AC Issue

[z28cp]

1992 C3500 Crew Cab Dually 6.5TD

OK, the AC light is flashing on the HVAC display when I start the truck. Manual says that this indicates that the Low Pressure switch is seeing low pressure. Fair enough.

Hooked up my AC gauge, thinking system is probably low. Does not appear to be. Tried bypassing Low Pressure switch. Compressor clutch still does not engage. No power to the compressor clutch, even with the low pressure switch bypassed.

Checked fuses under dash, all OK. So, does one of the relays on the firewall right above the accumulator go to the low pressure switch? And if so, can anyone tell me which one? I don't have a Helms manual for the truck yet, and the wiring diagrams in the Chilton manual leave a little bit to be desired...

It's pushing 100 degrees here, and I'd like to drive the truck without sweating...

Thanks!
DaveZ.

[Robyn]

Power comes from the dash control to the low pressure switch and then to the relay and from there to the Clutch.

The 92 dash controls should be the same as the 94 if memory serves.

I am suspect that the solid state contol unit in the dash has taken a poop.
If your Low side pressure is above 50 PSI with the unit off, the clutch should engage. Since it is not and the system pressures are enough that it should then I suspect the controler.

The digital display units can be a PITA. the later ones with the 3 dials were better BUTTTTTTTTTT.

See if you can scrounge up a controler that is known to be good and swap them out for a test.

Does all the other parts of the HVAC system work right??

Hope this sheds a little light on the issue.

Robyn

[Slim shady]

My suggestion would be to get a better wiring diagram from a local repair shop. Someone that has all data or some other fairly good wiring diagram. Controllers go bad but generally it is a simple problem, always easier when you have parts to swap BUT that isn't really diagnosing the problem or practical for most people.

A lot of so called techs throw parts at it rather than looking at potential problems with sensors and switches first. I can't comment on the system without looking at the wiring diagram but check the EASY stuff first.

I have a wiring diagram for the 95 and have access to an all data system so let me know and I may be able to get you a wiring diagram that is complete.