the Termo housing should work as it is dependent upon the a/c coolant pressure switch to work. My rudimentary understanding of the 20 pressure upper limit switch was that it just turned off the a/c compressor to save the system as you have described above, but that it doesn't effect the aux fan switch at the receiver/drier. If this is not the 15/20 pressure switch, what is the upper limit of this switch whereby it cuts off system? You stated if it goes under 2.0 after running , it will shut off the compressor, correct?
Now I'm confused...

The aux fan switch on the Thermo housing has its own relay and , as you said, when the switch opens at 105, it will override the aux fan switch at the receiver/drier and turn AUX fan on high. Question, I was told that when this switch kicks in, AUX fan should stay on until temp gets back to around 90? Is this correct? As I said, mine turned on around 105 when tested and then went off again around 102.
Closes at 105. Correct. I don't know how much the temperature needs to drop before it goes off again. I wouldn't worry; it sounds like it is working correctly.

Sorry I'm having such a hard time with this. So, either my a/c coolant temp switch at the receiver/drier is bad and/or my system isn't pressurized correctly?
Why do you say that? If the compressor turns on then the system has at least enough charge to fool the low pressure switch. Telling any more than that requires pressure gauges.
And, since I converted to 134A, what now should be the amount of coolant put into my system? I was told capacity is 45 and since 134A weighs less, to obtain the same pressure I needed to use about 15% less capacity in order to obtain the same pressure rations as the R12 did so the system will function correctly. Kindly advise.
I haven't worked on any systems commercially since before there was R134A, so i don't know for sure. I fill my own systems by pressure, not weight. You want your low side to have the correct pressure for just a touch above 32*F, so your evaporator doesn't doesn't freeze up.

Further, fluctuations in fan speed and temp of a/c possibly attributable to either blower motor resistor? acc unit ? malfunctioning mono control valve?
Depends on what is happening. Unless the temperature control is on max cool (or heat), the controller will vary all those things in an attempt to make you comfortable by Mercedes' standards.

HTH
 
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