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

you have to be more descriptive about your problem.
do a search on "a/c cuts out," or "acc." some useful
insights.

More descriptive -- OK let me try to clarify this further. The climate control works perfectly except for the fact that when trying to engage the air-conditioning mode, the air-conditioning compressor fails to engage. When either pushbutton for air-conditioning mode is selected, the compressor will turn on once I tap with my finger on the relay located in the engine compartment. This is the relay that supplies power to the air-conditioning compressor clutch. It is definitely not a defective relay or the wiring leading to the relay.

There is voltage present at the relay socket trying to engage the relay, however the relay is not getting enough current to actuate it. The voltage is about 12.6 to 12.7 V, although once again, the relay does not engage. This is what led me to believe that perhaps there is not enough current (amperage) available to trigger the relay, although this seems unlikely as the relays do not take very much current to trigger them.

Therefore, I am trying to determine the source of the voltage that is supposed to trigger the relay. If the “electronic switching unit for temperature control” supplies the voltage it may be switched via a solid-state circuit within that unit that may be crapping out. I do not know if the electronic switching unit tends to be a problem, like the pushbutton control units are. If the voltage is a direct connection made by a simple switch within the pushbutton control unit, then I am completely clueless as to how to proceed from this point forward.

And yes, I have done a fairly extensive search yet have not come up with any similar problem.

I appreciate your response, and hope that someone else out there may have either experienced the same problem, or may be able to point in the right direction.

Thanks again!

RTH

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

I have funny climate control problems in one of my cars - how hard is it to get to the buttons to clean the contacts?? I'd like to try that.

__________________
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey

SOLD:
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino

I was actually speaking of cleaning the contacts within the ETR switch, which is what controls the evaporator temperatures.

If you are planning on trying to repair your pushbutton assembly, it can be disassembled and the contacts can be cleaned using electrical contact cleaner. I have used brake cleaner in a pinch, as it also leaves no residue. TV tuner cleaner leaves some lubricity for moving parts that don’t carry current. Do NOT use carburetor cleaner, it will attack the plastics.

There are four little double pole relays on the circuit board – try cleaning the contacts by sliding some plain paper soaked with a bit of contact cleaner between the contact points. The other parts to clean are the sliding contacts for the buttons themselves.

I won’t start up another debate about the re-soldering of all the joints on the circuit board. IF there is a cold joint or one that has detached, this may solve some problems. Or, conversely, you could toast the components on the board depending on your finesse level with a soldering iron. Be aware that either way, once you have messed with your board, it will not have any core value for at least one of the reputable rebuilders that I know of.

Good Luck

RTH