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Front Cool, Rear Cold -- HELP!

TheBobit on Wed August 05, 2009 11:25 PM User is offline

Year: 1992
Make: Mazda
Model: MPV
Engine Size: 6.0l
Refrigerant Type: R134a
Ambient Temp: 81
Pressure Low: 45
Pressure High: 240
Country of Origin: United States

Two years ago I converted this system from R12 to R134a. The front used to blow cold and now the rear blows colder than the front. An "instant reading" thermometer placed in the front middle vent read 78°. The rear vent; 68°. Even though I had checked the blend doors while the system was apart, I opened up the evaporator "box" and the temperature right out of the evaporator was the same 78°. With an Ir thermometer, the front was 62° and the rear 48°.

Things done previously to the system: Compressor completely dismantled, thoroughly cleaned, new oil and "O" rings installed. Condenser flushed. New receiver/drier. Rear evaporator flushed and expansion valve cleaned. Crushed and leaking refrigerant line to rear unit repaired. All system "O" rings that I could get to were replaced. The required amount of Ester oil was added to each flushed component.

Things done this week: New evaporator and expansion valve installed in front unit. System was evacuated for 2+ hours and left over night. Re-evacuated for another hour+ before charging the system. Large fan blowing on the condenser while charging.

I generally charge systems to high side being 2.4 times the ambient temperature and this usually works out great. This time I did it by weight of 2.2 lbs [based on the amount used in a 1998 MPV with dual systems]. The last time I charged this by pressure/temperature, the high side rose at least 20-30 PSI (I don't remember the reading now) after stopping the charge at the 2.4 times point. The same thing happened after charging by weight. I didn't think to do it at the time but later checked the low and high side tubes to the front evaporator. At 74° ambient, the low side was 80° and the high side was 119°. The system equalizes to about 100 PSI when shut off.

I am usually pretty good at figuring a/c problems out, but since the rear unit still blows considerably colder than the front, this has me stumped. Any help would be greatly appreciated!

mk378 on Thu August 06, 2009 11:50 AM User is offline

I think it is undercharged, and your fan clutch is tired so the high side comes up more than it should for being undercharged. Capacity of the '92 with dual air was 49.5 oz R-12, if all parts are stock '92 type, you should try to get close to that amount of R-134a.

Prospeeder on Thu August 06, 2009 7:54 PM User is offlineView users profile

remember you only use like 80% of R134a to the R12 capacity.

1990 Pontiac Turbo Grand Prix
1990 Pontiac 6000 LE V6
Both have R12 A/C

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