Originally Posted by Doktor Bert

Jeremy5848
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Join Date: Mar 2006
Location: Sonoma Wine Country
Posts: 7,426

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Fixed!

I am pleased to report that with a used-but-good Klima relay (part 001 545 74 05) from SoCal Mercedes Parts and about 4 hours of work, I have been able to take the final step in returning my 85's climate control system to factory condition.

A PO had dealt with the Klima relay failure by wiring around it -- a simple relay that turned on the compressor whenever the climate control system was on and calling for cold air. I had to remove the extra relay and reverse all of the wiring changes that were made. One of the things I discovered was that the "abandoned" cable I found some months ago -- someone here theorized it might have been for an alarm system hood switch -- turned out to be the original wiring to the compressor solenoid.

In the beginning, the hard part was not knowing how the system was wired originally (hence my whine that started this thread). The answer, which I have uploaded here, lies in the wiring diagram and notes that the factory published in a manual entitled "Model Year 1985/Passenger Cars/USA Version/Introduction into service." This information, which is apparently unique to the 1985 model year W123 300D/CD/TD, does not appear in any other FSM of which I am aware, not even the climate control manual.

According to the manual, the Klima relay has three functions:
(1) when the engine is started, the relay keeps the compressor solenoid off until 10 seconds after the engine speed has reached 600 RPM. This "improves engine speed stabilization after startup."

(2) with engine speed less than 1050 RPM, when the accelerator is floored, the compressor solenoid is released until the engine reaches 2150 RPM or the accelerator is released. This function is controlled by a microswitch on the valve cover (I thought that microswitch was the kickdown switch -- it is not).

(3) if freon pressure is lost, a pressure switch tells Klima to turn off the compressor solenoid, saving the compressor from damage.

None of these functions work if you have a work-around relay in your system.

The Klima relay is connected to the EGR controller (that's how it gets the RPM information, I suppose) which is hidden behind the passenger's righthand kick panel. This probably means that the relay will not work if the EGR controller has failed; a clue might be loss of tachometer function. It also means that, when removing EGR parts "for testing," you want to leave the EGR controller in place and connected.

Jeremy