Need quick help charging A/C with freon

I posted this on the Jimmy forum and only got 1 reply which did not help. I know this is not the correct forum but the problem should be the same on most GM vehicles and the people here seem to be a little sharper. I"m pasting it here in hopes that someone can give me a hint.

How do you trick the compressor into coming on so freon can be added? Last year it got low and had to get an AC man to add freon. He said he couldn't find a leak but this year it will not cool again and the compressor won't come on just as it did last year. Any tips? How can I make the compressor come on so I can add the freon? My girlfriend is leaving in about 2 hours and I need some quick help with her truck.

What year Jimmy, it makes a difference.

What year Jimmy, it makes a difference.

1998, thanks.

Turn the AC to the max position and high fan speed. Add the refrigerant at the low pressure port at the firewall area but if it is very low on refrigerant charge it may take some time for the compressor to kick in and start cycling. There is a low/high pressure switch by the accumulator that senses system pressure and lets the PCM know when to turn on the compressor, etc. The charge must come up to a certain level. Also, make sure the electrical connector to the pressure switch is on properly and tight.

BTW - It may be a Jimmy but all GM AC principles are the same.

In a worse case scenario the pressure sensor can be bypassed (not recommended) but you'll need to use a resistor that represents a pressure in a range between low and high. It cannot be jumped outright like on older AC systems. The PCM feeds around 5 volts to the sensor and the sensor will send a signal proportional to the system pressure back to the PCM for compressor turn on and cycling.

I connected the gauge and a can of freon and it finally started cycling the compressor but it took about 10 minutes. I got the only can I had in and the compressor started staying on a lot longer and the air started to get a little cool. Didn't have enough time to go get another can but will add it this weekend. I didn't realize it took so long to get the compressor to cycling. Thanks for the replies. I knew I could count on you all.

I connected the gauge and a can of freon and it finally started cycling the compressor but it took about 10 minutes. I got the only can I had in and the compressor started staying on a lot longer and the air started to get a little cool. Didn't have enough time to go get another can but will add it this weekend. I didn't realize it took so long to get the compressor to cycling. Thanks for the replies. I knew I could count on you all.

The length of time depends on how low the freon was. If it took that much you have a pretty significant leak. Check the connections at the compressor, drier, and condensor. If you see an oily residue, there is a leak there. Also check around the front of the compressor behind the pulley. If you see nothing you may need to have a tech look at it with a sniffer to check the evap case. Also watch the pressure when you are filling it, If they are out of whack you have a blockage. If you need the hi-low numbers someone here I'm sure can help ya. Theory, fundamentals, and parts I know, the important stuff one of the pros here can help ya with.

It has a slow leak and last spring we had it serviced by someone who has worked on AC for many years and predominantly does AC only. He could not find a leak and charged up the system. It has leaked down again and it took about a year. She has about 165,000 miles on it and I told her just to add some freon every year until it craps out. It is a very slow leak.

Glad we could help out. :bigthumb:

I'd put money on the compressor for the slow leak.

Glad we could help out. :bigthumb:

I'd put money on the compressor for the slow leak.
I'm pretty sure you are right about the compressor. As long as a couple of cans of freon every six months will keep it going, I expect that is the route we will take.