Automotive Air Conditioning Information Forum (Archives)

TransAm525 on Sat August 25, 2012 1:18 PM

Year: 1978
Make: Pontiac
Model: Trans Am
Engine Size: 400
Refrigerant Type: R-134a
Ambient Temp: 88
Pressure Low: 90
Pressure High: 350+
Country of Origin: United States

I just completed a full a/c installation on my 1978 Trans Am. The system was placed on a vacuum leak check for several hours during the morning. One Aeroquip line had a slight fitting leak, of which was solved. No further leaks were discovered and the Snap-On charging station gave the system a passing leak check. The system was initially charged with 1.8 lbs of R-134a per vintage Air's general instructions. Once started, the high side pressure started to rise. Upon reaching 350 psi, the vehicle was turned off. During this time, the air from the vents was cool, but not cold. The low side lines were cool, but not cold. The high side hose, condenser, and liquid return line became so hot you couldn't touch them. Every single component of the air conditioning system is new. Essentially, the entire system is a stock setup now with a parallel flow condenser (same size, direct fit drop-in), a Sanden compressor, and Teflon/braided lines instead of rubber (also slightly shorter than stock length hoses). All O-rings are new and R-134a compatible, the orifice tube is the stock white GM part (also new).

After the first run, we dropped the refrigerant level from 1.8 to 1.2 lbs. The vent temperature felt the same, however the high side pressure continued to rapidly rise again, and at 350 psi we turned the engine off to prevent any damage. During both runs, the pressure cycling switch cycled the compressor clutch on and off. The shop was nearing closing time, so we left the 1.2 lbs in the system, unplugged the compressor clutch wire, and stopped work until next week.

The original system, with the A6 compressor and R-12, held 3.25 lbs, however I don't have any stock low or high side specifications listed in any of my books. Vintage Air recommended an initial charge of 06-12 psi (low), 160-250 psi (high), 1.8 lbs of refrigerant, and a duct temperature of 36-46°, however these specs are probably tailored more for their complete under-dash drop-in units.

With the above in mind, where should we go from here? Should we try using the stock 3.25 lb charge? I also have a set of electric fans (3300 cfm combined), wired to a thermal switch to engage the fans at 170° coolant temperature. During both runs, the engine wasn’t warm enough for the fans to engage, which may have contributed to the rising high side pressure (may have ro re-wire to engage with a/c clutch). Any recommendations or suggestions would be highly appreciated.

-Replaced evaporator core (new GM)
-Replaced receiver/drier (stock replacement)
-Installed Classic Auto Air direct-fit parallel flow condenser (stock-dimensions)
-Replaced liquid return lines
-Converted from thermal cycling switch to binary pressure cycling switch (switch is R-134a specific)
-Installed Vintage Air Sanden 508 rear exit compressor
-Aeroquip FBF A/C braided stainless/Teflon lines and fittings with O-rings
-White (stock) GM orifice tube
-Replaced all a/c controller and blower motor switches and relays

mk378 on Sat August 25, 2012 3:23 PM

The fans must run whenever the compressor is on. Running a condenser with no airflow will get exactly what you experienced. It has to reject heat to be able to condense, the more air the better. More sophisticated systems have a switch on the high side that closes at about 200 psi to start the fans, so when the car is moving they don't run unnecessarily, but there is not much harm in activating the fan relay directly from the compressor circuit.

Also make sure the condenser is plumbed properly, in at the top out at the bottom.

Is this a true CCOT system? You should run a TXV after a receiver on the high side, not an OT. OT systems would have an accumulator between the evaporator and compressor.

Edited: Sat August 25, 2012 at 4:33 PM by mk378