A/C Low Pressure Switch Question

1991 1/2 ton with 5.7L manual reg. cab

My a/c was not working so I figured it needed a charge. I put the gauge on the low side and it read about 45 pounds so I then unplugged the connecter from the LPS and jumpered it. The A/C Compressor kicked on and the pressure read about 24 pounds. Is the problem the Low Pressure Switch or could it be something else? Will I loose my freon charge if I replace the switch? Any help would be greatly appreciated.

1991 1/2 ton with 5.7L manual reg. cab

My a/c was not working so I figured it needed a charge. I put the gauge on the low side and it read about 45 pounds so I then unplugged the connecter from the LPS and jumpered it. The A/C Compressor kicked on and the pressure read about 24 pounds. Is the problem the Low Pressure Switch or could it be something else? Will I loose my freon charge if I replace the switch? Any help would be greatly appreciated.


I would say that your switch is bad (See http://autoacrepairs.com/cycleswitchinfo.htm)

By the way, you can just remove the old one and put on a new one since there is a schrader valve under the switch.

//2000CAYukon

I put a new LPS in truck today and a/c comp. still did not kick on but it is 14 degrees out. I will try when it is warmer, whenever that may be. I thought that the a/c comp. should kick on when I use the defroster to defog the windshield.

I put a new LPS in truck today and a/c comp. still did not kick on but it is 14 degrees out. I will try when it is warmer, whenever that may be. I thought that the a/c comp. should kick on when I use the defroster to defog the windshield.

I have a 90 K1500 5.7 Auto and the AC does come on with the defrost. Perhaps with the cold weather, the low pressure was not high enough to kick it on.

When a gauge on the low side, the AC compressor should kick on with the low side above 44 and stay on until 25 it will then kick back on when the pressure reachs 44 if the LPS is working properly.

//2000CAYukon

Does the Low Pressure Switch "look" at both Low and High pressure? The reason I ask is my system was low (ac light flashing and gauge read low and ac comp. not on) and I added freon by jumpering the LPS. I reset the flashing ac light and the ac still does not kick on and I replaced the LPS and the ac light still flashes. Could I have over charged it. It is reading around 45 psi when comp. not running.

Does the Low Pressure Switch "look" at both Low and High pressure? The reason I ask is my system was low (ac light flashing and gauge read low and ac comp. not on) and I added freon by jumpering the LPS. I reset the flashing ac light and the ac still does not kick on and I replaced the LPS and the ac light still flashes. Could I have over charged it. It is reading around 45 psi when comp. not running.

The Low pressure switch turns off at one pressure and back on at another pressure of the low pressure line.

Are you still using R-12 or did you switch to R-134a?

If you jumper the LPS and the compressor kicks on, the pressure is still too low to kick on the system. Perhaps your gauge is off or you have the wrong LPS. There is a different one for R-12 and R-134a.

I can take a look later at what the pressure is on my 90 (still R-12) when the compressor is off.

//2000CAYukon

The AutoZone guy gave me a LPS ( MT0204) for R-12 which is what was original. I converted the truck over to R-134A in 1998 and have not had a problem until now. Do I need a LPS for R-134A or will I have to adjust the R-134A pressures to work with R-12 switch.

The AutoZone guy gave me a LPS ( MT0204) for R-12 which is what was original. I converted the truck over to R-134A in 1998 and have not had a problem until now. Do I need a LPS for R-134A or will I have to adjust the R-134A pressures to work with R-12 switch.

The MT0796 is the LPS for R134a; however, the difference is on the low end. The MT0796 cuts off at 21 where the MT0204 cuts off at 25. This is due to the difference between R-12 and R-134a.

This still does not explain why the compressor is not coming on. The wrong LPS switch will probably cause it to cycle too much so you may want to replace it at some time.

I will look at the factory manuals later tonight (West Coast Time).

//2000CAYukon

The MT0796 is the LPS for R134a; however, the difference is on the low end. The MT0796 cuts off at 21 where the MT0204 cuts off at 25. This is due to the difference between R-12 and R-134a.

This still does not explain why the compressor is not coming on. The wrong LPS switch will probably cause it to cycle too much so you may want to replace it at some time.

I will look at the factory manuals later tonight (West Coast Time).

//2000CAYukon

Thanks for all of the help. I still think that I need more freon.

I added another can of freon and it works just fine. I must have a leak but nothing is obvious.