What is my A/C problem

The short version: My return line is frosting, the A/C is worthless.

The long version:

The car is a 1989 Jeep Cherokee, well maintained. Five years ago when I bought it, the R-12 needed a topping up. This summer it was at that point again, so I did it again. I don't have regular gauges available anymore, (SEE BELOW, AMENDED INFO WITH GAUGES) so I just used the sight glass. If I put in enough freon to stop bubbles, the suction line was frosting! Hmmmmmm.....

I also noticed that I got my best cooling at an idle, and it was progressively worse as the engine speed went up.

Before we go further, let me affirm that indeed, the auxilliary fan works, the fan clutch is a year old, the compressor clutch is engaged except when it should turn off (seldom, now), there are no compressor noises, the belt isn't slipping, the blower blows hard. Yada yada.

Mulling things over, I thought that the TXV (thermal expansion valve) was bad, since it meters throughput. And I swear, when I rapped it hard (there's a right sized hammer for every job), things improved slightly. Hence my conclusion.

Although I have a fair stash of R-12 available (thanks, Dad!), I thought this would be the time to bite the bullet and switch to R-134a. Especially true since I was in a heat wave and my R-12 was 2000 miles away. No more guilty conscience about releases, etc. I bought a conversion kit, swapped the TXV (what a b*i*t*c*h!), sucked out lots of oil from the compressor, blew compressed air backwards and forwards, pulled down the vacuum for 3 hours using my two stage air compressor, ports reversed and sequential, and added the R-134.

I did not replace the receiver/dryer or any other components. My thinking was that without any mechanical failures or opening of the system, it should be pretty pristine in there still. If the dessicant has some moisture, three hours of vacuum should pull it out. Immediately after turning the vacuum pump on, I felt the bottom of the dryer, the temperature never changed, hence I believe, no moisture.

Exact same symptoms!

ADDENDUM, GAUGE READINGS: I got some gauges today. Outside air temp,

68 degrees (got a major rain wave here in Denver yesterday.) At an idle, suction is 25, high side, 160, outlet temp barely colder than ambient (about 60 degrees.) I presume these to be a bit low, even for the temperature, but I have frost all the way from the evap to the compressor. I'm afraid that if I add more freon, I'll slug liquid into the compressor.

Now here's where it gets interesting. When I rev the engine to maybe

3000 RPM, the readings change to 10 and 180! As I see it, a restriction.

If the problem is in the receiver/dryer, I should have a temperature difference between the inlet and the outlet, maybe even condensation or frost between the RD and the evap.

I replaced the expansion valve, and I'd like to think that it is OK. Probably my old one wasn't bad, the symptoms are exactly the same.

Sooo............... I'm thinking that the evap has a restriction somewhere towards the outlet. Some evaporation of the freon is taking place in the evap, thus getting some slightly cool air. But most is taking place after the evaporator in the suction line. There isn't any difference before or after the muffler/filter bulge.

Man, I love replacing evaporators..................

Reply to
pzo
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slightly.

R-134a.

The desiccant is the wrong type for 134A. Probably turned into goo and plugged the system up for you.

Reply to
Steve W.

Sounds like the door is stuck closed over the evaporater to me.

Steve B.

Reply to
Steve B.

Make sure the expansion valve is actually opening - I had to replace mine on a 1985 Corolla - and ALL local shops gave me the wrong part. I had to special order mine from Denso to get the right piece.

I would have replaced the drier when you had the system open. Not only could it be the wrong type for R134a, I don't think vacuuming pulls moisture out of the dessicant.

If you have a stockpile of R12 stick with that. It will give better cooling performance (assuming everything in the system is functioning) than R134a because it is more efficient. Cars designed for R12 usually will have underspecced condensers to handle the extra heat generated when R134a is used.

Reply to
Masospaghetti

Let me first say that a frosting of the return line on a semi-humid day is perfectly normal. As for your pressures on the 68 degree day... they seem normal. Your engine reving pressures of 10 and 180... also normal. What is the air temp exiting the vents (after system is running for 5 minutes)? Does the compressor cycle? Is your evaporator drain clear? What temp do you feel on the input and output of evap?

Professor

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Reply to
Professor

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