My AC is fixed. I bought a good used klima relay and it did not help. Not long after I started the thread I noticed erratic tach behavior where it would work intermittently. Most of the time the tach would read zero, but if I rev the engine sometimes it would register but only go up to 1000 rpm and not more. If I jumper pins 5 and 7 on the klima relay, I can get the compressor to kick on and then the tach would work normally (register the correct rpm). Knowing that the RPM signal has no connection to the klima relay on the 85 300D, this got me really baffled. The only place the RPM sensor goes to is the EGR computer at the passenger side kick panel. When I was in there a while ago, I had the EGR computer apart (to learn more about how the tach works), I brought out pin 25 EGR amplified tach output signal (with a wire) so I can measure the voltage w/o disassembling the EGR computer. This was documented in this thread
Quick Fix: 1985 w123 Tach & A/C after EGR Failure
Anyway, with a DVM on the tach signal wire from pin 25 I saw 1/2 volt AC, if I jumper pins 5 and 7 of the klima to kick in the AC compressor, the voltage goes up to 2 volts AC. This makes no sense to me.
So I pulled the EGR computer off the mount and as I lifted the connector off the socket, a lot of water gushed out! There was much corrosion around the pins and no doubt it was shorting out some of the signals. I scraped all the pins clean and blew dry with compressed air and everything is back to normal. AC and tach works normal again.
Where is the water coming from? It must be coming from the harness! There is a main (fat) cloth tape wrapped bundle (which did not feel wet) and there is a skinny bundle that has a sheath (for the RPM sensor wires) that I think is where the water is coming in from. There is a disconnect for the RPM sensor by the middle of the firewall where I can see the sheath is open and that maybe where the water is entering. I will goop that opening up with Silicone and see if that stops the water. The internals of the EGR computer was totally dry, good it was designed hermetically sealed.
I still am not sure why the tach signal affect the AC compressor. It may have something to do with the OVP relay.
In the diagram below, does anyone know where the OVP relay external pin 1 goes? The diagram says "C circuit 30" but that tells me nothing. There is a circuit 30 withing the relay but where does that wire actually go to? Does it go to the AC compressor clutch? I wish there is one wiring diagram that shows everything.
