1987 300E A/C problem (compressor won't run properly)
 

I thought I had a bad A/C relay (002 545 13 05). I replaced the A/C relay, but the compressor still does not engage. I initially thought it was the relay, because when I removed it and hot wired port #5 to port #7 the compressor clutch engages. So, I guess the problem is whatever tells the A/C relay to activate the compressor. Can anyone help?

When I jump the A/C relay ports 5 to 7 the air conditioning runs fine. But, the compressor runs constantly, and that is certainly not a satisfactory fix.

If you would do a search here, you would find AT LEAST 100 posts describing your situation.

There is a low pressure switch that interrupts the clutch circuit when the refrigerant pressure is low. This is to protect the compressor from a low refrigerant situation because lubricant is not circulated through the compressor under this condition.

The first thing that should be done in almost all a/c diagnostic cases is to hook up the manifold gauges and check the pressures.

Good luck,

All pressures check out fine.
 

When I bypass the relay, all pressures check out fine, the compressor run and the air conditioner produces cold air.

Did you put an ohmmeter on the low pressure switch to see if it was making contact? If it is not, then it must be replaced.

You need to get a wiring diagram and do some basic electrical troubleshooting. Are you getting a signal from the low pressure switch to the relay?

Good luck,

The pushbutton unit on the dash sends a +12v signal to the Klima relay telling it to activate the compressor. I don't have a wiring diagram handy, so cannot tell you which pin it is on the relay socket. The +12v signal passes through the low pressure switch mentioned by Larry. That switch is located on the dryer.

There are also some other inputs to the Klima relay which can shut off the compressor. High coolant temperature (>115C) will shut down the a/c system. On diesel cars there is also a full throttle switch which shuts down the compressor - not certain if gasser cars have this function. You need to check these inputs as well.

You really need the climate control shop manual, a wiring diagram, and an inexpensive DVM to troubleshoot this system. It's not really all that complex, but without that basic information you won't get too far.

- JimY

Thank you (More suggestions are welcome)
 

Thanks to LarryBible and Jim Y. I will use your info and let you know what the results are.