First off, I’m no AC expert. I have a on and off cooling problem on my 88 with Electric Climate Control unit. 2 things to know first, the AC system was converted to R-134a back in 05 (17,000 miles ago) after my AC clutch went bad and C68 head unit was opened up last winter, checked and cleaned by a company that does this service (the reason for cleaning was erratic control function lights lighting up and the inside temperature display would flash sometimes). When I re-installed the head unit in February everything worked great. Now that summer is here I have developed a problem. The AC will blow cold air for about 5 minutes and then starts to blow outside temperature air. The AC clutch is working properly and the engine fan comes on when the AC is turned on. At first I thought it was low on Freon. I connected a small pressure gauge that came with the charging kit and the dial indicates it is over charged (gauge was in the low end of the RED zone). I know there are 2 electric switches under the hood that are connected into the AC system but I do not understand how they work. The other shrader valve shows up to 75 PSI on my AC pencil type gauge. Can anyone help me here? From August of 05 up to last week the AC has worked great.

 

After doing some investigative work on the Net I have decided to have it inspected by someone who knows what they are doing. Sorry for the post.

 

I can send some links for basic A/C information for the DIYer.
A small investment in a few tools can save bucks or at least let you know what is wrong.
One question: Does it blow cold anytime you turn the A/C on and after 5 minutes stops blowing cold?
Let me know.

 

I overcharged my AC system and that caused the evaporator to freeze. The air was cold at first and then I got hot air until I let it defrost for a while. Really sucked while driving home across the desert from Las Vegas in the summer. I had to turn the AC on and off for the whole trip.

 

Sometimes you can turn it off recirculate and chill outside air coming in will help unfreeze.
I always put in less refrigerant than what I "think" is needed, drive around for a while to see if cold enough or overfilled.
When charging try to simulate the real world. Drive around and get engine warmed up and it helps for it to be a hot day too. Put a fan in front of car(fan speed on high) to simulate airflow, while adding refrigerant.
If you have high and low side gauge it can help not to overcharge.

 

I have a leak behind the steel band that holds the accumulator on the fan side. I put in a can of Freon and saw it leaking out. Some how the system manages to keep enough Freon to blow cold air for a few minutes. I should take it to the AC shop and have them replace the $100 accumulator + re-charge the system because I have no non-messy way to remove the Freon that is still in the system. 100 PSI?

 

If you have a leak you can "see", then definitely fix that. If you take the system down, also check for other leaks with a vacuum check. If O-rings, gaskets and high/low valves are old, replace them now, they are cheap.

 

AC accumulators look to be wielded together with top part having the hose connections and the bottom the Reservoir. The leak is in the wield.

 

If you can see it leaking out, there will be nothing to remove by the time you get it to the shop. The accumulator is a $15 part if you go aftermarket.

 

But I only see the leak when the car is running with the AC on.

 

An overcharged system doesn't make ice as it pushes too much liquid into the Evaporator for it's pressure to drop to freezing (about 22psi - R134 - or 25psi -R12 - with superheat).

Freeze protection is the Low Pressure Switch. It cuts power to the Compressor when it senses the above pressures. Assuming it's correctly charged, the Switch may be bad. Check it out by holding idle at a steady 1200 to 1500 rpms with the System on Max and with your Gages hooked up, disconnect the Blower Motor. That will cause the Low Side to drop and if the Switch is right, the Compressor will cutoff at the threshold which you will see on your Gage. If it doesn't, you should see the ice and you'll need to replace the Switch or you can try to adjust it. Most have a screw between the terminals and turning that screw counterclockwise lowers the threshold; clockwise raises it. Try a quarter turn at a time and retest as above. Note - most new switches are set for R134. If you've stayed with R12, you will need to raise the threshold or it will make ice.

Absent ice, cold/warm cold/warm is usually a temp door flopping around or not enough gas causing it to cycle off. There is a possibility of an overcharge, particularly with an R134 conversion that crammed too much into it causing it to bounce up to the High Limit (400 psi) which will also shut it down. I've seen some incredible numbers posted around here for the Low such as a 60 psi which can push 75 to 80 degrees out of the vent and those were probably close to hitting the high Limit. Know that R134 freezes, absent superheat, at 28 psi (you'll probably never see that until it's 60 degrees); R12 30 psi. Mid 30's to Mid 40's is more typical. If you want to see actual temp, put a Digital Thermometer on the Evaporator outlet. That temp, less temp from your pressure/temp chart is the superheat - should be about 8 degrees. The Center Vent, considering thermal losses, maybe 2 to 3 degrees above the Evaporator Outlet.