Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

GM 99 truck clutch won't engage when low side press sw jumped

roaniecowpony on Mon June 15, 2009 1:41 AM User is offline

Year: 99
Make: GMC
Model: 2500 truck
Engine Size: 6.0
Refrigerant Type: 134
Ambient Temp: 75F
Country of Origin: United States

I'm in the process of recharging after evacuating the system on a 99 GMC 2500 6.0 ltr. In the past, I simply filled the first half can or so and then started the engine and jumped the low pressure switch by disconnecting the plug and connecting the two contacts of the plug with a paperclip in the end of the connector. My recollection was that was all it took to engage the clutch. I do it now and na-da. The clutch never engages.

Could the clutch be out of adjustment? Any other things to check?

Many thanks


Dougflas on Mon June 15, 2009 3:20 AM User is offline

Try disconnecting the battery for a few minutes to reset the control head/computer

GM Tech on Mon June 15, 2009 10:04 AM User is offline

Those control heads don't set codes- like other GMs-- so unhooking battery probably won't help.

here is what I'd do.

Do you have 12v at compressor?

if yes, is air gap too wide? bump it while running.- then adjust it permanently.

other things to check- is HPCO connector plugged in on rear of compressor? does jumping it help?

does jumping a/c relay in underhood fuse/relay center help? does relay have a ground source from PCM?

is control head light for a/c on? (says a/c request is present)

you have already tried jumping LP cycling switch.....

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

roaniecowpony on Tue June 16, 2009 1:10 AM User is offline

It's been a yr or two since I've been on this site and I've forgotten all the names of the guys here. But, to answer your questions in reverse order:

Yes, I did try to jump the low pressure switch.

Yes, the control head light is on and switched to ask for A/C, recirc, cold, high blower.

The othes I'll have to work my way thru.

I 'm guesing clutch gap since this clutch has been on for over 70,000 miles. I'll do the check to dislodge it to get it to engage first and then check the gap. Do I have to remove the compressor from the truck to adjust the gap?


Chick on Tue June 16, 2009 7:47 AM User is offlineView users profile

No, most you'd have to do is remove the fan shroud out of the way, and of course the adjustment tools.. But with that many miles, it is looking like a clutch gap adjustment..Hope this helps.

Ps: Find and fix your leak, constantly topping the system off is NOT good..

Email: Chick


Freedoms just another word for nothing left to lose

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.