Automotive Air Conditioning Information Forum (Archives)

mikedpp on Mon April 13, 2015 10:31 PM

Year: 2008
Make: Pontiac
Model: Grand Prix
Engine Size: 3800 V6
Country of Origin: United States

Good Evening Everyone,

I have decent experience as a DIY auto machanic, but have never had to work on A/C system. I understand the basics of the how an A/C system works, which is why I am having some difficulty understanding the scenario I have on hand right now. Let me explain what I am looking at. I have a 2008 Pontiac Grand Prix with 3.8L V6. Until Saturday, the A/C system was functioning well. Upon the next time, I started the vehicle, there was no cold air coming out of the vents. Since there was a ticking coming out of the dash, I assumed that the ticking blend actuator finally died, so I replaced that No more ticking, but still no cold air. Next, I verified that the compressor clutch was engaging. When the A/C system is turned on, the clutch engages and stays engaged. It doesn't start and stop as it would if the system was low on refrigerant. I looked for relays and fuses and found 2 fuses, 1 relay, and 1 diode in the two fuse blocks related to the A/C system. I am not sure what all of them do, but I assume that if the clutch is engaging, that they are all working properly. That could be a misconception on my part, but I can not find a resource to tell me what each of those electrical components is responsible for.

I have not taken any readings on pressure because I didn't want to go spend money on the manifold gauge set yet, so I wanted to see if anyone had any suggestions from your own experience. Please let me know what you think. Thanks for your help in advance.

Mike D

mikedpp on Tue April 14, 2015 1:02 AM

Quick update...I went out to double check a few things. I started the car, turned on the A/C system. The clutch engaged and all appeared to be working properly, but no cold air. I found the low and high pressure lines going into the firewall and neither felt cold to the touch. I then decided to check the fuses and relays. After I pulled and replaced the diode in the under-hood fuse block, it started blowing one of the fuses right next to it and the clutch did not engage. Please let me know what you think.

Thanks again.

mikedpp on Tue April 14, 2015 1:19 AM

A couple more things...I tested the diode and it is wide open in both directions, so I assume that is why the one fuse keeps blowing, but would that stop the clutch from engaging? Now, prior to removing the diode and blowing the fuse after reinstalling it, i physically inspected the clutch on the front of the compressor. I am wondering if there is a short in the clutch. One more question...is it possible for the clutch to engage , or appear that it is engaging, and have the compressor itself not work?

Dougflas on Tue April 14, 2015 7:44 AM

Did you accidentally put the diode back in the wrong direction? That compressor may be a varialbe displacement and not meant to cycle. If it is varailbe displacement, the control valve may be stuck. You need to fix the fuse problem and see what the pressures are.

mk378 on Tue April 14, 2015 9:54 AM

When the clutch engages and compressor turns-- the lines should get cold. You will need pressure gauges to diagnose. But first you need to get the electrical system back to where you started. It seems to be a refrigeration problem not an electrical problem. It does appear to be a TXV system with variable compressor, those will not cycle.

Edited: Tue April 14, 2015 at 9:59 AM by mk378

GM Tech on Tue April 14, 2015 10:51 AM

The electrical system was just fine until you messed with it. The compressor is a variable stroke, does not cycle with low refrigerant! The number one cause of mobile a/c failure is loss of refrigerant, due to a leak. So play the odds, extract and weigh the charge- probably more than half is gone- then look for dye (it is in there from the factory) and then fix the leak, and evacuate and refill to factory spec. If you don't have the tools to do this then, take it to someone who does.

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

mikedpp on Tue April 14, 2015 5:55 PM

Quote
Originally posted by: Dougflas
Did you accidentally put the diode back in the wrong direction? That compressor may be a varialbe displacement and not meant to cycle. If it is varailbe displacement, the control valve may be stuck. You need to fix the fuse problem and see what the pressures are.

No, it will only insert in one direction. I am not sure how the diode went bad, but I tested it and it is bad. I went to a Chevy dealer today for a replacement. They had to order them for $4 each. I ordered 3 of them. The old GM part number was 1540021, the new one is 88987713, in case anyone else needs that info. I will get the diode tomorrow, and restore the electrical system, and borrow some gauges and see what the pressures are.

I will let you know what I find out.

Thanks for your help.

Mike D



Edited: Tue April 14, 2015 at 5:57 PM by mikedpp

mikedpp on Tue April 14, 2015 5:57 PM

Quote
Originally posted by: mk378
When the clutch engages and compressor turns-- the lines should get cold. You will need pressure gauges to diagnose. But first you need to get the electrical system back to where you started. It seems to be a refrigeration problem not an electrical problem. It does appear to be a TXV system with variable compressor, those will not cycle.

I am not sure what a TXV system is but will research it after this. I just wanted to ask what you meant by "those will not cycle." If you don't mind, please let me know what you mean by that. Do you mean that they will not cycle when low on refrigerant?

Thanks for your help,

Mike D

Edited: Tue April 14, 2015 at 6:13 PM by mikedpp

mikedpp on Tue April 14, 2015 6:03 PM

Quote
Originally posted by: GM Tech
The electrical system was just fine until you messed with it. The compressor is a variable stroke, does not cycle with low refrigerant! The number one cause of mobile a/c failure is loss of refrigerant, due to a leak. So play the odds, extract and weigh the charge- probably more than half is gone- then look for dye (it is in there from the factory) and then fix the leak, and evacuate and refill to factory spec. If you don't have the tools to do this then, take it to someone who does.

You are very correct, the electrical system was fine until I messed with it. I believe I understand that the compressor will not cycle with low refrigerant, but does that mean that that the clutch will still engage? On my 2002 Ford, when the system was low refrigerant, the clutch tries to engage but stops. I thought that was how the low pressure sensor worked to keep from ruining the compressor. So, you end up with a clutch that engages every few seconds and stops with a couple of seconds, and keeps repeating that cycle.

Are you saying that the clutch will engage and stay engaged when low on refrigerant? I though that once the clutch engages, the compressor was turning/cycling. Please let me know if that is correct.

Oh, and yes, I get what you are telling me. I need to get some gauges and check the pressures. I will do that, but please tell me what you think on my other question any way, if you don't mind.

Thanks,

Mike D


Edited: Tue April 14, 2015 at 6:07 PM by mikedpp

mikedpp on Tue April 14, 2015 6:04 PM

Hi guys,

Thanks very much for you help so far. I know I must sound like a total idiot, but I am trying to learn and obviously save money in the process. I appreciate you help. I really do.

Mike D

Dougflas on Tue April 14, 2015 8:23 PM

By cycling, the clutch will turn on and off as the temperature of the evap changes. What happens is the Ac system has a pressure/temperature relationship. The colder the evap gets, the lower the pressure. A sensing switch (can be either temperature or pressure ) interups the clutch power supply. The clutch will turn on or off as needed. This is what is termed 'cycling clutch" system. Try googling the operation of vehicle Ac systems to learn more or you can purchase the book this site's sponser sells. You will learn a lot.

mk378 on Tue April 14, 2015 10:28 PM

The variable compressor has an internal mechanical control that detects a low inlet pressure and makes it slow down or stop pumping, even though the clutch stays engaged and the shaft is still turning with the engine. This is important because if the system is allowed to run too hard, the water that condenses out of humid air, which normally drips off the evaporator and runs out under the car, will instead freeze in place. This freeze-up blocks the evaporator from working and cool air will no longer come out inside the car. So it is important that every car have some mechanism to keep the evaporator from over-cooling. There are several different designs which are a trade-off between cost and luxury. The simple on-off that is common on trucks can be an issue on cars because the driver is more likely to notice the compressor cycling.

The simple pressure switch system found on your truck has the side effect that the compressor will cycle rapidly when the charge is low, even though the evaporator doesn't get cold. It really isn't an intended feature, it is just something that happens because of the design. It is not always the case with other designs.

Edited: Tue April 14, 2015 at 10:31 PM by mk378

GM Tech on Tue April 14, 2015 10:34 PM

Your system has a 3 wire transducer that reads system pressure- if pressure goes below 47 psi the compressor will not come on- to get 47 psi at 80 degf temp- you can have as little as 5 ounces refrigerant in a 32 (or whatever your spec is) ounce system. Five ounces is way too little amount of refrigerant to cool a system.

Your car has "NO charge" protection to protect the compressor, but not "LOW charge" protection- which can be just about as harmful to compressor.

Variable stroke compressors as is yours (a CVC) do not cycle to avert evaporator freeze- they destroke- ie: they are continuous running and yes, the compressor clutch stays engaged whenever a/c is requested.

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

mikedpp on Tue April 14, 2015 11:29 PM

Thanks for the great info. After I get the diode and borrow some gauges, I will post some pressure reading in a day for so. Thanks again.

mikedpp on Wed April 15, 2015 9:52 PM

I have an update:

New diode installed and clutch engaged and stayed engaged.

Attached gauges to high and low fittings, and collected the following readings:

Engine off: High = 115 PSI
Low = 0 PSI
Engine on & clutch engaged: High = 115 PSI
Low = 0 PSi
Engine on & Clutch not engaged: High = 115 PSI
Low = 0 PSI

After I got the pressure readings, I discovered what the dye in the system looks like as some seeped out of the high side fitting when I attached the hose for the gauge. I have not seen any of that dye anywhere along the a/c lines, but I did not take the dash apart to look in there yet.

I also noticed some slight metallic noises coming from the clutch after I turned the a/c system off. Not grinding, but a light pinging noise. In any event, I doubt that is normal.

I am not sure what the high reading should be, but I thought the low side should pull a slight vacuum which allows the charging of the system through that side. Since I did not observe any difference in the pressure readings between the system being on or off, my thoughts are that if the clutch is not engaging properly into the compressor or the compressor is dead.

What are your thoughts?

Jag987 on Wed April 15, 2015 10:30 PM

With the system turned off, the high and low sides should be the same. At 115 on the high side, the ambient air temperature would have to be very high. With the low side showing nothing, I would have to wonder about the quality of the gauges and if they are working properly. An easy way to check them is connect them to a car with working Ac and see what they say. The low side should not pull a vacuum while running. The low side pressure is lower than the pressure inside whatever you are charging from, so the refrigerant flows from the high pressure charging pressure to the lower pressure ac system.

-------------------------
I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!

mikedpp on Wed April 15, 2015 10:44 PM

Quote
Originally posted by: Jag987
With the system turned off, the high and low sides should be the same. At 115 on the high side, the ambient air temperature would have to be very high. With the low side showing nothing, I would have to wonder about the quality of the gauges and if they are working properly. An easy way to check them is connect them to a car with working Ac and see what they say. The low side should not pull a vacuum while running. The low side pressure is lower than the pressure inside whatever you are charging from, so the refrigerant flows from the high pressure charging pressure to the lower pressure ac system.

I am in Mesa, AZ, so ambient air temp today was about 85 and considering the engine was hot, it could have been over 100 degrees under the hood. What would be a typical pressure reading be in both high and low hoses with the system off?

I thought about the possibility of the low side gauge being inaccurate, so I will test them and report back.

Jag987 on Thu April 16, 2015 1:20 AM

We got snow today where I am, so I just think everyone should be cold like we are here. At 85* it would be expected to be about 95 psgi, at 100* it would be about 124, so your 115 could be right where it should be. with it off, it should be the very same, but because the gauges are scaled differently (the high side goes to a larger number on the same size gauge), it is usually hard to tell if they are the very same (at least it is for me). So if they appear to be close to the same, I just *cough* assume they are the same. But remember, as GM Tech stated above, even with only 5 ounces of refrigerant in the system, you could be seeing these same numbers with the system off.

-------------------------
I bought a can of 134a at w**-mart that had a stop leak, oil, and dye in it. It also had a hose and a gauge, so now I'm an AC pro!

mikedpp on Thu April 16, 2015 10:19 AM

Good Morning. So, with the system off, both the high and low sides should be the same. Is that correct? If so, what difference should I see when the system is on? Even before I test the gauges on another vehicle tonight, my assumption is that when the system is on I should see a difference in the high side reading. Correct?

Thanks,

Mike D

mk378 on Thu April 16, 2015 11:58 AM

Correct, the two should be the same before you start. When started, high side should go up and low side should go down. The high side will run 200-300 depending on the outdoor temperature. Variable compressors try to regulate the low side to about 25 psi. If pressures are 115 and don't change at all when the compressor starts, compressor is likely bad-- stuck at zero displacement.

mikedpp on Thu April 16, 2015 3:15 PM

Is it possible that the clutch could just be bad? It would be easier and less costly to replace the clutch that the whole compressor. I am asking since the clutch was making some pinging sounds, as I noted previously. Since the clutch activates and appears to be engaging the compressor, I didn't know if it were possible for it to only appear to engage without it actually engage and drive the compressor. Anyone have a thought on that?

Thanks,

Mike D

AC_Doc on Thu April 16, 2015 6:07 PM

-------------------------
Sometimes you must accept things at faith value!

AC_Doc on Thu April 16, 2015 6:17 PM

First,
With a high side reading of 115, the low side shouldn't be at zero, A/C or A/C off.
I would check to make sure the low side port hose connector is actually depressing the service port valve.

If you have a manifold set, exchange the blue and yellow hose positions, then open a charged refrigerant can on the other end of the yellow hose.
The low side gauge should indicate the ambient pressure of the can. If you look at where the needle crosses the scale marked R-134 it will tell you the ambient temp of the can.

Second,
With the high side nailed at the static ambient temperature pressure with the clutch engaged or not, says the compressor isn't doing anything.
Make sure the clutch is actually engaging the pulley and the compressor shaft is turning. Check the rotation of the nut in the center of the clutch face.

AC_DOC





-------------------------
Sometimes you must accept things at faith value!

mikedpp on Thu April 16, 2015 6:22 PM

Quote
Originally posted by: AC_Doc

Make sure the clutch is actually engaging the pulley and the compressor shaft is turning. Check the rotation of the nut in the center of the clutch face.
AC_DOC

Ah, I did NOT check that. I will do so in a couple of hours. That should tell me if the clutch is bad or the compressor, correct?

Thanks,

Mike D

mikedpp on Thu April 16, 2015 10:25 PM

Update...

I reconnected the low side gauge and I must not have pushed it on hard enough yesterday, because today it gave me a reading equal to the high side with the system off. Also, I have the same readings with system on (110 psi), so it would appear that compressor is not working even though clutch engages.

I tried visually inspect the rotation of the nut at the center of the clutch with the system engaged, but there isn't one on this compressor. There is a threaded bolt hole in the center. I am not sure if that means that a bolt that is supposed to go there is missing or if that is a bolt hole for a puller to take the clutch off.

Can anyone think of another way to determine if just the clutch is bad, or the whole compressor?

Thanks

Mike D