AC troubleshooting

[leusgs]

Air Conditioning will not turn on. 91 Sy.

If I jumper the compressor clutch leads to the + and - battery terminals, the compressor engages. Low side pressure goes to 30 PSI and the vents blow cold (40F). So the system is working mechanically.

On the clutch electrical plug, it reads 12.5 volts. But when plugged in the clutch still does not engage. Only when you hotwire to the battery does it engage. Is 12.5 volts not enough? Is there voltage there but maybe not enough current to engage the clutch?

Jumping the low pressure switch plug at the accumulator does not make a difference.

ideas?

 

[randy merritt]

Re: AC troubleshooting

Just a stab in the dark, but have you tried jumping only the neg side of the clutch lead? This would at least tell you it is a ground problem.

 

[Emmo]

Re: AC troubleshooting

Any chance the switch in the dash needs to have terminals cleaned w contact cleaner? Just pondering. My clutch was bad and burned up 23 years despite pressure being good in system per color gauge I bought for low side , I suspected worn bearings and years of service so replaced w new and a dryer. In your case no mechanical binding so maybe copper contacts not getting full volts?

 

[Static]

Re: AC troubleshooting

Any chance the switch in the dash needs to have terminals cleaned w contact cleaner? Just pondering. My clutch was bad and burned up 23 years despite pressure being good in system per color gauge I bought for low side , I suspected worn bearings and years of service so replaced w new and a dryer. In your case no mechanical binding so maybe copper contacts not getting full volts?

This is what I was thinking as well. My switch was physically broken inside the dash and I ended up replacing it. However having 12v at the clutch would rule this out.

 

[mattw]

Re: AC troubleshooting

This drawing may help. Did you test the ground like Randy suggested?

 

[leusgs]

Re: AC troubleshooting

Tonight tried jumping just the negative, no difference.

I monitored voltage at the plug with AC on and engine running, and sometimes it was around 14 volts, but most other times it's around 12.5 volts. When at 14 volts, it would engage the clutch. at 12.5 volts it would not. I had it unplugged once at 14 volts, plugged it in, clutch engaged, pulled it off, then it read 12.5 volts, plugged it back in, would not engage. Mostly it's at 12.5 volts and not working. I had the negative terminal (right hand) connected to battery ground the whole time to eliminate the ground issue. Should the volts jump around between 14 and 12.5 at the + green wire?

Then later I was playing with it with the truck off. at 12.7 volts it engaged when jumpered. With headlights on volts dropped to 12.0 and wouldn't engage. It seems like below 12.5 or so it would not engage.

Either 1) the wiring to the clutch has issues, maybe bad contacts at AC control in the cab; or 2) something is wrong with the clutch and it's taking too much voltage to engage. I read somewhere that this style of clutch should engage at 10.5 volts or so.

Thanks for the diagram above. I also swapped the Fuel pump and AC relays (same relay #) which should eliminate that possibility.

 

[leusgs]

Re: AC troubleshooting

Ok I think it's the clutch. All wiring seems fine. If you push the clutch with a stick it engages then after a while it cycles off, then 12v comes back to the plug and with a little push the clutch engages again.

Is there an adjustment or does it need to be replaced?

Resistance across the clutch terminals is 7.1 ohms. I read on another GM forum that it should be 3-5 ohms for compressors of similar vintage.

 

[neildemo]

Re: AC troubleshooting

There is a adjustment screw. If your switching from r12 to 134a this needs to be turned down for the clutch to kick in. Otherwise it will not engage because of low coolant pressure.

 

[neildemo]

Re: AC troubleshooting

Found this out from Davep.

 

[Couger1968]

Re: AC troubleshooting

Sounds like the air gap on the clutch needs to be adjusted. There are videos on youtube showing information about diagnosing and adjusting it. I there is a tool you can rent from car places to help do it.

 

[leusgs]

Re: AC troubleshooting

There is a adjustment screw. If your switching from r12 to 134a this needs to be turned down for the clutch to kick in. Otherwise it will not engage because of low coolant pressure.

Thanks. I think you're referring to the low pressure switch on the accumulator which does have an adjustment which I have turned down a bit. This is not the issue. Physically the clutch will not engage without a push, or if you hit it with a bigger 13-14 volt shot directly from the battery.

 

[leusgs]

Re: AC troubleshooting

Sounds like the air gap on the clutch needs to be adjusted. There are videos on youtube showing information about diagnosing and adjusting it. I there is a tool you can rent from car places to help do it.

Thanks. I'll look into this.

 

[leusgs]

Re: AC troubleshooting

I rented an AC clutch tool kit at the parts store. pressed the clutch on to 0.020" and now AC works.

Thanks Couger1968 for the wise diagnosis!

 

[Couger1968]

Re: AC troubleshooting

Glad it worked.