OK, I had the A/C upgraded to 134a, and now the compressor does not come on when I push the button on the dash. However, if I hot wire to the battery, it comes on and runs perfectly. If anyone has any ideas, please help.

Probably the low pressure switch, assuming that the compressor is not even trying to come on (cycling on for a second or more and then shutting off). If this is so, the method you used for charging the system may not have been correct.

 

BTW, it will help to know what vehicle this is on.

did you get all the R12 out?

if the system is under or over charged the compressor won't come on.

check your fuses and relays, the fuses to the a/c system are subject to blowing, i'd particularly be sure to look at them since they are so easy to inspect visually.

GG has good points.

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if the system is under or over charged the compressor won't come on.

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IIRC, overcharged will allow the compressor to come on until the overpressure occurs, and then cut the compressor. Is my memory (and my brain) scrambled-eggs this morning? :-\

i think your right northwet...

 

it runs until it builds up too much pressure, then shuts off.. but at static conditions its not "overpressurized" just overfilled... so it will turn on, for a little bit.