Compressor relay bypass??
 

Hi have 1988 300SE W126. The a/c is cold and works fine but the compressor disengages after a few miles and won't come back on until I turn off the car and restart it. Again in a few miles (if I'm lucky) it is off again. After reading several posts about all the reasons the compressor will disengage, I tried a few but it keeps happening. I then thought I would wire in a temporary switch between terminals 15 and 87 at the relay connector and leave the compressor relay out. Turning the switch on made the compressor start, but it still quit after a few miles. Now I can't get it to come on at all. There seems to be at least 5 wires going into the terminal on the compressor. Does anyone know if I can bypass everything and supply 12volts to the compressor energizing coil to engage it? I live in Phoenix at 105 degrees. I would really like to just force it on when I need it. Thanks!!

Don't run it without an evaporator temperature sensor in the circuit, or you could freeze your evaporator into a block of ice.

Check for 12V at the compressor when it fails. If you have 12V, tap the clutch with a screwdriver handle to see if it engages.

Which wire at the compressor do I check for 12V? There is at least 5 of them.
Thanks
Erik

You did mention that; I missed it.

You'll have to find out which of them goes to the clutch coil. Of course, that many wires indicates that you have a speed sensor, so if the clutch slips the current to the clutch coil will be cut.

How big is the clutch gap? It should be around .020". If that's fine, another possibility is a dirty speed sensor.

If you have a sw bridging 15 and 87 @ the relay , then speed sensors , pressure limit cut-out, temp cut-out, etc. circuits are all out of the circuit b/c that jumper is direct 12v from fuse F/7 to clutch and by-passes all sensor inputs to relay.
So, if the clutch is dropping , you have a poor contact at fuse 7 or a bad upstream feed to that fuse, or a bad clutch/gap.
The test would be to put a test lamp at 15 and ground and see if that goes out when clutch goes out. IF YES, that would indicate fuse/feed upstream problem..
If NO, that would indicate downstream problem....clutch/coil/wiring..

Send me a pm-mail OR e-mail & I will send you instructions.

Sorry, I haven't got back on this. That day after work I went out and the clutch engaged when I flipped my switch. It stayed on all the way home (45 miles) and seems to be working ever since. However if I turn the switch off while it is running, it doesn't seem to engage when I turn the switch on again unless the car has sat for quite a while. It sure is nice to have it work! I'll send a pm to MBDOC.

I'm not too worried about creating ice on the evap coil because the air here is very hot and dry.

Thanks for the input!