99 Tahoe a/c compressor wont engage

[wistler1]

I just bought a 99 Tahoe thats been wrecked and sat for about two years I was told. Anyway, while trying to get the a/c working again, Iv replaced fuses, relay, frayed wires, and gave it a shot of freon. However I cannot get the compressor to kick in. I can jump the relay and it kicks in. The problem I noticed is the ECM will not send the negative command to activate the relay. Iv got 12v+ from the HVAC switch through the high press cutout switch, good grounds through the low and high press switches. Iv jumped all switches and have power to the relay. Is it possible for the ECM to be bad? Or does anyone have any other suggestions.
Iv ohmed all the wires going to ECM which checked out fine.

 

[SunlitComet]

It is possible but it appears you may have the wrong diagram. So look at these diag chart and diagrams and work thru it again the be sure. Keep in mind not every pressure switch is N.O. or N.C.

Print this out cause the pictures will change soon.



Compressor Clutch Does Not Engage
Symptoms - HVAC System - Manual

Notes

HVAC Compressor Clutch Does Not Engage (Gas) (Part 1 Of 2)

Notes

HVAC Compressor Clutch Does Not Engage (Gas) (Part 2 Of 2)


HVAC Compressor Control Schematics
HVAC System - Manual

Notes

HVAC Compressor Control Schematics (Part 1 Of 2)

Notes

HVAC Compressor Control Schematics (Part 2 Of 2)

 

[wistler1]

SunlitComet, your diagrams appear to be of a 2000 or newer model. Mines a 99 old body style. But thanks anyway.

 

[SunlitComet]

If you say so. I won't bother with it then.

 

[DefendHawaii]

LOL... Yeah had same issue. For some reason mine was not engaging. The clutch would not kick on at all. Then one day I was changing fluids it kicked on for a few minutes. realized I had JUST the rear AC on. It sucked an entire huge can of 134A and now I am told it just needs to have the dash control assembly replaced. Might want to check on that too.

 

[wistler1]

The schematic Im going by shows the hvac sends a 12v signal through a switch on the back of the compressor then to the ecm. Iv got good power through the switch. Its mind boggleing.