AC clutch not engaging

[GreenMonkey]

I recently got the AC charged in my 98 k3500. I drove it home from the shop and everything worked great. Got in it a few days later and the air wouldn’t get cold. The compressor wasn’t engaging. My knowledge is limited on AC issues, but here’s what I tried. First, i tapped on the relay, then swapped in a known good relay from another gmt400. Still not working. I checked the fuse on the side of the dash, and the two in the engine bay fuse panel. They all looked good. Then I bypassed both the low and high pressure switches and it still wouldn’t engage. Next, I tried getting power directly from the battery to the compressor with no luck. Is it possible that I lost all my Freon in 2 days of sitting? I don’t see anywhere that looks like it leaked out (no dye on anything). And is there anything else I can easily check before taking it back to a professional. Thanks in advance.

 

[wirlybird]

I think the odds are on a leak maybe. It is suspect that shortly after getting it charged it stopped working.
Just had to do a compressor replace on my 97 Tahoe. Compressor was leaking.

 

[GreenMonkey]

Thanks whirlybird. That’s what I was afraid of. But oh well, it’s only money. Lol

 

[GoToGuy]

You don't bypass the high press switch, it's normally closed till press exceeds 450psi, then opens, breaking the circuit.
You could get a test light, start truck, fan high, max ac, go to low press switch, back probe the switch with test light, if you have power on both wires on installed switch, system contains at least enough pressure to close low press switch. Get the idea.
You say you bypassed and jumped things. But you don't mention any actual testing for power at any area that completes the circuit to the compressor electromagnetic clutch.

 

[GreenMonkey]

You don't bypass the high press switch, it's normally closed till press exceeds 450psi, then opens, breaking the circuit.
You could get a test light, start truck, fan high, max ac, go to low press switch, back probe the switch with test light, if you have power on both wires on installed switch, system contains at least enough pressure to close low press switch. Get the idea.
You say you bypassed and jumped things. But you don't mention any actual testing for power at any area that completes the circuit to the compressor electromagnetic clutch.

I did test for voltage at connectors to the low and high pressure switches with a multimeter with the key in the on position, but truck not running. Each had around 11.5 volts. Guess I'm not fully understanding of how the switches work. I'll do more research and try to do what you mentioned. not exactly sure what the "back probe" terminology means. Thanks for your help.

 

[pressureangle]

This is where a 'Power Probe' rules over a DMM.

Jump the compressor with B+ and ground to verify the clutch actually works.
If yes, verify clutch connector ground.
Verify power to clutch connector. If no, verify power to high switch on compressor. If no, verify power to low switch on accumulator. If no, move to controls/fuses/connectors in dash.
If you have power to the low switch but not high switch, check sufficient system pressure. If pressure is +25psi you should have power to high switch. If no, replace switch.
This circuit is confusing mostly because the low/high switch normal states are opposites and the low switch can be in an open state due to either low refrigerant or failed switch. Separate the 3 devices and test them individually, it becomes simple.

 

[GoToGuy]

Back Probe, a technique to check for power , or ground, or complete circuit. Either probing the wires or using a fine wire probe inserted along the wire in back of the connector to test for power or ground at device. You can guess, but all diagnostics are a waste if you don't have system knowledge. Fan high, AC button light on, no cooling, even at 2000 rpm , no cooling. AC clutch not engaged. Out of box failure? Happens. Example: up to 20% of all rockets, missiles, bombs fail to detonate. Expensive and brand new.
Download the OE Service manuals here, search in forum for links there Free. Shurkey and others donated there time to share these. There not hard to understand.
Two electric switch protections. A low pressure switch - opens when sys' pressure drops below set value 25 to 30 varies with model.
High pressure switch- opens ( removes power) when compressor output pressure is 450 psi or greater. Therefore is normally closed.
Your choice, good luck!

 

[Schurkey]

Download the OE Service manuals here, search in forum for links there Free. Shurkey and others donated there time to share these.

Not I. I can't take any credit for posting service manuals in that thread.
www.gmt400.com/threads/88-98-service-manuals.43575/

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

I USE those links, but I didn't originate any of 'em.

 

[GoToGuy]

Ok, I gave incorrect credit, but it was a big thanks for those who did do that project. Thank you!

 

[Erik the Awful]

Either probing the wires or using a fine wire probe inserted along the wire in back of the connector to test for power or ground at device.

Rob some of your wife's quilting T-pins. You can stick them in the backside of a connector to probe with your test light or meter.
www.amazon.com/Mudder-Pack-T-Pins-Plastic-Silver/dp/B06XS48HDK