94 GT - No Air Conditioning

Last summer A/C worked fine but this year sons mustang has no A/C. We went to recharge. Had the engine running with A/C on Max. I noticed the compressor was not engaged and stopped the activity before any R134 was added. Thinking about it more I wondered if the compressor would engage even with low Freon. Is there a sensor or something that detects low pressure and will not allow the compressor to engage? I guess a better question is how best to trouble shoot an A/C problem. We already have the R134 kit but not much else. I know how to turn a wrench but have not had very good luck with the couple of A/C problems I have encountered in the past.

Here is what I have in mind:

1. Try jumping the compressor clutch to see if it can engage.
2. If it can engage then try adding some Freon and hope for the best.

Not a very good trouble-shooting plan, I know. Would love comments/advice from others who have had a similar problem. Chilton was not much help.

Thanks in advance.

First of all, be very careful due to potentially high pressure gas/liquid in the AC system.

If your recharge kit is one with a pressure gauge you should be able to get some idea of what is going on just by connecting it to the low pressure return line (larger one going back to the compressor). If the pressure without anything running isn't at least above 30 PSI the low pressure cut off will not allow the compressor to come on - this is just a general value, I am not sure what the actual setpoint is. If the system has a full charge, I would expect to see a pressure between 70 and 120 PSI with nothing running and depending upon the ambient temperature. If the system off pressure is low, you will need to add refrigerant until the pressure is high enough for the low pressure cut off to stay on. Be careful about adding liquid refrigerant to the system; keep the can upright for gas.

If the system off pressure looks OK I would check for a blown fuse or faulty relay. If these are OK then check for 12 volts at the compressor. If that is 0 volts, check to see if there is a voltage drop across the low pressure cut off - this will require back probing the connection. The low pressure switch should be on the return line, possibly on or around the accumulator. Again, this is a general statement as I am not familiar with your specific model. There is most likely a high pressure cut off that could be faulty as well.

Please, if you are unsure of any of this, take it to someone with more experience - not worth personal injury or costly damage to your system.

Good luck, John

Thanks for the feedback and advice ride2fix. We did check the pressure and it was low, added r134a and compressor started kicking in. 2 Cans and it worked great. No mess or injuries.