I'm hoping that someone out there may be willing to help me by looking up a wiring diagram for the climate control system in a 1984 300D. I'm trying to determine which part is defective. Unfortunately, the service CD contains a very poor scan of the wiring diagrams, and it is all but useless for tracing the individual wires. As I quote the "item numbers" from the wiring diagram, I am looking at the 1983 schematic, since that is the one that is contained in the CD

I am trying to determine what supplies voltage to trigger the air conditioning compressor relay (item # 28 in the diagram, located in the engine compartment -- the one which engages the electromagnetic clutch on the AC compressor). Is the voltage for the relay supplied from the push button control unit (item No. 4 in the wiring diagram) or from the "electronic switching unit for temperature control" (item No. 9 in the wiring diagram)?

To clarify, I am not looking for the source of voltage for the compressor clutch itself, (which is supplied via a fuse) I am seeking the source of voltage that triggers the coil in the relay (terminals 85/86).

Thank you in advance for your help – it is greatly appreciated!

RTH

RTH - when I was troubleshooting the one on my '85 model I ran a switched "hot" wire from the radio power supply to the "supply" side of the evap temp switch. I could turn my comp on/off by making the switch. Seems that the power is fed from the pushbutton unit and mine was not working so until I replaced the pushbutton unit this is how I turned the comp on/off. The power goes from the pushbutton unit to the evap temp switch and then through the pressure switch to the Klima relay at least on the '85 model. Try putting power on the wire between the evap temp switch and the pushbutton unit. Make sure the wire from the pushbutton unit to the evap temp switch is disconnected first. You probably have a bad pushbutton unit.

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Jim

Before checking anything, make sure you have freon. If it has leaked out the clutch will not operate.
Not sure where that is on your model, but may be at drier.

I got the wiring diagram - the relay is supplied via the pushbutton control unit, and then the voltage goes through the ETR switch to the relay. It ends up being that the contacts inside of the ETR switch were a bit dirty. This explains why there was voltage present at the relay socket, yet because of the dirty contacts there was too much resistance to allow enough current to pass and actually trigger the relay.

I actually disassembled the ETR switch and cleaned the contacts, and now it works fine, except for the fact that it is probably now out of calibration. I will actually get the new switch, but for now this at least allowed me to track down the problem and verify the cause.

Thanks again to everybody!

RTH

Awsome display of investigative exploration tenacity! I v'e never done surgery on an ETR switch before. Where did you find the wiring diagram? I have a two drawings that don't match where my wires go (I am slowly and painfully mapping them out by hand and harness molestation. The brain teaser........one wire goes from #6 socket on klima relay to a terminal bolt on the fusebox floor, thru the gas pedal switch, through and onto the transmission. Why in the wolrd does it go there for??????
The second mystery....... #10 relay socket wire goes to freon pressure switch, through ETR, through neutral safety switch, thru A/C push button switch and onto the fuse. Why in the world ..............
I fear that somewhere in a previous life, the wires may have been rearranged in the connector plug from the harness to the NSS.
Any Ideas>