Compressor will not cycle with LPCO jumpered

[robmaxfli]

I'm starting with all new AC parts - everything - on my 1992 R-12 system.

I've added 18oz. of R-12 so far (38oz capacity) but I cannot add more because the comp. clutch will not come on again. At first, I was able to add some r-12 and the clutch cycled very quickly a few times. I could see the Low side build pressure and then drop when the clutch went off - pretty normal. Then the gauges show the system start to equalize as if I had turned it off. The pressures were normal - on the low side - while charging. However the lines never felt cold with this much Freon in.

But then the Clutch stopped cycling and I cannot get it to come back on - even if jump the Low Pressure Cutoff. Static Pressure is about 90 on Low and High here in hot Dallas. so there is plenty of pressure and I shouldn't need to jumper the switch at all.

I checked all the fuses and relay and they are all good. I'm not sure where to go with this but I have attached a link to the wiring diagram if you are so inclined.


https://www.justanswer.com/uploads/port ... iagram.gif

[71403]

There's also the high pressure cutout switch on the back of the compressor. Might be easier to back-probe at PCM can verify a/c request and compressor relay control at the same time.

Is this an early 90's full size FWD Buick? Been a while but recall there were problems with moisture/corrosion in the bulkhead connector near the accumulator.

[robmaxfli]

Why yes - its a '92 Park Avenue...What was the solution to the bulkhead connector issue?

I did a continuity test on the high pressure switch on the back of the compressor and it passed - so it seems good.
I did the same test on the low pressure switch (both installed with pressure) and it gave continuity too - but I don't think it is supposed to have continuity at rest with pressure (like the high side).

thanks!

[71403]

At static pressure both switches are closed / have continuity.

Check there's +12v at the LPCO on the light blue wire when it's connected to the switch (back probe) and a/c is requested. If so both switches are good, pressures are in range and head unit is supplying +12v. If that passes check at at PCM pin D11. If +12v present - test light preferred - the PCM is seeing the request for A/C.

If that checks out move onto the clutch control side.

[robmaxfli]

I am getting only 9.7V at both wires at the LPCO and the same on both wires at the HPCO. To trace upstream, I traced/pierced the light green wire ( that is supposed to connect to a DRK GRN/YEL wire) up through the loom up right before the firewall - and got about the same 9.50V. ...

I suppose the only thing I can do is try to find where the light green and drk grn/yel wires meet and see how to looks - will take some digging to find in that passenger firewall area inside.

thanks

[robmaxfli]

For no reason whatsoever, the clutch decided to come on when I randomly turned on the AC again!!

When it did I checked the voltage again at the LPCO. I noticed that when the clutch came on I got 12V on the DARK BLUE wire and 0V on the light blue. I swapped the 2 wires on the back of the LPCO and it still gave me 12V on the DARK BLue only. I also noticed that when the clutch stopped, the 12v dropped to 9.5V until it kicked back on again.

[robmaxfli]

Well, now I can't get the clutch to come back on...something is flakey...I'm going to try to get to the control head and test the volts leaving that ...