W124 AC Troubleshooting Confusion - PeachParts Mercedes-Benz Forum

Bengoshi2000 · #1 05-07-2022, 10:18 AM

At the end of last summer, the AC quit on my 91 300D. Research pointed to the Klima Relay so I replaced it and enjoyed a couple weeks of AC before it went out again. Now I'm getting serious about getting the AC operational. I'm working my way through the 50 step guide someone posted years back (that I printed, but can't find the link to).

What I've done so far:

jumper pins 5 and 7 on the Klima relay socket, the compressor clutch engages and I have the nice cool air in the cabin that I'm used to.
between pin 5 and the Neg battery terminal I get 13.6v (at idle).
between pin 10 and the Pos battery terminal I get 13.25v (at idle with the PBU on a setting calling for AC). Same reads .46v when PBU is off. So the PBU is sending the proper voltage signal.

With this information I got a Panasonic relay (again suggested in another thread, can't find it in this morning's mind fog) and wired it like this:

Pin 10 to relay 86
Relay 85 to pin 1 (ground)
Pin 7 to Relay 87
Pin 5 to Relay 30

This should throw the switch between pins 5 and 7 when pin 10 is hot (ie the pbu calls for ac). But alas, no dice.

I know that this configuration bypasses all of the safety/kickdown protections the Klima provides... but I'm starting with basics. If I can't get this simple configuration to work the other features are a moot point it seems.

I have pulled apart the old Klima relay and there are several burned spots on the trace leading to pin 1, which is the ground IIRC.

FWIW, Pin 1 to the neg battery terminal reads 0v and about 1 ohm of resistance.

Do I have a bad ground at the Klima relay socket?

Not sure how to troubleshoot from here. Any suggestions?