proper voltage for A/C compressor clutch?

[skyking]

My buddy's GMC pickup A/C quit yesterday. I took a quick look, and noted only 2.7 volts to the clutch. It was not turning when the AC was turned on. I tested the coil, it had continuity and was not shorted to ground, so that was OK.
I figure full battery volts are normal for that clutch, and there is a snafu in the dash controller or something between here and there.
Ideas?

 

[91TTZ]

yes

 

[no0b]

first idea (dumb) put 12V to the clutch see what happens,
second idea find a car enthousiests(sp) website and ask dere

 

[skyking]

Originally posted by: no0b
first idea (dumb) put 12V to the clutch see what happens,
second idea find a car enthousiests(sp) website and ask dere

I'm thinking that 12V won't be a problem, and the thing will run as it should! Next problem: where did the votlages go?? It has an LCD dash unit, and probably some relay technology in there.

 

[duragezic]

First idea...where did Roger go??

 

[jlee]

Zenmervolt might know too -- don't lose hope on ATOT yet

 

[skyking]

He got pissed by some petty bickering and namecalling, and got tired of giving out free GOOD advice to the ungrateful. He still lurks under a different username.

 

[duragezic]

I thought an old guy like him was bulletproof! But I suppose sh1t can get bad here, some people get way too serious and involved.

 

[radioouman]

Quixfire might be able to help you too.

I've messed with compressors before, and I am really thinking that the connector on your compressor doesn't have power going "to it". It is actually connected to a relay which is connected to the switch in your dashboard. You just need to take a pair of alligator clips or something metal and jump those terminals together to get the compressor to run.

Please check with a mechanic before following my advice though.

 

[bmacd]

get a service manual!!!

-=bmacd=-

 

[Eli]

I'm pretty sure that all automotive AC clutches use normal system voltage (12.7V minimum).

 

[skyking]

yup, thinking that too. Now I think I'm stuck without a schematic to trace that stuff back. My gut says the GMC dash heater control module.
This guy asked me if I knew any good AC service guys. "hold on, don't do anything hasty now!!"

 

[Ronstang]

The AC clutch of most cars operates on 12 volts (all the ones I have worked on at least) so if you are only getting 2.7 volts it simply is not going to engage. Check the operation of the clutch and AC system quickly by running a temporary jumper wire from the battery to the clucth. You can determine which prong is positive and which negative by using a volt/ohm meter on the harness plug and checking on which one goes to ground. If the AC functions properly when jumped then there is an electrical probelm upstream of the compressor. I would check the AC relay first in this case as it is most likely the culprit.

 

[skyking]

took a look around for that relay, and did not find it with the rest. I'll get a schematic or service manual.

 

[Ornery]

My son's '95 G. Marquis had a compressor clutch that wouldn't engage. I checked voltage coming to it, and it was 12V when the A/C called for it to come on, and zero when it cycled off. I figured the clutch was bad, and took it to a shop to have it replaced (I'm getting lazy!)

Fortunately, my mechanic checked it out and found the switch was bad, which was FAR cheaper to replace than the clutch. I'm guessing even though it was passing 12V to the clutch, there wasn't enough amps to do the job, probably due to corrosion or arcing & carbon buildup. Anyway, check the switch.

 

[Ronstang]

Originally posted by: Ornery
My son's '95 G. Marquis had a compressor clutch that wouldn't engage. I checked voltage coming to it, and it was 12V when the A/C called for it to come on, and zero when it cycled off. I figured the clutch was bad, and took it to a shop to have it replaced (I'm getting lazy!)

Fortunately, my mechanic checked it out and found the switch was bad, which was FAR cheaper to replace than the clutch. I'm guessing even though it was passing 12V to the clutch, there wasn't enough amps to do the job, probably due to corrosion or arcing & carbon buildup. Anyway, check the switch.

That is exactly what happens when a relay is old and has cycled many times which burns the contacts over time. It is not going to be the switch but the relay as most manufacturers I know of use relays in the AC system to pass the high voltage to the clutch to protect the switch. In this case the relay IS the switch.

 

[Shawn]

cars are 12v. everything should be 12v.

 

[91TTZ]

Originally posted by: Shawn
cars are 12v. everything should be 12v.

Many of the sensors are less than 12v, they might use 5v for instance. The voltages coming from the ECU aren't all 12V. And the spark plugs obviously don't work off 12V.

 

[Shawn]

Originally posted by: 91TTZ

Originally posted by: Shawn
cars are 12v. everything should be 12v.

Many of the sensors are less than 12v. The voltages coming from the ECU aren't all 12V. And the spark plugs obviously don't work off 12V.

Well the clutch is getting it's power from the battery. It should be 12v.

 

[91TTZ]

Originally posted by: Shawn

Originally posted by: 91TTZ

Originally posted by: Shawn
cars are 12v. everything should be 12v.

Many of the sensors are less than 12v. The voltages coming from the ECU aren't all 12V. And the spark plugs obviously don't work off 12V.

Well the clutch is getting it's power from the battery. It should be 12v.

It should be, yeah.

 

[Ornery]

...most manufacturers I know of use relays in the AC system to pass the high voltage to the clutch to protect the switch. In this case the relay IS the switch.

I think you mean pass the high amperage to the clutch to protect the switch. When he told me it was the "switch", I didn't know what to think, but was thrilled it would be a cheap repair either way. Maybe his part supplier called it a "switch", so he did too, but I know what you mean about it actually being a relay of some sort.

 

[ruffilb]

were

 

[Quixfire]

Originally posted by: skyking
My buddy's GMC pickup A/C quit yesterday. I took a quick look, and noted only 2.7 volts to the clutch. It was not turning when the AC was turned on. I tested the coil, it had continuity and was not shorted to ground, so that was OK.
I figure full battery volts are normal for that clutch, and there is a snafu in the dash controller or something between here and there.
Ideas?

Most A/C systems provide 12v to the clutch coil for engagement. If you are only getting 2.7 volts at the connector then there is a problem from the A/C relay to the clutch coil connector circuit. The problem with GM systems is they use a diode in the circuit to control system load to the circuit. This diode can fail allowing voltage to leak to ground without blowing a fuse or damaging the relay. I would check the voltage at the relay to the coil. If you have 12 volts at the relay then the diode is bad and you will need a replace connector & pigtail with the diode. If not then the problem could be the relay or the power source to the relay.

 

[skyking]

Originally posted by: Quixfire

Originally posted by: skyking
My buddy's GMC pickup A/C quit yesterday. I took a quick look, and noted only 2.7 volts to the clutch. It was not turning when the AC was turned on. I tested the coil, it had continuity and was not shorted to ground, so that was OK.
I figure full battery volts are normal for that clutch, and there is a snafu in the dash controller or something between here and there.
Ideas?

Most A/C systems provide 12v to the clutch coil for engagement. If you are only getting 2.7 volts at the connector then there is a problem from the A/C relay to the clutch coil connector circuit. The problem with GM systems is they use a diode in the circuit to control system load to the circuit. This diode can fail allowing voltage to leak to ground without blowing a fuse or damaging the relay. I would check the voltage at the relay to the coil. If you have 12 volts at the relay then the diode is bad and you will need a replace connector & pigtail with the diode. If not then the problem could be the relay or the power source to the relay.

Thank you! I need to locate those items, I did not find the relay on my first cursory inspection. There is a black cover with relays under it on the firewall, above the heater core. I thought it would be in there, but none were marked as A/C relays.

 

[Ornery]

1. ? ?