I was struggling with this issue late last summer, but got frustrated and put it aside. Last e-mail contact with Classic Auto Air indicated 24 oz R134. Which I put in after evacuating the system.

With the summer temps approaching again, I'm revisiting this and hopefully can resolve it. I have a thirdgen R4 compressor, so figured someone here may be able to help.

My setup:
1971 Camaro
CAA perfect fit series system
350 engine
Aluminum radiator
New GM R4 compressor (I have a serpentine set up)
Ford Taurus cooling fan (moves a TON of air)
Auxilliary fan on condenser

Here's how the system is currently performing

Outside air temp - 95F

Pressure values after a drive:

High = 250 psi @ idle
High = 300 psi @ 2500 rpm

Low = 38 psi constant

A/C vent temperature 65F!

Also, when the compressor is on, there's a huge drag on the engine (compressor is using a lot of power).

It's also causing the alternator to work really hard. I have 150 amp alternator. With compressor off, it's putting out over 14V. With compressor on, it drops to barely over 12V.

Any thoughts on what's going on?

Thanks!

 

Not in any particular order...

Your 150A alternator might do that at 2000-2500 engine RPM, or 6000 alternator RPM, but is more likely to be making about 40A at idle.

The normal electrical load of the engine, compressor clutch at about 3A, interior fan at maybe 10A (even more on high speed), and TWO external electric fans at probably 12-15A each are just about using up all the power the alternator can produce. If your Taurus fan is significantly larger (over 150W) it places even more load on the system.

The mechanical compressor load and additional alternator on the engine should be compensated by either a target idle RPM adder programmed in the PCM or an idle speed kicker solenoid on a carbureted engine.

Running R134A in what was originally an R12 system will likely result in lower performance unless the evaporator AND condenser sizes are increased. This is due to the differences in latent heat capacities of the refrigerants, their expansion controls, and the necessarily 10-15% lower charge (by weight) of R134a in a given system. As a result, more heat exchanging area is necessary to transfer the same amount of heat, and especially in the condensing phase. Moving more air will only provide a minor improvement, and that will limited by differential temperature.

R134A performance at a delta T of around 100°F will make full condensing back to liquid difficult at normal system pressure limits. In other words, a lot more surface area is required to condense R134A back to liquid at any given pressure than is required for R12. A more efficient (thicker, denser, parallel-flow, or dual) condenser will be necessary to create the same effective heat transfer. You can test this theory by artificially cooling the condenser with a garden hose running cold water when you test the system at 95°F ambient. If the cooling becomes acceptable with proper condensing, you may be able to get by with the original evaporator.

 

Thanks for the reply.

I guess a little more info on my system...

The Classic Auto Air system was designed for R134A, so austensibly everything is sized for that.

During non-A/C operation, the Taurus fan barely moves the needle on my voltage gauge, even on the high speed- this is even with my EFI, stereo system, and at night with the all the lights on. I can't imagine the little aux fan on the condenser + the compressor clutch being that much extra of an electrical load.

It was suggested on another forum that I may have overcharged the system. WHat I'll probably do is let a little charge out at a time and get that 300 psi down until I start getting the air vent temps down to the right temperature.

On the alternator, the voltage barely climbs when I'm driving at speed.

 

Don't forget the load from the interior blower.

If the '71 is the one in your attached photo, you can ship it to me and I'll gladly evaluate the symptoms and come up with a reasonable solution within 48 months, guaranteed!

 

Check the compressor temp. Center of the case should be a maximum of 150 degrees when working at full tilt. Also, did you put enough oil in the system? A little extra is better than not enough.