I know that this isn't a truck but I am having a problem with my 1987 Mercury Cougar XR7. The a/c system has been converted over to R-134a a few years ago. Since then the air has not been blowing as cold as the R-12 and as cold as when first converted over. So I threw my manifold gauges on to get readings of 25psi on the low side, I don't remember what the high side gauge read as I was suprised in the drop in pressure. So I added about 15 ounces of R-134a to it and brought the pressure up to 37psi and the air blew cold like it used to. I thought that I was flying high until I started to car up today and turned the air on when I was moving it around in the garage two days later. I noticed the air was not even working at all and the compressor was cycling on at a high rate. So I attached the gauges again and got a reading of 20psi on the low and 100 psi on the high. I thought to myself, "oh great I f'ed it up again." Well I just recieved a case of refrigerant the other day so I went to put in another can and I realized that the schroder vavle on the high side was sticking slighly and perhaps that is why the system leaked down. Well that was fixed and then I reattached the manifold gagues and proceeded to fill the system with oil and refrigerant. Once I opened the first can into the system the pressure jumped to 45psi, on the low side, and the compressor kicked on and then dropped to 20 and then kicked off. The high side read around 100psi. It keeps on doing this and will not take anything more than one can after I evacuated the system before I began. I am at a loss here and don't know where to go after this. Someone please help me as it is getting really hot here in Michigan lately!

 

your system is cycling on low pressure. depending on where your suction port is, where you are reading pressure may not be the actual pressure as to where the low pressure switch is. the best advise i can give to you is to vaccum down the system and wiegh in the refrigerant. after a good leak check of coarse to verify your prevoius repair.if you don't want to do it that way, jump the switch or hot wire the clutch.

 

Both high and low pressure ports are located right behind the compressor about 4-6 inches. Where is the low pressure switch located at? On the evaporator in the dash?