Here's a little problem with my ’84 300D that has me stumped. The climate control system works perfectly, except that the air-conditioning compressor works intermittently. I have ruled out the pushbutton controls, as I have temporarily swapped them with another unit that I know is good. I have traced the problem to the relay (controlling the compressor) in the engine compartment. If I tap on the relay, the compressor will come on. I slightly crimped the electrical conductors in the socket to assure good contact. I also replaced the relay, assuming that the old one was sticking. The behavior was the same as before however. I swapped the relay with another one, just to be sure that I had not received a defective new one. Again the behavior was the same, the air-conditioning compressor will come on when I tap on the relay with my finger.

I checked for continuity in the wiring leads and they are fine. I pulled the relay out and checked the voltage going to the coil (terminals 85 and 86) -- it was between 12.6 and 12.7 V. so it seems as though there's enough voltage to trigger the relay, but perhaps not enough amperage? Strange. Has anyone else ever come across this problem?

I looked in the service CD, but the wiring diagrams are pretty useless -- apparently they are scanned into a PDF file, but the resolution is really lousy. I noticed in the wiring diagrams that there is an "electronic switching unit for temperature control". Question: is the AC relay supplied with voltage via the “electronic switching unit for temperature control”, in which case this may be the culprit? Or is the voltage supplied directly by the pushbutton unit, in which case I don't know where to turn to next.

Any information would be greatly appreciated!

Thanks everyone!

RTH

Man - RTH you sure come up with some doozies . Earlier this year when troubleshooting the a/c on the 300D I could push on the Klima relay and the compressor would run. This problem went away when I replaced the pushbutton assembly. Seems when I pushed on the relay I forced the contacts closed that pulls in the compressor circuit. If you take the cover off of the Klima relay you can see the contact that you are "making up".

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Jim

As far as coming up with "doozies” I usually solve the more mundane problems myself, and then I post the brain twisters on the forum :-).

Unless I am wrong, I don't believe that my car has a Klima relay; so I must deduce that they started using them in 1985. (?)

Due to the fact that the wiring diagrams on the CD are illegible (for the climate control circuitry, at least) I'm kind of lost here. Oftentimes when it comes to problems like these, other people's experiences with the idiosyncrasies of a particular car can help tremendously.

I have given some further thoughts to the problem since my first posting -- I'm wondering if it could possibly be a poor ground connection. I'm not sure where this particular cluster of wires grounds to the chassis; so it may be time to rely on my degree in Mickey Mouse engineering from Disney University. As a temporary test I think I will try to run a ground wire from the appropriate to terminal on the relay socket directly to the chassis and see if that alleviates the problem. I am still wondering if anyone out there has ever had to replace the "electronic switching unit for temperature control" due to a problem like this -- or perhaps if this is a normally bulletproof part, maybe someone can tell me that I'm barking up the wrong tree.

If anyone else out there has any ideas, I would greatly appreciate hearing them!

Thanks again!

RTH

I had a similar problem reguarding the sunroof on my 83 300SD, i would have power everytime i checked it, but the sunroof would only work intermitantley. I traced it to a screw that attaches the posotive wire to the fuse box(it attaches wire to fuse). every time i tapped the fuse box in general it would eaither work or stop working, this is what lead me to this. the only problem with this theory is that on my car the blower is also on the same fuse as the compressor, however if your compressor is a fuse in itself you may want to check the wire connection to the fuse. Hope this helps
Ryan

RTH - call the MB 800 number and order the book titled "Electrical Troubleshooting Manual for the 300D/CD/TD". It will help you with everything you need for electrical/vacuum. OK now for some more help.
You need to find the pressure switch at the dryer and see if it has 12 volts on it. If it does not then you will need to trace that circuit out and try to figure out why it doesn't have power on it. When I got real close to resolving my problem I ended up putting a hot 12 v supply to my evaporator temp switch because the pushbutton unit was not supplying this power. I would turn my compresson on/off by flipping this switch. This supplied power (which was not coming from the pushbutton unit like it should) through the protection circuit to the Klima which would allow the compressor to come on.
I may be wrong but I thought your car had a Klima relay. Look on the inside fenderwell on the driver side in front of the fuse box/brake booster area. It should be a black plastic cover a little larger than a pack of cigarettes. Remove this cover and you should see your Klima.

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Jim

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