svchama

The A/C on my 1994 500 SL started blowing hot and cold at the same time. If I select the center position it blows cold but the upper or lower positions blow hot. Any suggestions?

bobterry99

I will guess that the in-dash climate unit has lost control of the valve which regulates the flow of hot coolant into the heat exchanger located behind the center of the dash, and I will guess the most likely cause is a failed valve.

I would run diagnostics as explained here. The unit may report a fault that aids in your diagnosis. If it does not, then look at the what the manual calls "Test mode A" and check the values that are displayed. Assuming they are all plausible to you, if my assumption about the control valve is correct you will find that the heater core temperature (item #08 in the menu) is much, much higher than the ambient outdoor temperature.

svchama

Hello Bobterry99,
Thank you very much for answering my question. I did the tests but I’m not certain how to interpret them or where to find the remedy info. I used a cheap outdoor thermometer for the actual temps inside (31°c) and outside (30°c) the car.
The results were;
Test A Possible Cause
02 33°c In-car temperature sensor
Test B
01 No stored DTCs in system memory
En d
07° Evaporator temperature sensor (B10/6) open circuit
70° Aux coolant pump (M13), open circuit
73° A/C compressor signal, open circuit
74° Aux fan signal, 2nd stage, open circuit
75° Aux fan signal, 1st stage, open circuit
En°d
Test C Nominal value
02 33°c Indicated value may deviate by no more than ±1 °c
04 33°c Indicated value may deviate by no more than ±3 °c
06 32°c Indicated value may deviate by no more than ±3 °c
08 31°c Indicated value may deviate by no more than ±3 °c
12 33°c Indicated value may deviate by no more than ±3 °c
16 23u
18 21u
20 07u
22 11u
24 38u

bobterry99

You have 5 stored intermittent faults. I suggest you erase them, and after driving the car for a bit read codes again and see which return.

Test C shows a coolant temperature of 33 degrees. The test needs to be done with the engine at operating temperature and with the climate control blowing hot.

svchama

Hello bobterry99,
I ran the test again with the system at operating temps. Here are my results. Do you think my problem the in-car temp sensor? If so, where can I find the info on how to fix it.
Thanks again

Temps inside (28°c) and outside (32°c) the car.
The results were;
Test A Possible Cause
02 32°c In-car temperature sensor
Test B
01
En d
74°
En°d
Test C Nominal value
04 39°c Indicated value may deviate by no more than ±3 °c
06 27°c Indicated value may deviate by no more than ±3 °c
08 34°c Indicated value may deviate by no more than ±3 °c
12 80°c Indicated value may deviate by no more than ±3 °c
16 23u
18 22u
20 07u
22 10u
24 38u
26 39u
En°d

bobterry99

No, I do not think so.

Your A/C compressor should be running continuously, and assuming that it is the evaporator temperature of 27-degrees is too high. This would suggest that the system has a faulty expansion valve or is low on charge. A set of pressure gauges connected to the system may be the proper way to diagnose your problem.

SRHsl600

the Uni valve, a water control valve corrected the problem for the 600. However you are getting advice from one of the best with bobterry99 answering!

svchama

I got it fixed! It was just a loose connection on the uni-valve. Thank you both very much for your input.