1998 SL500 R129 Climate Control No A/C
 

Hi All:

Having an issue I'm a little stumped on where to start diagnosing next. As it is now, the climate control unit works for heating the car just fine but no A/C, no engaging of the compressor. Refrigerant levels are low but still should be enough to run. The 1 thing is that I'm unable to activate forward direction airflow as well. Ran the codes with Xentry and came up with the following:

KLA Fault Memory
B1416 Heating water circulation pump; A31m1
B1232 Refrigerant pressure sensor; B12
B1233 Refrigerant temperature sensor; B12/1
B1228 Left heat exchanger temperature sensor; B10/2
B1230 Evaporator temperature sensor B10/6
B1233 Refrigerant temperature sensor B12/1
B1226 In-car temperature sensor B10/4

Of these I can't clear any so I took a shot at the Evap Temp Sensor replacement and still have the same issue. Ran sensor values direct from the Climate Control unit and the errors from the Xentry Fault Memory are all reading OPE showing open circuit on the control unit which was my suspicion of a domino effect from either fuse/relay vs all of these sensors being bad. Fuses look good but a not sure about any relays directly affecting only these sensors. (or if 1 of these sensors is the culprit for the remainder)

Any help would be much appreciated! Thanks in advance for your time.

I'm on the run for the moment so can't type much. Be aware that the HVAC dash unit is the same as a 97 era C class. You will need to change the version coding by the scan tool or some button presses on the unit.

Buttons can break underneath.I've had this problem 10 years now.

Pulled the codes again tonight which are displayed below. The actual readings for the open circuit looks like it's pointing at the in-cabin temp. sensor. Does that seem correct? (ie; -//- means circuit OPEN)

If that's the identifier then that'll be the likely culprit. Once that's closed the remainder should follow suit unless there are additional bad sensors within the loop.

Also included the CCU version information just in case.

http://proudfoot-design.com/samples/...612_184657.jpg

http://proudfoot-design.com/samples/...612_184838.jpg

http://proudfoot-design.com/samples/...612_184858.jpg

http://proudfoot-design.com/samples/...612_185417.jpg

http://proudfoot-design.com/samples/...612_185420.jpg

http://proudfoot-design.com/samples/...612_185424.jpg

http://proudfoot-design.com/samples/...612_185721.jpg

http://proudfoot-design.com/samples/...612_185850.jpg

http://proudfoot-design.com/samples/...612_185725.jpg

It looks more like sensor power or ground has a problem and that is preventing all of the sensors from operating. You need a wiring diagram then hang a volt meter on the wires and see what is happening.

I have had situations on other brands of cars where sensor power is pulled to ground then the power supply shuts down to prevent melting something.

Makes sense! Thanks!

Doing some research and found this:

The cabin temp. sensor is located in the upper control panel along with the switches for the dome light. There is also a wire hooked into that circuit that goes to the power top computer in the trunk. It turns off the AC when you have the top down.

Is this true in the R129 and something to look into? I wouldn't consider this system registering 'faults' in the computer if that was part of the design?

FIXED!
 

DeOxit cleaning of all the cables and re-connecting of the CCU. All is perfect!

Could you explain DeoXIT and exactly what you did. I'm having similar problems with my 1997 SL 500.

DeoXIT is a brand of contact cleaner. Apparently he pulled the climate control ( the button panel ) from the dash and cleaned the contacts on the wire and unit end.

Thanks, I guess it's worth a try !