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Grand Prix won't build pressure

RobStercraw on Thu August 14, 2008 12:08 PM User is offline

Year: 1998
Make: Pontiac
Model: Grand Prix
Engine Size: 3.8 N/A
Refrigerant Type: R134a
Ambient Temp: 85
Pressure Low: 40
Pressure High: 40
Country of Origin: United States

System had been dead for the 2 years I've had the car. Partially charged system and compressor ran and sounded okay. Leak-checked and found to major leak from front seal. Replaced compressor with new ACDelco unit. Replaced receiver dryer and orifice tube (had a little trash in it but not blocked or anything) as well as all the seals I could get to. Blew out the evaporator and condenser.

Vacuumed the system with an electric vacuum pump for a couple hours - system held a full vacuum for several hours so I'm pretty confident it's sealed.

Went to charge the system and could not get the compressor to engage. Do not have a scan tool so I couldn't verify that the PCM had received the A/C request signal. Thought about jumping the low pressure switch but it's a 3-terminal and I couldn't find it in the wiring diagram so I decided not to mess with it and walked away for the day.

Last night, decided to try it again - it had cooled off, so I stuck the can of refrigerant in a bucket of warm water to up the pressure a bit.

after about 5 minutes - the compressor did actually engage and start running, but won't build any pressure above the can pressure (about 40 PSI) high side is the same as the low

of course at that pressure I have no cooling

I do have the proper charge of oil in the compressor, I turned the clutch by hand about 20 revolutions before installation, the system holds a full vacuum.

Neither of the cooling fans are running though - IIRC, when the compressor runs, at least one of the fans should run. This isn't why I am not building any pressure - but seems weird. The factory manual SUCKS for A/C diagnosis and description and operation so I'm kind of at a loss. I'm an ASE-certified tech, but not in A/C obviously and I know just enough to be dangerous...

Any ideas?

RobStercraw on Thu August 14, 2008 12:41 PM User is offline

Emailed a friend of mine who sells A/C parts and he ran my problem past one of the techs there - they said this compressor is variable displacement and that it can't be charged from zero from a can - that it will need to be charged using a charging station with refrigerant under pressure. I've never dealt with these type compressors before so this has never been an issue.

Hopefully that's all it needs...

GM Tech on Thu August 14, 2008 12:49 PM User is offline

Your buddies are dead wrong- as long as you pulled an adequate vacuum you can charge from cans-- but it will take 47 psi static pressure to enable the a/c request to turn on compressor--

Also all GM "W" cars "wait " on head pressure to turn on the fans at 175 psi-- so you IIRC boys are dead wrong as well- Most GM vehicles do enable cooling fans with a/c request-- just not the W-car line-up.....any more misconceptions that need corrected???

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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......

RobStercraw on Thu August 14, 2008 1:01 PM User is offline

Quote
Originally posted by: GM Tech
Your buddies are dead wrong- as long as you pulled an adequate vacuum you can charge from cans-- but it will take 47 psi static pressure to enable the a/c request to turn on compressor--

Okay, so they're wrong. So either I am doing something wrong, or I have a problem because it won't build any pressure. Any suggestions?

Also all GM "W" cars "wait " on head pressure to turn on the fans at 175 psi-- so you IIRC boys are dead wrong as well- Most GM vehicles do enable cooling fans with a/c request-- just not the W-car line-up.....any more misconceptions that need corrected???

That part was my misconception apparently. Just about every other car I've dealt with that had two fans turned one on whenever the compressor was engaged. Like I said, I knew that wasn't the reason for not building pressure - just an observation.

GM Tech on Thu August 14, 2008 1:07 PM User is offline

It will build pressure when you get the proper charge in it-- what is the suction side pressure with valve shut to can? is it sucking down to 30 psi? what is high side at same time? an 80 psi can will flow into a 30 psi system easily-- your can is either empty, or your not connecting to the system properly.…make sure your valves are both shut off when measuring pressures...

If it still won't pump- then your control valve may be amiss..


This is why I always install new shaft seals- on old compressor- no need to replace anything except the seal- I keep the a/d, and OT- never change them for a leak failure mode- which is the most common on a V-5.…

-------------------------
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......

Edited: Thu August 14, 2008 at 1:08 PM by GM Tech

RobStercraw on Thu August 14, 2008 1:40 PM User is offline

With the valve shut to the can - the pressures on both sides are about 40 psi.

The can definitely is *not* empty.

mk378 on Thu August 14, 2008 1:46 PM User is offline

Warm up the can. With the engine off, turn the can upside down and open both valves on the manifold. Refrigerant should rapidly flow into the car as a liquid. Repeat with another can. When it stops taking it that way, or you've reached the full specified weight, close the valves, double check they are closed, spin compressor to be sure it's not hydro-locked, then start the engine. There should be enough pressure to engage the compressor now. Complete the charge thru the low side if necessary.

GM Tech on Thu August 14, 2008 1:55 PM User is offline

Quote
Originally posted by: RobStercraw
With the valve shut to the can - the pressures on both sides are about 40 psi.



The can definitely is *not* empty.

How about the valves on your gage set? they have to be closed- otherwise you are running in a bypass mode- through the gage set and not through the system.

-------------------------
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......

RobStercraw on Thu August 14, 2008 2:20 PM User is offline

Yep, valves on the gauge are closed (except when charging - then I opened the low side valve - I can see refrigerant flowing through the sight glass on the gauge). (there are seperate valves on the hoses @ the connectors - those are open)

RobStercraw on Tue August 19, 2008 6:03 PM User is offline

Guess my buddy was *not* correct after all. Taking the car to the shop and having them charge the system using a charging station did *not* work and I thoroughly am *not* enjoying the 39 degree air that was *not* coming from my vents with a 94 degree ambient temp.

Nope, I'm *not* enjoying that at all.

mk378 on Tue August 19, 2008 6:19 PM User is offline

Sounds like your new compressor is defective. It's pretty common for those V series to get stuck at zero displacement. They spin but won't pump. Replacing the control valve (available seperately) almost always fixes it but you should go for your warranty on the whole unit.

iceman2555 on Wed August 20, 2008 2:18 AM User is offlineView users profile

Glad you were able to get the system charged and it is now operational. It is always best to static charge a system prior to clutch engagement. However, have seen many variables charged with individual cans, or with a 30lb'er and using pressures. Seems as if you had 'one' of those that needed it all before it worked. Many late model GM fans are operated with a pressure switch instead of a constant on with clutch engagement. My son's Chevy is the same....the fans are on/off dependent upon pressures.
All in all...your system now works.....

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

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