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Old 01-02-2003, 10:04 AM
Posts: n/a
Heater/AC control - beating a dead horse

I am sure this has been covered. I have searrched some of the archives, but could not find the exact thing, but nothing is exact when it comes the the automated heating system control.

Vehicle: 1987 300D

Problem: for first few hundred miles of driving, the automated heater/ac control system works. After that, things get crazy.

The temp control is set at 18 and heat all of a sudden starts blasting in. Then it corrects itself and cold starts blasting in. Went to manual control with same results. Then it works for a time, but fan always stays on about mid setting and never changes speed, except to do the blast thing. Outside temp is approx 25 F.

The present control unit is a rebuilt one that was on the car when purchased.

I cleaned the sensor in the roof as it had a lot of fuzz around it. Plus it "seemed" to work better when I would turn one of the dash vents toward the roof. Sampler fan appears to be working.

Question 1: could the sensor be bad?

Question 2: is it the control unit itself? Any way to test unit?

Question 3: can a new control unit be purchased anywhere? I have only found rebuilt ones - Programa appears to be the only company rebuilding. Any suggestions on other rebuilders.

Question 4: is it just a forgone conclusion that one must purchase a rebuilt unit every so many years?

Any thoughts appreciated - thank you Ron
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Old 01-02-2003, 11:41 AM
Posts: n/a
The most likely culprit is the little blower fan that sucks sample air from the vent near the dome light, through a tube to the temp sensor. This fan should be running any time the ignition switch is on.

It is physically located in several different positions in the 124 chassis depending on year. Mine is behind and to the right of the blove box.

One way to see if it is running is to put a little smoke near the intake grill to the right of the dome light. You can use a cigarette (Yuck!) or some other source of smoke, maybe lighting a match then putting it out and quickly hold the smoking match near the grill. If it does not draw air into the grill, you will need to investigate.

Good luck,
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Old 01-02-2003, 01:09 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
One other thing to think about is the aux water pump. The heater valve is pulse width modulated. This means that it pulses more or less to control water flow. On 124 cars the heater core cools off at idle due to lack of flow. The aux pump needs to move the coolant in these times. If it doesn't the valve is pulsed further and further open and then when the car leaves the light the rush of natural circulation gives a burst of heat before the valve can compensate.

Took me a while to find this the first time. It won't show up at road speed.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 01-02-2003, 06:13 PM
Posts: n/a
Larry and Steve - thank you for your input.

Larry - the aspirator/sampler motor is working. I pulled the glove box and checked.

Steve- the problem seems to occur only at road speed and not when at rest.

I just wish there was some way to bypass the the auto system. Oh for the good old days of cable pull on and off controls. These did not fail, or at least in the vehicles I have owned.

At any rate- thanks guys - Ron
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