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Tbird S/C 134a Swap

88pony on Wed May 26, 2010 8:05 PM User is offline

Year: 1990
Make: Ford
Model: Thunderbird S/C
Engine Size: 3.8L
Refrigerant Type: 134a
Ambient Temp: 85

Basic question here...

I've got a 1990 Thunderbird Supercoupe. (Supercharged 3.8L) The car had been converted to 134a before I bought it, but it had no charge. I had it professionally leak checked and found the evap was leaking. I replaced the evap, accumulator/drier, and had a friend who is a commercial/residential HVAC contractor charge it with 134a. He left it on his vacuum pump for several hours before charging. That was late last fall so the A/C didn't get much use after charging other than confirming it was working and blowing cold.

This spring when taking the car out of storage and trying out the A/C, we found it still blows cold, however the pressure release valve on the back of the compressor keeps venting refrigerant. Also when the clutch engages with the engine at idle it will stall or almost stall the engine. Implying that the pressure is very high. The compressors spins smoothly and freely by hand. During the evap replacement I filled the new accumulator/drier with the Ford service manual recommend volume of new 134a friendly oil.

I don't have gauges handy, and my buddy is several hours away. Any suggestions before I haul the car over to see some gauges?

Note: We did almost the same conversion on my 1988 mustang 5-6 years ago with parts and help from this site and it is still working great.



mk378 on Wed May 26, 2010 8:34 PM User is offline

Assuming properly charged, overpressure like that happens because the condenser overheats. Are the fans working?

88pony on Wed May 26, 2010 9:56 PM User is offline

Ran a quick test, yes the (electric) fan comes on (seems like high) when the A/C is cycled on. Also we've been driving the car (less A/C) in 80+ temps with no overheating so I'm confident the fans are working properly.

The engine almost stalls the first time the compressor engages, even after sitting overnight, so I assume the condenser wouldn't be hot yet. Once the compressor engages the car only runs for about 10-20 seconds before the pressure valve releases. Here's the weird part, it seems to be getting worse, last fall we could use the A/C with no issues, early this spring I could run the a/c for a minute or two, long enough to get cool air from the registers, before the pressure valve would vent, now it immediately vents as soon as the clutch engages. It was pretty cool last fall when we charged it, but I would think with the venting it's been doing it would have relieved itself of any overcharge. I'm wondering if the pressure switch on the accumulator/drier needs adjusted? Maybe some debris has clogged something? The condenser and orifice lines look newer, and I know the evap and drier are new. The only older looking component in the system is the compressor, and with 60k miles on the car, and no noise from the compressor I'm hoping it is ok.

Thanks for the feedback.


bohica2xo on Thu May 27, 2010 12:45 PM User is offline

Pull the orifice tube out & have a look.

Stalling the engine @ idle with an FS10 is usually a sign of a failure. The good news is AMA has brand new replacements that are great parts.

That system only holds so much refrigerant, and if it is the HPRV venting it can only happen a couple of times before there is not enough refrigerant left to run the system. HPRV's that old are famous for not re-sealing after a venting episode.

The fact that it still cools after a venting cycle makes me wonder if you heard something else - like a belt slipping on a locked up compressor.

Unless the system was grossly over charged initially, it should not be cooling by now.

We really need pressures to diagnose this system.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

88pony on Tue June 08, 2010 12:08 AM User is offline

Not a belt slipping, I saw the cloud of refrigerant one time when it vented (hood up), and I can see fluorescent dye stains on the hood liner where it was blowing with the hood covered. The belt also looks great. I'm going to put some gauges on it and most likely go ahead and crack the system open to check the orifice tube. It may not be blowing cold any longer it vents before it runs long enough to get cold.

A few more questions:

1. Is there a way to check the compressor once it is removed? Or just replace it? Everything else in the system is already new.

2. I'll go ahead and replace the orifice tube at the time of the compressor, what about the drier? It was new last year and the system has been sealed, I won't crack it open until I have the parts in hand to put it back together to minimize the time it is exposed to atmosphere.

3. What are the trade off of going to a reman compressor vs. the AMA new unit? there is about $100 difference in price. The car is a weekend toy in pristine condition so I don't mind spending a little more for a quality part, but almost double seems a bit steep.

4. When I charge it how much 134a should I add? The sticker on the rad support calls for 2 lbs 8 ozs. Any rules of thumb for 134a?

5. How much oil should I add to the new compressor?

Thanks again.


88pony on Tue June 08, 2010 12:10 AM User is offline

Clarification, the radiator support sticker calls for 2 lbs 8 ozs of R12, I'm converting to 134a.


Dougflas on Tue June 08, 2010 5:03 AM User is offline

you stated that you added the Ford amount of oil to the accumulator. Exactly how much oil did you add to the system? You may need to flush the entire system and start from scratch.

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