Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

[marvin5881]

What I've tried so far.

Verified all Fuses

The Power connector to the low pressure switch gets 12v when the car is on. If I jump it, it doesn't cause the AC compressor clutch to engage

The Power connector to the high pressure switch gets 12v when the car is on. If I jump it, it doesn't cause the AC compressor clutch to engage

I Jumped both High and Low pressure switches at the same time

Power connecter on the AC Compressor gets 13v only if I jump the AC Relay

Only when I jump the AC Relay does the AC Compressor clutch engage

AC is cold and working when the relay is jumped

Pressure at the Low Pressure A/C Service port is 30 PSI but I believe it shouldn't matter because I jumped the low pressure switch.

Tried known good relays

The wiring underneath the engine bay fuse box is all good.

I replaced the ECM with a known good because apparently the ECM is what sends the ground signal to the pin on the A/C Relay to so that it can work.

I replaced the AC Temperature Controller in the dash.

 

[kolgeirr]

Have you tested the relay socket to see if the relay's getting the signal to switch closed?

 

[marvin5881]

To confirm, I should be able to test that buy getting a test light like this? https://www.harborfreight.com/6-12-volt-circuit-tester-4288.html

And then connect to clamp on the test light to battery positive and then stick the other end on the socket where the ground should be?

 

[GoToGuy]

Compressor pressure switch normally open.
High pressure cutoff switch normally closed.

 

[kolgeirr]

To confirm, I should be able to test that buy getting a test light like this? https://www.harborfreight.com/6-12-volt-circuit-tester-4288.html

And then connect to clamp on the test light to battery positive and then stick the other end on the socket where the ground should be?

If it's a switched ground yup. For any switched positives just do the opposite, clamp on battery negative. You can also do this with a multimeter - if you don't have one, buy one, they're invaluable on diagnosis of electrical issues in these trucks as they will tell you actual voltage and resistances of things.

 

[marvin5881]

Have you tested the relay socket to see if the relay's getting the signal to switch closed?

From what I understand: Applying power to terminal #30 will engage the compressor clutch provided all the safety pressure switches are satisified. Check for power at #86 and #87, they both should have power in run. #86 is power for the relay coil, and #87 is the power for the clutch circuit, this is what is switched to #30 to power the clutch circuit.

86 & 87 have power when the vehicle is running

On the relay circuit slot #85 that goes to the ECU and also through both pressure switches then to ground, I get 08.4 mΩ This result is the same even if I jump both pressure switches so I won't bother doing that anymore.

When I put the relay in the socket and I squeeze my multimeter pin underneath the relay on #30 I still don't get voltage. I'm not sure why the relay isn't activated to switch power.

 

[marvin5881]

I found this online from someone else with a GMT400 truck with working AC

"With the truck running and AC button pressed, the resistance to ground was .555 mili ohms (basically closed or minimal resistance)
With the truck running and AC off, the resistance to ground was 8.55 mili ohms"

With the AC Set to on or off I still get 8.4 mili ohms regardless so it looks like this is where my problem lies. Not sure how to tackle it from here or what this means for my continuity.

 

[kolgeirr]

I found this online from someone else with a GMT400 truck with working AC

"With the truck running and AC button pressed, the resistance to ground was .555 mili ohms (basically closed or minimal resistance)
With the truck running and AC off, the resistance to ground was 8.55 mili ohms"

With the AC Set to on or off I still get 8.4 mili ohms regardless so it looks like this is where my problem lies. Not sure how to tackle it from here or what this means for my continuity.

Try using a piece of wire to short from the relay ground to battery ground. If that gets your compressor running then you definitely know where to look, it'll either be a wire issue or something with what's on the other end of the ground. Beyond that I'm afraid you'll need someone with more knowledge than I have on the AC relay/ECM interactions as I woulda figured a powered relay would engage even with that small amount of resistance on the ground.

 

[marvin5881]

A breakthough, I got a piece of long copper wire and I touched it to the negative terminal on the battery, I then brought the other end of the piece of wire and I pressed it into the relay socket on the pin that's supposed to be ground while the relay was installed into the socket and the A/C Compressor turned on. So I guess it's time to check out the wiring or wherever that ground is.

 

[thebmrust]

At some point please draw this up somehow so we can all see the diagram, of sorts in case anyone else runs into this problem in the future.