Compressors not engaging

[Lil-Pony]

My 2001 4.6L expeditions compressor won't engage. Checked all fuses and relays have tried jumping both the cycling switch and pressure switch but not at the same time. The compressors ground is good. But I'm getting millivolts on the other wire for the compressor. I've tried replacing the wot relay but no dice. Any help/advice?

 

[UrbanRedneckKid]

Back up the circuit, and find out where it stops being 12v/(battery voltage).

 

[Lil-Pony]

So all the switched are good relay is good but doesnt engage, relay is controlled by the ecm for wot conditions. Ive rigged it with a relay bypassing the ecm control and it work and cycles properly. Is my ecm bad, ive also have a miss that im trying to fix but that shouldnt be causing this, could it

 

[Lil-Pony]

 

[Lil-Pony]

The violet wire is the turn on switch in the dash

 

[adsm08]

If bypassing the WOT relay's connection to the computer makes it work the computer is probably seeing something that makes it think there is a problem.

I have seen lots of things cause that, bad temp sensors, bad pressure sensors, bad wiring (BTW, if a mouse chews the wires off your high side switch the computer sees that as 400PSI in the system and shuts it down). I really doubt the computer itself is at fault. Much more likely to be a bad input making the computer thing something is wrong. Just like every other computer controlled system it has fail-safes in the programming to keep it from just going going going boom.

 

[Lil-Pony]

So have the ECM Diagnosed

 

[adsm08]

More or less.

 

[.:Rafael]

Do you have enough refrigerant ?

If the system doesn't "see" the minimum refrigerant needed it WONT cycle the compressor (preventing it from burning)

Check that first.

 

[adsm08]

Do you have enough refrigerant ?

If the system doesn't "see" the minimum refrigerant needed it WONT cycle the compressor (preventing it from burning)

Check that first.

He said he jumped the low pressure switch and it didn't kick in.

 

[RonD]

The PCM controls most things like relays(spark and injectors as well) by grounding, not by supplying 12v to the component.

So tracing the 12v is good, but it could just be a loose ground at or near the PCM.
Test the relay sockets for a good ground when PCM should have them grounded, i.e. when device should be on, so engine running, A/C set to ON, make sure PCM is grounding or not grounding the relays as it should.

 

[Lil-Pony]

Thats what i meant by jumping the relays ground. I shoved a wire in the relays negative socket the one with out battery power and shoved the relay back in when i grounded this wire the compressor engages and will cycle on and off properly. Im going to have to have the ecm diagnosed to see what sensor is telling the ecm to open the wot relay.

 

[RonD]

Older thread with a fix for his WOT A/C issue
http://www.therangerstation.com/forums/showthread.php?t=86686

Many auto parts stores have the nicer OBDII scanners that you can view all the sensor data in real time.
TPS of course, but that would give you running issues as well
ECT, if this sensor is falsely showing a high engine temp then A/C is turned off by PCM