Tahoe AC problem

have a ac problem on a 98 tahoe 5.7.the ac clutch is not engaging.system charge is good.the ac has 3 switches the one on the accumulator(low pressure switch?) has 12v at switch.I tried jumping switch clutch still wont engage.the switch on the block on back of compressor(high side switch?)12v at this switch,tried jumping it clutch still wont engage.the switch on back of compressor itself(clutch cycling switch?)no votlage to this switch,even tried jumping it.clutch still wouldnt engage.have swapped ac relay with another one.help please.........

have a ac problem on a 98 tahoe 5.7.the ac clutch is not engaging.system charge is good.the ac has 3 switches the one on the accumulator(low pressure switch?) has 12v at switch.I tried jumping switch clutch still wont engage.the switch on the block on back of compressor(high side switch?)12v at this switch,tried jumping it clutch still wont engage.the switch on back of compressor itself(clutch cycling switch?)no votlage to this switch,even tried jumping it.clutch still wouldnt engage.have swapped ac relay with another one.help please.........
Auto or regular ac controls?
First thing check all fuses.
What is the charge pressure readings?
Jumper the clutch and see if it kicks in.
Let us know what that does?
Are the switches 3 wire or two wire?

Switch on rear of compressor is HPCO..normally closed
The switch on the manifold block at compressor is for recirc door activation when head pressure rises too high- does not affect compressor operation.

Have you tried bumping clutch with hammer handle?, if clutch engages, your air gap between clutch driver and pulley is too wide ad needs to be pressed on to a width of .020"

if above result is negagtive, then:

Jump the a/c relay- this will tell you if problem is an input or output issue...if clutch engages, you have input issues, if no engagement, you have a wiring issue at compressor or bad coil on compressor--jump 12v battery hot directly to compressor to determine if coil is good.....

system is regular ac controls
all switches are 2 wire
all fuses are good
not sure about pressures but ac will work for about 15 minutes when it has set for awhile(like 4 or 5 hours)but once clutch kicks out it never kicks back in.



I have put 12 volts on compressor and it kicks in and cools good.I have not jumped relay but did replace replace it with a know good relay.I will try jumping relay tonight.the switches are 2 wire switches.the wires at HPCO switch dont have any power. Also there isnt any power at the clutch.I have put 12 volts to wire at HPCO switch and it works good.where does HPCO wire get its voltage from?thought about running a jumper wire to wire at HPCO switch and leaving it but not sure if it will hurt compressor or cause ac to freeze up???

thanks guys

system is regular ac controls
all switches are 2 wire
all fuses are good
not sure about pressures but ac will work for about 15 minutes when it has set for awhile(like 4 or 5 hours)but once clutch kicks out it never kicks back in.



I have put 12 volts on compressor and it kicks in and cools good.I have not jumped relay but did replace replace it with a know good relay.I will try jumping relay tonight.the switches are 2 wire switches.the wires at HPCO switch dont have any power. Also there isnt any power at the clutch.I have put 12 volts to wire at HPCO switch and it works good.where does HPCO wire get its voltage from?thought about running a jumper wire to wire at HPCO switch and leaving it but not sure if it will hurt compressor or cause ac to freeze up???

thanks guys

The light green wire on HPCO switch goes to the ac switch in HVAC control module.

Make sure as switch is switching on threw the light green wire.

is the HVAC control module the switch panel thats in the dash??where you turn on and adjust ac/heat?

This forum might help if you can't get it figured out w/ advice here.

http://www.autoacforum.com/categories.cfm?catid=2

is the HVAC control module the switch panel thats in the dash??where you turn on and adjust ac/heat?
Sorry about late answer internet down here.
Yes it is.
No on connector B light green wire feeds AC switch on signal to low pressure cut out switch.
From Low pressure cut out switch dark green/white feeds to VCM engine computer connector c3 pin 25.

From vcm connector c2 pin 3 dark green/white connector c2 pin 3 dark green/white to ac compressor relay B6 dark green/white wire.

If switching ac request there and relay has power on A4 pin pink wire ign mini fuse and orange wire B4 ac compressor fuse.
Your clutch should turn on.

Caution some control circuits to relays are ground controlled.
If some do not have 12 volts switching on them check for grounding control.Looks like the AC clutch relay may be one of them with ground control.
Let us know what you find.
If all else fails get a.
http://www.alldata.com/products/diy/index.html

For finding ground control snd ground circuits.
Lise part no 24550 makes a computer safe double light test light.
Green light is neg ground and red light is pos circuit.
Or you can reverse a computer safe test light to find ground circuit.
But be sure to always use computer safe test light on and computer circuit.

ok I think its the hvac control module.I was checking wiring on back of it and when you hit ac will work for a little while.when it stop hit again and ac will come back on.so for now I just run an ign hot wire to light green wire on HPCO switch.will get a module next week.

If you are going to use it jumper ed always run it threw a fuse and be careful where you are jumping into circuit as some circuits are ground circuits and computer low voltage circuits.
Good Luck and let us know how it goes.

thanks.I run it through the safe side of a ign hot wire,will post once I get a havc module and let you if that was the problem

thanks.I run it through the safe side of a ign hot wire,will post once I get a havc module and let you if that was the problem

since you fed the clutch with 12volts , at the compressor and it did not pull in I suspect that the clutch coil is defective. since this is an old vehicle replace the compressor...use new or a scrap yard part.

sometimes coils when they fail,,, slowly loose the ability to create a strong magnetic field. this is due to the windings shorting out. as the temperature increases so does the resistance. then there is insuffient magnetism to pull the clutch in after a period of time...

since you fed the clutch with 12volts , at the compressor and it did not pull in I suspect that the clutch coil is defective. since this is an old vehicle replace the compressor...use new or a scrap yard part.

sometimes coils when they fail,,, slowly loose the ability to create a strong magnetic field. this is due to the windings shorting out. as the temperature increases so does the resistance. then there is insuffient magnetism to pull the clutch in after a period of time...

I put power to the hot side of the hpco switch and the clutch did pull in.the problem is Im not getting power to the hot side of the hpco switch

I put power to the hot side of the hpco switch and the clutch did pull in.the problem is Im not getting power to the hot side of the hpco switch

this high press cutout switch does not have 12volts to it . this is a signal that goes from the hvac control module to the switch then to the PCM input. if the switch has high pressure it opens. then the PCM prevents the ac clutch from getting power. the pcm monitors many other componets when out of range this will prevent ac clutch from actuating.. short out this switch. then using a ohm meter with the ac running monitor the switch resistance it should be zero ohms.if not then it is bad or the pressure is too high. measure the high side pressure.

.. short out this switch. .

I would never "short out a switch" jumping the signal to ground may damage your PCM.

The thing to do is to "jumper" the electrical connector that attaches to that switch- most use a paper clip or a fused jumper line. Do not allow the paper clip to touch any metal or to "short" out.

The term "short" usually refers to a 12v hot signal being introduced to a body ground. Let's not confuse the original poster.

I would never "short out a switch" jumping the signal to ground may damage your PCM.

The thing to do is to "jumper" the electrical connector that attaches to that switch- most use a paper clip or a fused jumper line. Do not allow the paper clip to touch any metal or to "short" out.

The term "short" usually refers to a 12v hot signal being introduced to a body ground. Let's not confuse the original poster.


I did not correctly state this. It should be short out the connector that goes to this switch should be shorted out. then with the ac running measure the switch resistance.

shorting out or bypassing this switch , is what I was hoping to help with on finding this problem ...,

grounding out a circuit is not what I stated or indicated.

I knew what he was saying.I have jumped the connector at the high pressure switch.ac clutch didn't kick in.but tonight I will get ac clutch kicked in and measure resistance at high pressure switch.

I knew what he was saying.I have jumped the connector at the high pressure switch.ac clutch didn't kick in.but tonight I will get ac clutch kicked in and measure resistance at high pressure switch.

the hvac control module is most likely the problem ..with all the testing you did , the results I have read , and this looks more like this is the problem. many have had this occur at this time,with this model year.

got a hvac control panel from a junkyard,and it fixed the problem,thanks for all the help guys

got a hvac control panel from a junkyard,and it fixed the problem,thanks for all the help guys

thanks for the report . this will aid others .