R12 system on 79 Trans Am

[warrenseale]

I had the A6 compressor rebuilt on my 79 Trans Am. I got the compressor back from the rebuilder located in Tampa, installed it, evacuated and charged it with R12 (3.75 lbs). Prior to that, the system was flushed with an aerosol power flush and blown out with compressed air to clear the solvent. However, the system is not working as it should. The static pressure is 80 PSI. With the compressor running, the low pressure side is 65-70 psi at idle with the engine just started and everything cold and an outside temperature of 75 deg. As the ambient temperature rises in the engine compartment the pressure increased to 80 psi. When the engine runs at 2000 rpm the low side is 55-60 psi. The high side is 100-110 psi at idle and increases slightly with rpm. There is a temperature difference between the input and output pipes of the orifice tube although the cold side is not ice cold. The accumulator is cool to the touch as well

I discussed this with the rebuild shop and they said it sounds like the orifice tube is not there when in fact it is there. They couldn't help me beyond that. Before I vent the system and waste a bunch of $, I want to be sure I have a good understanding of what is happening here. Can anyone offer some advice? Is something going on with the orifice tube?

[warrenseale]

I'm using 20+ year old 12 oz cans of R12. I suppose it is possible that they really are no longer 12 oz due to small leakage at the top of the can. Do these pressures indicate that the charge is a little low?

[Tim]

Looks to me you're not pumping very much refrigerant. 60 & 110 @2000. Ambient is a littlew low but think if working correctly you would see a range closer to 35 & 200 @ 2000 RPM.

[warrenseale]

Looks to me you're not pumping very much refrigerant. 60 & 110 @2000. Ambient is a littlew low but think if working correctly you would see a range closer to 35 & 200 @ 2000 RPM.

Do you think I need to add more refrigerant? I just don't want to waste putting R12 $$$$$$ into it if it won't make a difference.

[Cusser]

I'm using 20+ year old 12 oz cans of R12. I suppose it is possible that they really are no longer 12 oz due to small leakage at the top of the can. Do these pressures indicate that the charge is a little low?

My own 20 year old cans of R-12 are still fine.

Before I vent the system and waste a bunch of $, I want to be sure I have a good understanding of what is happening here.

One does not VENT refrigerant, one recovers it and re-uses it. It's illegal to intentionally vent refrigerant.

[Tim]

One does not VENT refrigerant, one recovers it and re-uses it. It's illegal to intentionally vent refrigerant.[/quote]

This is true!

Does this system use an orifice tube? If so are you sure it was placed in the proper direction? Not there yet, but have concerns your compressor is pumping correctly. Of course, the rebuilder will say it is. They always do and unless you have a good working relationship with them. You can send lots of effort into tracing things down to no end. Kind of sad they stopped supporting you.

I don't have refrigerant capacities on this vehicle. If 3.75 is correct. I think the pressures should have a larger gap.

[warrenseale]

This system does use an orifice tube. It was replaced with a new one and in the proper direction. 3.75 lbs is what the system requires. I agree that the pressures should have a greater difference but is this because the system is still low on refrigerant? Right now I'm trusting that the compressor is in good working order. The rebuilder does have a pretty good reputation.

[Tim]

Good to hear. Ambient temp being on the lower side is not helping. Add a little more if you like and see if the pressure chnaged as well as cooling.

[warrenseale]

I figured I would first look to see if the GM factory service manual for my car would provide any ideas. The trouble shooting chart says that if the charge were low then I would be getting a very very cold (possibly with frost) condition on the evaporator inlet after the orifice tube, and low suction side pressures <25 psi. This never happened anytime during the charging process. I periodically felt the evaporator inlet pipe during the charge. The chart says that if the suction side pressure is higher than the desired 30psi, the evaporator inlet pipe is the same temperature as the output (which it is) and the compressor does not cycle on and off then either the orifice tube is missing (which it is not) or the inlet screen of the compressor is damaged or restricted. At this point I don't know about the condition of the compressor screen but I can't imagine that it could be restricted given that the system was flushed and since the compressor was just rebuilt the screen shouldn't be damaged. So what is left, and internal compressor problem?

[Dougflas]

The input and output lines of the evaporator should be close in respect to temperature. If they are cold, then start looking at a reheat problem. Clamp off the heater hose or disconnect the heater core. The engine needs to be at 1500 rpm to 2000rpm. If the lines are cold, it is possible the evaporator has worn out seals and the air is blowing around the evaporator instead of blowing thru it. I hate to say it but it may be your "newly rebuilt" compressor.