I'm not much of an a/c tech so I'm looking for a little help here. Everything was working just fine 2 weeks ago. When I turn on the blue dial I don't hear the compressor engage. No slight drop in engine rpm. Obviously no cool air through the vents. '84 400i. Front & rear radiator fans are working. Both compressor belts are there and the pulley is turnng without any noise or "screaching". Fuse is good. Checked the relays by replacing them one at a time with a working one. There is a single wire that goes to the compressor. This sounds like a really stupid question but; am I correct in assuming that this is the power wire?
Yes -- when that wire is at +12V, the EM clutch on the compressor will engage and transmit power from the belt to the compressor (there are actually two wires, but the other wire will be a short one that is connected directly to ground). However, there are pressure switches in series with that +12V wire -- if the refrigerant pressure is too low or too high, the corresponding pressure switch will be open and prevent the +12V from reaching the compressor EM clutch. The pressure switch is on top of the dryer/filter (from the 400i SPC drawing it looks like you have a "combination" pressure switch) -- as a quicky test, you can unplug the pressure switch and then connect the two terminals in the wiring harness together with a jumper wire (simulating a "closed" pressure switch) and see if it works or if you can see refrigerant flowing in the sight glass on the dryer/filter. Alternatively, you can just apply +12V to that single wire at the compressor from another +12V power source (like the "+" battery terminal) and see if it works. (However, it is possible that you've lost all of the refrigerant so even if the compressor runs you may not see anything in the sight glass nor get any cooling.)
Hmmm..... there was a bit of R12 put in last October and a bit more put in at Tim Stanford's place this Easter. Makes me wonder if there isn't a small leak. I think I understand. If there was and the pressure was too low, from lack of freon, would the low pressure swtch keep the compressor from firing off?
Yes -- using a test light would be fine (even better because if you have no refrigerant at all you can still detect/not detect something). If you "fail" that test (i.e. no +12V present), short the pressure switch closed (as described in the prior post) and recheck.
Dave Going back to your original comment about the blue knob. When all is okay, the AC system is active with the blue knob in any position. All that is required is to turn the yellow know to any fan position. I think your problem is that Houston is too damn hot! (I lived there 8 years.) Ken
Nice posting dstacy I would also check for power at the combination switch before you bridge it out as I have seen thermostats fail and the +12v feed to the binary/combination switch comes off your thermostat
Ok, finally got around to checking things out. There is power INBOUND to the switch on the left side of the drier. I think that is the low pressure but I'm not sure. No 12v outbound from that switch. Sounds like its going to be a simple matter of throwing in a pound of R12 and away we go. Thanks for the help so far, I'll let you know.
