A/C Diagnosis

[newbold1]

Now that the case half seals have been replaced I am on to the very last outstanding issue on my daughters 1996 DeVille.

The A/C seems to work but just barely. Rather than to just tell you all that it runs cool but not cold I did a bunch of amateur tests hoping that these will help someone with better knowledge about this to determine a likely diagnosis.

I don't know if it really makes any difference but just so you know, this car is not equipped with dual climate control.

Okay, here's what I know...

The temperature when I checked the car was 87 degrees and humidity was about 60%. The service manual indicates that at 90 degrees and high humidity (>= 40%) the Low Pressure side should be at 55-65 psi, the High Pressure side should be at 310-340 psi, and the vents should produce temperature 70-75 degrees.

I checked my car with the engine running at different RPMs to see how that affected the results. Here is what happened.

Idling at 675 RPMs --- Low Pressure = 63 psi --- High Pressure = 180 psi --- Driver Side Vent = 77.0 degrees --- Passenger Side Vent = 84.2 degrees

Idling at 1,375 RPMs --- Low Pressure = 52 psi --- High Pressure = 230 psi --- Driver Side Vent = 70.1 degrees --- Passenger Side Vent = 76.8 degrees

Idling at 2,150 RPMs --- Low Pressure = 44 psi --- High Pressure = 225 psi --- Driver Side Vent = 66.7 degrees --- Passenger Side Vent = 72.1 degrees

After I had the engine at 2,150 RPMs I went back to regular idle and it took about 5 to 10 minutes but the numbers did finally all go back to the same ones that they were as described above for 675 RPMs. When I shut off the engine the Low Pressure and High Pressure sides slowly balanced out and settled at about 102 psi on the guages.

So, comparing to the service manual it appears that the High Pressure side is very low compared to where is should have been. Also, the driver side vent was close to within specs but the passenger side vent was pretty far from it.

Here are some other physical things that I noticed that I assume might be helpful to know.

1. The accumulator was fairly cold and sweating which I believe is a good thing.

2. The entire accumulator out and in pipes (fatter pipes) were cold and sweating somewhat as well.

3. Where the evaporator connector is located that skinnier pipe was a little cold.

4. From the orfice tube all the way to the high side service port the skinnier pipe was fairly warm to hot.

5. The compressor seemed to keep running the entire time and I had the A/C set to AUTO and 60 degrees.

Any A/C folks out there that have an idea as to what needs to be done to get the A/C to operate within specs? Do I just need to have the system evacuated and then recharged? Or, does this sound like symptoms related to a mechanical problem?

Thanks for any help!

[Cadillac Jim]

I'm not an A/C man but the pressure readings and the differential between the two sides look like slightly low Freon to me. I would also clean or change any filters, such as the cabin air filter, and make sure that you don't have dust accumulation to put a felt cover over part of the evaporator core. Then, make sure that you don't have any codes regarding the temperature and vent control doors in the vent systems.

The system only holds two pounds of Freon. It may be a half-pound low. In any case I would bet that a full pound of Freon would over-fill it.

[KHE]

The car is low on refrigerant - try adding a few ounces of R-134a and monitor the vent temps and high/low pressures.

[newbold1]

I stuck a thermometer in the middle air vent to watch the results. I bought a 12 oz can of R134a and a valve tap tried to regulate the rate at which I was adding so as not to do too much.

The low pressure side psi increased ever so slowly but the vent temperature in the car didn't improve. It went a little lower, little higher, little lower... basically just bounced around within about a 1-2 degree range. The high pressure side psi barely budged at all.

I ended up using the whole can and from what I can tell it made almost no difference. If I rev the engine the vent temperature slowly drops down to about 70 degrees. That's not as low as before but on the other hand it is about 95 degrees today which is 8 degrees hotter outside than when I tried this yesterday.

All in all, I am about where I was even with the 12 oz of R134a added to the system. I have to assume something isn't working right. The high pressure psi is way lower than the service manual says it should be. At idle it doesn't even get to 200 psi.

What do you all think may be wrong? Any chance the whole system needs to be evacuated and then recharged? Or, do these symptoms sound like a repair is in order?

[KevinW]

While the system is under load (moderately high fan setting, no recirculation or a warm interior with recirculation), a cold accumulator surface suggests there's sufficient refrigerant to cope with the operating conditions at hand.

It is possible that the air mix door is stuck in a position that is allowing some of the incoming air to pass through the heater core (after first being cooled by the evaporator). The presence of a trouble code may assist in this diagnosis. The air mix door actuator is located behind the glove compartment and can be revived in some cases after some disassembly and the application of a drop of oil on the motor shaft (test both directions using a battery afterwards).

[newbold1]

While the system is under load (moderately high fan setting, no recirculation or a warm interior with recirculation), a cold accumulator surface suggests there's sufficient refrigerant to cope with the operating conditions at hand.

It is possible that the air mix door is stuck in a position that is allowing some of the incoming air to pass through the heater core (after first being cooled by the evaporator). The presence of a trouble code may assist in this diagnosis. The air mix door actuator is located behind the glove compartment and can be revived in some cases after some disassembly and the application of a drop of oil on the motor shaft (test both directions using a battery afterwards).

Sorry, I had meant to include in my last post that I had checked the codes and there weren't any that had been triggered.

[KevinW]

Sorry, I had meant to include in my last post that I had checked the codes and there weren't any that had been triggered.

The presence of codes may assist in the diagnostic process, but the absence of codes cannot rule-out problems relating to the air mix door. If the linkage is disconnected somewhere, the output shaft of the actuator may be responding as commanded, but the air mix door may be stuck in a position that is forcing some air to be heated. You might try running the system in 'Economy' (no A/C compressor activation), and measuring the vent temperature. The vent temperature will be slightly higher than the temperature of the air at the inlet (base of windshield) due to the ductwork and insulating ability of the air mix door, but I'd expect no more than a couple degree differential at highway speeds, though some cars with circuitous or poorly insulated HVAC intakes are considerably worse under the best possible conditions.

[joeb]

i just replaced the compressor on my 96 STS. i could never get the high side over 125psi. bad compressor. i watched the pressure on the high side when i added freon. at about 230 the fan speed will go from low to high. than the pressure drops down to 200 or so and the fan speed slows down. this is at idle. i never really checked the pressures at high idle. the vents were about 50 deg at idle. i am probably a little low on freon. it sounds like your compressor is starting to fail or not perform well. is your condenser clean? how about fan speed? does they seem to run at low speed or high? try spraying water on the condenser and see if the pressures drop.

[KHE]

If you have verified that the mix doors are functioning properly, the next thing to do is to recover and weigh the refrigerant charge to make sure you have the proper amount of refrigerant in the system. Charge 2.0 lbs of refrigerant the refrigerant back into the car and check the pressures and vent temps with the climate contol panel set to AUTO 60 degrees. If the high side pressure is still low and the system is not cooling very well, the compressor is shot.

[newbold1]

I had the A/C system completely evacuated and properly recharged. The result is that the temps coming from the vents is improved but still only really cold when the RPMs are higher. Sitting at idle the air is just barely better than warm. Drive for a minute or two and it's pretty cold. Stop, and in a couple minutes the air is back to cool if not warm.

Driving around today the car was comfortable enough... hated stopping at red lights though.

Since this is going to be my daughters car I think we can live with the result for now. I am guessing that the compressor is probably pretty worn out. We'll wrap up this summer as is now and next spring we may see about repairing the system. Hopefully, it will hang in there that much longer. I wonder if the components are all original. If so, it's pretty impressive that the system works at all after nearly 13 years and 178K miles.

[BodybyFisher]

Did you have a look at the orfice tube when you had it depressurized?

[Cadillac Jim]

A lot of things can happen to a compressor, but one of the more common ones is the reed valved breaking up. If that happens the ACM will likely shut it down with a "Low Freon" message but if it stays on then metal fragments will be released, and you will need to change the dryer/accumulator. So, if it keeps on giving problems, you probably should consider changing the compressor.

[KHE]

You stated the system cooled adequately at speed but not at idle - are the fan(s) coming on when the A/C is requested?

Don't stop the troubleshooting process at this point - now that you know the system has a full charge of refrigerant, hook up the gages and read the high and low pressure sides at 1500/2000 rpm and at idle and post them. The pressure readings will indicate how well the compressor is performing.