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Discussion Starter · #1 ·
My AC Compressor clutch doesn't engage unless I directly connect it to the battery. It has plenty of refrigerant and I don't believe it is the low or high pressure cutoff switches. Here is the behavior I am seeing.
  • I see voltage across the control inputs on the A/C enable relay.
  • If I jumper the switched circuit on the A/C enable relay socket while there is a diode in D101, it immediately blows the 10A mini fuse.
  • With a diode in at D101, both sockets of the connector going to the A/C Compressor Clutch show continuity to ground.
  • Without the D101 Diode, only the black wire shows continuity to ground.
Is the ground connection I'm seeing expected behavior for a diode or does this sound like a bad diode? Perhaps maybe a ground short somewhere between the relay and the diode? I must admit I am fuzzy on the expected behavior of the diode in this circuit and as I am typing this I am wondering if I got the connections wrong on the relay when jumpering it. I don't think I did but am not going to do it again until I get some more fuses. When I first started testing this, the 10a mini fuse was fine. I did not blow it till I jumpered the relay.

Any suggestions on how to further debug or fix this issue would be appreciated.

Thanks

Product Rectangle Schematic Font Slope
 

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Discussion Starter · #7 ·
Progress but it still isn't fixed. The diode is bad. It shows continuity between the pins no matter which way I test it. I am using the continuity setting which beeps when the probes are connected. The diode in the slot above shows no continuity either way so that is a bit perplexing. Either they both are bad in different ways or I need to learn more about testing diodes. My local Napa does not have replacements so at the current time I have just removed it and stopped blowing the 10a fuse.
  • I found that the connection to the compressor clutch is flakey (maybe the ground). With the relay jumpered I can get the clutch to engage by jiggling the connector.
  • I found a position that had the clutch consistently engaged with the relay jumpered but when I swapped the relay back in, the clutch does not engage even though the control circuit on the relay shows 13.1V.
  • With the plug not connected to the clutch my DMM shows 13.1 volts at the plug with the relay jumpered and fluctuating millivolts with the relay in place.
  • I replaced the relay with a new one bought at Napa and it still doesn't work.
  • I'll have a new plug for the clutch and 2 replacement diodes tomorrow. The diodes I found on amazon.

Is it possible the relay is "noticing" a poor ground or lack of diode on the switched circuit (that my DMM does not) and refuses to connect it? Any other suggestions on next steps?

Thanks

Circuit component Gas Control panel Cockpit Electrical wiring
 

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Discussion Starter · #9 ·
Thanks for the info and pictures.

I agree that it is hard to tell which way to plug the diode in. The white graphic on the back does seem to denote a direction for the diode but there is no indication in the fuse box as to which way to put it. The plastic body of the diode is keyed to the socket so that you cannot seat it the wrong direction but you can make an electrical connection without seating it. Probably best to pull the relay before inserting it if you don't remember which way it goes. That may very well be how mine got blown.

Took me awhile to figure out which diodes I needed to buy. Finally noticed a very hard to read part number molded into the back. GM #12135037. That is how I located them at Amazon. I actually took the van down to parts store without either diode installed so I can confirm that it runs without them.

Thanks again, hoping to get this fixed soon. It's supposed to be 98 here on Tuesday.
 

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Discussion Starter · #11 ·
Progress, but not working yet. This has me really confused. I think I need to reassess some assumptions.

I've replaced the diode and the connector on the ac compressor clutch. I found this resource for testing diodes for those interested.

How to Test Diodes with a DMM

I have tested the diode and relay while not installed in the fuse box and both are fine. I have tried both a new and the existing relay. When I monitor the control voltage without the relay inserted and jump the switched contacts, the clutch engages and the control voltage stays at 13.1 volts. The moment I insert the relay, I get zero voltage in the control circuit. There is voltage across the hot side and battery negative, just not the ground contact for the relay.

I know the VCM switches the ground depending on the state of the high and low pressure switches. If this is what is happening then it seems that the VCM only switches the ground when the relay is inserted. I have tried disconnecting and jumping the connector for the low pressure switch but it makes no difference. I have not done so with the high pressure switch. Can this be tested the same way?

I have the service manual and it has a series of troubleshooting steps involving testing voltages of various contacts within the VCM connectors . I have not done so yet. Can anyone give me suggestions for unplugging the VCM plugs, they seem not to want to be removed without some force and I do not want to damage them.

Any suggestions are appreciated.
 
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