Well, I have a climate control issue that is stumping me. The problem appeared when I got the car out after its winter snooze. Everything worked perfectly when I put it away last fall.

I get no heat in the automatic mode, e.g., when the temp wheel is set to a temperature as opposed to manual full hot (the "dot"). The system goes through the motions as if it is going to produce heat - the airflow moves to the floor, the fan speed increases, etc. - but no heat. Just A/C. If I put the temp wheel to full heat (manual), I get plenty of hot air.

I have:

- checked the blink codes - Code "1", no errors.
- checked the heater valve and pump and both work as they should.
- checked the electrical voltages to the heater valve and pump and they are correct (e.g., +12 VDC to pump and 0 VDC to valve in manual producing heat and the opposite in automatic with no heat - the control unit is not calling for heat).
- checked each thermistor in the system - all values are as specified by MB.
- checked the suction fan for the car ambient sensor - it is working.
- checked all vacuum flaps are working.
- replaced the complete climate control unit (same behavior for both old and new).

The engined wiring harness is new. The A/C works perfectly but, the auxiliary fan control for engine temps over 105 does not seem to be working.

So what's left. I'm stumped (and cold). Any suggestions?

Thanks in advance,

- FD

 

I will have to check which one but look at the fuses. Is anything else not working?

Horace

 

Horace -

Thanks for the reply. I've checked all the fuses and they are good. I have also now verified that the climate control system auxiliary fan activation based on engine temp over 105 C also works (I tested this with a resistor). So everything on the car is working is it should except for heat in the automatic mode!

 

The system seems to be working again, although I'm not sure why. For the benefit of some future thread searcher, "the fix" may have been the following:

1. I carefully "burped" the cooling system to purge all of the air on the theory that air in the system caused a delay in hot water flow to the core. Because of this delay, when the monovalve is "modulating" to provide regulated heat, it was never open long enough for water to reach the core. When in "manual" mode, the valve is always open so hot water eventually got there and made heat.

2. I ran the pump from an external 12 v source, both forward and backward to "loosen it up". The theory here is that the pump was drawing too much current (binding after sitting for the winter) and causing the control unit to go into over current shut down on the pump drive, thus killing the heat.

3. Spontaneous remission - You know how this works - what ever it was just fixed itself.

One of these three seems to have done it.

Thanks,

- FD

 

On the issue of climate control: I was just curious if anyone knows just how the system works as far as actual air temp coming out of the ducts. As the car interior cools, the temperature of the cooling air seems to be not as cold. Does the system cycle the compressor less as it reaches the preset temp? My system has been recently serviced, it cycles the compressor normally and the interior stays comfortable I but was just wondering if this is how the system is designed to work?

 

Well, the gremlin is back. Little to no heat unless the system is on full hot. Things seemed to go down hill after I replaced the pollen filters (see leaf story posted on another thread). I have checked everything - vacuum pods, thermisters, controller, mono-valve, pump, and heater core. Everything is 100% per MB specs. I still can't figure it out. Any guesses would be appreciated.

BTW, I did discover something while troubleshooting that I have never seen documented anywhere except for a note on a schematic that says, "to air conditioning". It turns out that in the Cabriolet, the soft top controller interfaces to the climate control system and disables the cabin air temperature sensor (the one that's in the mirror mount/interior light switch) when the top is down. This makes some sense because there is really no "cabin air" to sense - the driver is much better off with a more manual mode of operation. The thing to be aware of is that if your top controller is non-functioning or removed, your climate control will not work properly. But alas, not the source of my problem either.

The top controller also interfaces to the rear window defrost relay to turn it off when you put the top down (I have seen this documented).

 

Well, the automatic system is still not working right, but it now seems to be down to just the fan speed. The system seems to provide heat and A/C to bring the cabin temperature to the set temperature. But once it's there, the fan never slows down to "idle" like it used to. The fan speed module is working because because the manual fan speed buttons control the fan speed properly. Yet the fan speed settles down to only about 25% of max fan speed in "automatic" mode. This is way too noisy to leave alone (for now, I change to manual "low" mode after the car gets up to temp).

Question: When you car climate control system is running and the car is up to temperature, how close to the manual "low" fan speed does the fan go in automatic mode?

Thx!

 

You might need a blower motor resistor. This is located in the ducting and controls the fan speed in relation to the temperature of the air passing over it when the fan control is on AUTO.

 

hamrt -

I'm not sure if your post was in reference to the heater core thermistor (there is no blower motor resistor in the actual motor circuit in the W124), but it did give me an idea. According to the MB climate control troubleshooting documents (83-600) the heat exchanger thermistor only controls the actuation of the flaps (e.g., floor or dash). This makes no sense because from a control systems perspective, one would use that sensor to to modulate the opening and closing of the monovalve to control the heated air output temperature just as the evaporator thermistor is used to modulate the duty cycle of the A/C compressor to control the cooled air output temperature. The MB documentation makes no mention of this, but it appears it does indeed control the monovalve. I got to thinking that the climate controller is not adding enough heat because it thinks the output air is hot enough when, in fact, it is not. This, I thought, could cause the fan to keep running at higher speeds to satisfy the call for heat from the cabin air sensor.

I had previously tested the heater exchanger thermistor and it is right on MB specs. So, to test my theory, I modified my core thermistor by adding a 2.7 K ohm resistor in series with the thermistor element. The net effect of this is to make the climate controller believe the heater core is about 20 deg. C cooler than it really is. This, in turn, would cause the monovalve to stay open longer, resulting in hotter output air, resulting in lower fans speeds being demanded by the cabin air sensor.

OMG, I think it worked! At least so far.

If the engine is warm, cabin comes up to temp within a couple of minutes and then the fan settles down to near idle speed (1.4 volts, just as MB says it should). The floor air stays very warm until equilibrium is reached and then settles into just warmer than cabin temp. Shazam!

We'll see if the fix holds, but it's a really good start. Thank you for your post!

- FD