I there a HVAC doctor in the house

[Don in VT]

Hi All,

I have had problems with our A/C on our new to us 90 ZR1. The cooling frequently drops out and when I run the diagnostic program I get 09 (low freon). I was never sure how much 134a was in the system so I took it to the shop where I work part time and put it on the A/C charger. We use a very new Robinair 34788 which does it all. We, recovered the 134a, pumped it down, checked for leaks and added a small amount of oil. When we started to fill the system it failed after we added about 7oz. of 134a. We had dialed in a charge of 24oz.
We ran the program 3 times with the same result.
All of the above was done with the engine off as is our std procedure.

I took the ZR home and did the following. with the engine off I used my charging hoses and tried to add another 16oz. The system would not take any 134a at all essentially duplicating the shop results. With my 134a 1 lb can still connected to the low side I rebooted the A/C controller,set the controls to high and started the engine. Immediately the compressor clutch started to cycle and the system began to take the freon. The duct temp dropped like a rock and the system was cooling just fine. I did not run the system very long as I only added about 8 to 10 oz out the the 1 lb can making a total of 15 to 17 oz. in the system.

Here are the questions for the A/C knowledgeable folks,

Why did the charging system fail the freon fill.? It worked fine on the the next car.

Is it necessary for the compressor to be running to put a full 24oz. in the system.? If I go back to the auto charger should we start the motor and the A/C system to complete the charge ?

Any help from the forum would be helpful.

Thanks

Don in VT