Really odd - I answered this question a few minutes ago, and the answer just disappeared...

You charge the system by weight, not pressure. The specs for your car specify 1200 grams of R-12. You use approximately 80% as much R-134a as you do R-12, so you use about 960 grams of R-134a, or about 34 ounces. I normally install three 12-ounce cans of R-134a in my system.

 

The reason I asked about pressure was diagnostic. I want to make sure everything is working properly.

 

Wally, my car calls for 42.3oz of R12 btw

 

Actually, the tech spec book calls for 42.328754339 ounces of R-12. (1200 grams converted to ounces)

Pressures depend upon:
- Engine RPM
- Outside air temp
- Amount of air going over the evaporator
- Amount of air going over the condensor

So, the answer is "it depends". Perhaps 20-30 psig on the low side, 150-250 psig on the high side.

A better diagnostic might be to feel of the system.
- Tube from the compressor into the condensor should be hot enough to hurt.
- Tubes between the condensor and the receiver/dryer and between receiver/dryer and expansion valve should be moderately hot to pretty warm.
- Tube from the evaporator to the compressor should be cold.

 

I had it going today with low side pressure at 45psi high side was at 250psi. Outside air temp was around 50f. Does this appear in line with spec? Pressure was checked at idle.

 

One more thing- is it normal for an expansion valve to leak a little at first? I put dye in with the refrigerant charge and the pass side of the expansion valve shows a small leak.

 

That is not normal. The o-ring on that connection must be going.

Dan
'91 928GT S/C 475hp/460lb.ft

 

The o-rings are fine, it's leaking from the expansion valve intself. I hate to replace it again...

 

Low side is much too high -- too much freon. It won't cool properly like that. Recover the freon and fix the leak before recharging.

 

Where exactly do you see it leaking from?

https://www.928gt.com/p-1007-ac-expa...ve-865-95.aspx

The valve should not leak from anywhere except the o-ring fittings. If it is, it is defective.

Dan
'91 928GT S/C 475hp/460lb.ft

 

oddly it's leaking from the brass plug you see in the front of the picture. I should say it leaked for the first few minutes but I'll replace it anyway.
SharkSkin, I put the exact amount of R134 it calls for, so it cant have too much. What else could cause the low side pressure to be too high?

 

Leaking from that brass plug indicates a bad o-ring. I'm not sure that's user-serviceable, but on the 86.5 it should be relatively simple to replace(compared to the early cars, anyway).

If you put in the "right amount" for a car with rear AC, and you don't have rear AC, you put too much in. 1200g is for cars with rear AC, 1050g should be the right amount for cars without rear AC(up to July 88). Also that is the proper amount of R12; as Wally says you would use a lesser amount of R134.

Assuming that is not the problem, your expansion valve may be stuck open. Since it ought to be replaced anyway to fix the leak, I'd say put it together and re-test.

One other point. If your car was originally running R12, the hoses are not suitable for R134. You should get all of the hoses rebuilt with barrier hose rubber sections(fairly cheap at any competent AC shop), replace all of the o-rings, and make sure the oil is R134 compatible. Also, when converting a new dryer is recommended(fairly cheap).

 

Thanks for your help everyone. While I'm at it- how do I get in to the rear ac unit without breaking anything? I have not been able to figure out how to open it up...

 

The pressures at idle speed are not very meaningful. The pressures should be checked after they have stabilized at a steady 2000 RPM.