A/C question( yeh, another one)
 

Well, my first post and a little long, so bear with me. Bought 1989 911 25th Anniversary Edition, this past March. PO had neglected routine maintenance and other items, so I have been sorting thru. The A/C didn't work, so researching all I could, is how I found this site. Short version, I discovered a PO had upgraded system, New barrier hoses, rear condenser, electric fans, etc. Having read all I could find on this site, I took to Mechanic to flush both condensers, pull, clean and flush evaporator, install new expansion valve and dryer, flush all hoses, install hi and low pressure switches, install new compressor, etc. charge and check for leaks.
So far so good. Blows Ice cold air.....for about an hour or so and then air progressively gets warmer, to the point it blows warm air. The volume of air also seems to be reduced. After several hours, you turn on and blows cold again..etc. repeat above.
So what do you guys think.
Thanks in advance

Something is freezing up. Has to thaw to get back to normal. Of course this is me with little to no experience fixing such things. :)

I will wait on the sidelines with some G.H. Cretors this time.

Your evaporator is freezing up. Check your coolant charge.

Does that mean existing charge too much or not enough?? Any idea how much is approximately correct amount?? Shop I used has upgraded a few P cars and been in business 40 years, well respected, but owner doesn't check every step of all repairs going out door. Thought I read on here possibility of temp. probe in wrong location may cause this also? My first thought was it has to be icing, but cause??? Got 12 month warranty on repair so I know they will make good. Just trying to make a few suggestions for them to check. Don't want to come across as " know it all" to them but want to be able to talk to them somewhat intelligently. After reading problems others on here are having with "gunked up" evaporators, I think I will double check with them and make sure they pulled evaporator and cleaned it thoroughly, inside and out. Again, thanks for all advice.

+1 and +1.

Empty a little refrigerant out when cold. Not much - just a 2 second spray with a car key on schrader valve (and gloves). Test it. Rinse and repeat.

This is your problem.

I would first check to make sure the thermostatic switch is operational and properly calibrated.

Fire the system up, make sure the compressor clutch is engaged, turn the thermostatic switch fully CCW, the clutch should disengage.... Thermostatic switch operation verified.

PS: do not keep the rear lid open any more than necessary.

Checking the calibration, or calibrating the thermostatic switch, is a bit more of a challenge.

But... assuming the switch function is correct per above, operate the system with the control at mid-point for long enough to determine that the evaporator does not freeze up at that setting.

If it doesn't freeze-up then the switch should be calibrated.

If it does still freeze up then have pro have a looksee.

Same question regarding TXV "pigtail"....

Almost never agree with any of the crap that wwest spews out. In this case I do however.

Start with a simple test. Turn the cool/cooler control towards the least cool setting. Does it still do exactly the same thing? If yes, check the pig tail for correct insertion into the evaporator. If everything appears honkydory then I would start with a suspicion that the control is not function as it should and is causing the freeze up.

Thanks for reply. I did try turning Temp. control switch all way off while driving, after it had started blowing warmer air, still cool, but warmer. We were on Interstate and could not tell if compressor disengaged. No change noted in air temperature, when turned down though, so that may be culprit. Finally turned system off and drove home in 95 deg. temperature. Momma was not happy! Late when we got home, so didn't check anything further. Will explore further and take back my Mechanic. Have 12 month warranty on parts and labor for just this reason. Shop been in business 40 plus years and specialize in import cars. I know they have upgrade a few P car A/c systems, which is why I took to them. Want to be able to politely suggest where they may look for problem with out seeming like a "know it all". Will post update . Thanks.

Try running the system with the temp knob backed off from full cold. Just back it off 25% or 30% of a turn. If it does not freeze up your probe is not making the proper contact.

+1 on backing off the temperature control as a test. This same thing happened to mine after I replaced the entire system including the evaporator. Of course not everything came out perfect. The evaporator fan squealed which I removed and cleaned the bearings then re-oiled it. Fortunately it came out fine.

However like the original post, after an hour the temp started to climb and it would no longer cool. I discovered that the guide tube that went through the evaporator was missing so I fabricated a new one but that didn't do it either. Ultimately I replaced the control and thermocouple which solved the problem 100% no more freeze ups.

One thing I've observed is that the temperature stays lower if I don't run on the fastest fan speed position. I typically run on #3 which results in vent temps around 35 degrees. I can watch the compressor cycle and the temp rise and fall a few degrees so it seems that everything is working.

I'm sure with additional condensors the system would work even better. I used the Rennair system with the original rear condensor.