A/C Trobleshooting assistance needed

[Dave S.]

I have a 71 350/270 car witrh C60/air conditioning. We have checked the system for leaks and the R-12 is full and has been all along. Pressures are proper as checked with the gauges. System blows cool air but not cold. Evaporator inlet is cool but not cold. Diagnosis from my A/C guy is that the system is working fine but the Compressor is only performing marginally thus causing substandard air temperatures.
Before I pull the compressor I thought i'd see if anyone had additional suggestions. Getting ready for a PV and I think I need better performance from the system.
If anyone has a suggestion on where to send the compressor for a rebuild I would appreciate it. This is a 4 star Bowtie car so the rebuilded will need to keep the cosmetics as original.

[Larry M.]

Re: A/C Trobleshooting assistance needed

Dave:

Need more info before a diagnosis can be done. At 2000 engine RPM with windows down and AC blower on HI speed, what is the system suction and discharge pressure? Suction measured at POA valve (and also at compressor inlet if you have that connection). Discharge measured at compressor outlet block/muffler. And what is ambient air temperature going into the condenser? What is AC duct temperature? And does the sight glass on the drier have any bubbles at 2000 RPM indicating a low charge? Is the compressor hot, warm, very hot, or cool? Is the evaporator OUTLET and POA cool, cold/sweating, or warm?

I would suspect an expansion valve (or expansion valve plugged inlet screen) or a POA valve sticking rather than a worn out compressor. I would not touch the compressor without checking out these other two components. Also, if the compressor discharge is normal/typical it is probably pumping okay. A bad compressor would have a low discharge pressure.

Why does your friend/mechanic say the compressor is shot?

The biggest problem is that if you remove the expansion valve to check it, you have opened the system. At that point you wonder if you should send everything out for rebuild/checkout due to the high R12 cost and handling issues......you only want to do it once and not piecemeal. I understand the problem, but if you only concentrate on the compressor and nothing else, I think you will also be disappointed. The chance of it being the problem is probably less than the other things I mentioned.


Larry

[Edward J.]

Re: A/C Trobleshooting assistance needed

Dave, Im With Larry, Pressure Does Play A Big Part Of The Performance Of The Compressor And System Function, One Other Item, Does He Have A Machine That Will Evacuate The System And Measure The Amount Of Charge Thats In The System When Removed, That Would Be My First Step. Ive Owned A Repair Shop For 35yrs. And Those Older R-12 System Do Have A Site Glass To Look At For Air In The System While Operating. Hope This Helps Ed (49497)

[Dave S.]

Re: A/C Trobleshooting assistance needed

Dave:

Need more info before a diagnosis can be done. At 2000 engine RPM with windows down and AC blower on HI speed, what is the system suction and discharge pressure? Suction measured at POA valve (and also at compressor inlet if you have that connection). Discharge measured at compressor outlet block/muffler. And what is ambient air temperature going into the condenser? What is AC duct temperature? And does the sight glass on the drier have any bubbles at 2000 RPM indicating a low charge? Is the compressor hot, warm, very hot, or cool? Is the evaporator OUTLET and POA cool, cold/sweating, or warm?

I would suspect an expansion valve (or expansion valve plugged inlet screen) or a POA valve sticking rather than a worn out compressor. I would not touch the compressor without checking out these other two components. Also, if the compressor discharge is normal/typical it is probably pumping okay. A bad compressor would have a low discharge pressure.

Why does your friend/mechanic say the compressor is shot?

The biggest problem is that if you remove the expansion valve to check it, you have opened the system. At that point you wonder if you should send everything out for rebuild/checkout due to the high R12 cost and handling issues......you only want to do it once and not piecemeal. I understand the problem, but if you only concentrate on the compressor and nothing else, I think you will also be disappointed. The chance of it being the problem is probably less than the other things I mentioned.


Larry

Larry?Ed,
Thanks for the replies. I'm a bit in no mans land here as A/C is foreign to me.
My mechanic does not have the set up to check the pressures in the compressor. He checked the pressues in the system at about 20 psi at idle and 40 psi at 2000 rpm which he felt was OK. He let some air out of the system and it made it slightly better but because we are not getting ice cold at the evaporator he felt the compressor was not performing properly. He believes the system is full of R-12. This system was working about 6 months ago. I'd prefer not to take the system apart as it is a low mileage Bowtie car.

[Arland D.]

Re: A/C Trobleshooting assistance needed

Dave,

Just replaced the A/C compressor in the '82 today because of a leaky seal and when removed it was dry (as in no oil). The new compressor was fine on the pressure readings but the air coming out of the vents wasn't all that cool. A small adjustment to the air mixture / temp control door cable had a fairly dramatic effect on the temperature of the vent air. If your door is letting in any heated air, the temperature of the conditioned air will rise quickly and appear to be uncool Let us know what you find,

Arland

[Dave S.]

Re: A/C Trobleshooting assistance needed

Dave, Im With Larry, Pressure Does Play A Big Part Of The Performance Of The Compressor And System Function, One Other Item, Does He Have A Machine That Will Evacuate The System And Measure The Amount Of Charge Thats In The System When Removed, That Would Be My First Step. Ive Owned A Repair Shop For 35yrs. And Those Older R-12 System Do Have A Site Glass To Look At For Air In The System While Operating. Hope This Helps Ed (49497)

Ed,
He does not have the ability to evacuate the system. He tells me that I do not have a sight glass. I will call you tomorrow.

[Edward J.]

Re: A/C Trobleshooting assistance needed

Dave, My 72 Has A Site Glass, I Would Think That Your 71 Would To, Its On The Reciever Dyer On The Top, It May Have A Plastic Cover On It Being It A Bow Tie Car, No One May Have Ever Removed It To Look At The Freon Flowing Through The System.

[William C.]

Re: A/C Trobleshooting assistance needed

Dave, you should be able to find the reciever/dryer, close to the condensor. The sight glass is located in the top of the dryer, normally in a bowtie car with a small cover pressed in place to keep the glass clean.

[Jack H.]

Re: A/C Trobleshooting assistance needed

I agree with the analysis of others...it's a 'stab' in the dark to blame the AC compressor!

The in/out pressure profile tells you the health of the compressor as well as the state of refrigerant charge. If that meets spec (your copy of the Chassis Service Manual has a VERY thorough spec table), then your problem(s) are elsewhere. They can be manifold...

(1) Are the fins on the condensor straight/true or folded over from past road debris? Are they clogged with foreign material that's reducing heat dissipation? You can check that visually by looking with an inspector's mirror and/or blowing compressed air through.

(2) Is the POA valve throttling as it should? You'll need an experienced HVAC/auto A/C mechanic to comment on this one.

(3) Is the evaporator doing it's thing properly? This is one that many miss because the evap unit is INSIDE the Harrison air box. But, the bottom of the box has a rubber 'weep' bladder to exhaust liquid condensate. The old dealer mechanics trick is to pull the bladder and go through the weep hole using something like a bore scope to visually check the condition of the evap unit.

More on #3. Since your car is a '71, there's NO PROTECTION against airborne debris (dirt, dust, tree leaves, Etc) from being sucked through the cowl and ultimately BLOCKING air flow through the evap core. Later era Shark cars were improved to add screens high in the cowl on each side to thwart external debris from migrating into the Harrison air box...

IF, the evap IS clogged, there are two approaches to rememdy. The first is the factory standard approach...pull the air box and physically clean the debris from the evap core.

The dealership 'rate buster' mechanics often took a 'shortcut'... Remove the rubber bladder, go through the weep hole at the bottom of the air box with a flexible 'wand' extension on a shop vac and SUCK the debris out with the air box in place...

So, some food for thought for you. BTW, the Chassis Service Manual will tell you EXACTLY what the cockpit outlet temperature should measure for a healthy A/C system in tabular form. Bottom line, there's no need to guess about how cold should it be...

[Dave S.]

Re: A/C Trobleshooting assistance needed

I agree with the analysis of others...it's a 'stab' in the dark to blame the AC compressor!

The in/out pressure profile tells you the health of the compressor as well as the state of refrigerant charge. If that meets spec (your copy of the Chassis Service Manual has a VERY thorough spec table), then your problem(s) are elsewhere. They can be manifold...

(1) Are the fins on the condensor straight/true or folded over from past road debris? Are they clogged with foreign material that's reducing heat dissipation? You can check that visually by looking with an inspector's mirror and/or blowing compressed air through.

(2) Is the POA valve throttling as it should? You'll need an experienced HVAC/auto A/C mechanic to comment on this one.

Jack,
Thanks for the added info. The condenser is virtually as new and should be functioning. The evaporator seems to be working as there is quite a bit of condensate coming from the drain hole.

(3) Is the evaporator doing it's thing properly? This is one that many miss because the evap unit is INSIDE the Harrison air box. But, the bottom of the box has a rubber 'weep' bladder to exhaust liquid condensate. The old dealer mechanics trick is to pull the bladder and go through the weep hole using something like a bore scope to visually check the condition of the evap unit.

More on #3. Since your car is a '71, there's NO PROTECTION against airborne debris (dirt, dust, tree leaves, Etc) from being sucked through the cowl and ultimately BLOCKING air flow through the evap core. Later era Shark cars were improved to add screens high in the cowl on each side to thwart external debris from migrating into the Harrison air box...

IF, the evap IS clogged, there are two approaches to rememdy. The first is the factory standard approach...pull the air box and physically clean the debris from the evap core.

The dealership 'rate buster' mechanics often took a 'shortcut'... Remove the rubber bladder, go through the weep hole at the bottom of the air box with a flexible 'wand' extension on a shop vac and SUCK the debris out with the air box in place...

So, some food for thought for you. BTW, the Chassis Service Manual will tell you EXACTLY what the cockpit outlet temperature should measure for a healthy A/C system in tabular form. Bottom line, there's no need to guess about how cold should it be...

Jack,
Thanks for the added info. The condenser is virtually as new, clean and should be functioning. As I merntioned this is a low mileage Bowtie car so dirt and debris should be minimal. The evaporator seems to be working as there is quite a bit of condensate coming from the drain hole.

[Dick W.]

Re: A/C Trobleshooting assistance needed

Dave, make sure that the heater water shut off is working. Hot water will negate the best of A/C systems

[Dave S.]

Re: A/C Trobleshooting assistance needed

Dave, make sure that the heater water shut off is working. Hot water will negate the best of A/C systems

Dick,
Thanks. I need to check the hot water shut off as well as the temperature cable that Arland suggested. I found the sight glass and it appears to be free of bubbles.

[Jack H.]

Re: A/C Trobleshooting assistance needed

If I remember the vacuum truth table (C60 section of AIM), the hot water shut-off valve is only engaged when the system is in the OFF and MAX A/C modes...

That means the heater core was designed to be hot when the A/C was running in normal (outside makeup air vs. cabin recirculate) A/C mode. Plus, didn't they drop the hot water shut off valve in 1972 because its contribution to system efficiency was deemed to be of 'marginal' value?

Where I'm coming from is this... The system should still function 'properly' with the heater core charged/hot and that depends on the air diverter doors in the air box working properly to direct make-up air around, through or partially through the evaporator.

Of course, all bets are off if the diverter doors aren't in good condition OR the evaporator is plugged/partially plugged with road debris resulting in air flow restriction...

[Jim T.]

Re: A/C Trobleshooting assistance needed

Dave something happened to my 1970's air conditioning system in 1988, cannot remember the all the details, but it was not cooling.
Chevrolet dealer mechanics wanted to remove the evaporator. So, so glad they could not loosen the nuts.
Long story is in archives of previous posts. Anyway the desiccant container in the dryer bursted and allowed the desiccant to move to the screen in the expansion valve and stopped up the flow.
Very easy to fix, just remove screen and clean or if you can find this little steel screen buy a new one but is not necessary unless you damage it.
New dryer would be needed as well as flushing the lines from the dryer to the expansion valve remove the desiccant.
Your greatest loss will be the the R12 to remove the expansion valves screen to inspect it.
I trouble shot my problem by using my 1970's GM shop manual. I don't know why the Chevrolet dealer mechanics did not know how to do this.

[John H.]

Re: A/C Trobleshooting assistance needed

GM factory A/C systems are all "re-heat" systems - in "normal" outside air operation, after the cooled air exits the evaporator coils, some of it passes through the heater core, depending on the position of the temperature control, which operates the temperature door. If the temperature door cable isn't properly adjusted to close the door fully and/or the edge seals on the door are shot, some of the cooled air will still leak through the heater core and get heated up on its way to the outlets.

That door MUST have good seals and close all the way with the temperature control in the "COLD" position in order for the system to operate as designed.