A/C problem

Ditto on the same truck: Doc? Buddy had three complaints when he gave me the truck: No, A/C, cruise, or tunes. 1992 GMC Z-71 5.7 "K" engine

Hot as hell so I figured that I would give him A/C first: Converted the system to R-134A. Recover, recycle, recharge, no leaks. A/C flashed on the control unit. Noticed lime green wire from cycling switch on accumulor while checking relays goes to ECM. Reset ECM, A/C cold and normal.

Diagnosed radio, replaced amp with boneyard unit, would not play until ECM reset. Hmmm.

No cruise, replaced multi-function switch. No cruise. Replace Cruise control module with brand new 30 bucker I found on eBay motors, no cruise. rest ECM, no cruise. Well, two out oif three (I swapped out the new multi-function switch) in the meantime.

Are we seeing cracked boards, ECM updates, speed dens?

Bush

I've been down a similar path.

first got the truck it was fairly rough. Truck was already set for

134A. Repalced compressor, accumulator, condensor, orfice tube... all with GM-Delco parts... no leaks, pulled vaccum... runs cold. then intermittant problem with A/C light and a/c system shutting down. I replaced the cycling switch and the A/C control head...both with GM-Delco parts. I've resetted the ECM to many times to count at this point.

Surely the A/C cycling function is not controled by the ECM?? I don't have a shop manual on this truck but I've been thinking about getting one.

My A/c works and it does get cold but if you run the A/C all day , at some point in time the A/C light flashes and the A/C shuts down. I bleive its a pressure problem since the system does not cycle on and off like its supposed to.

Does your A/C cycle on and off ??????

My radio sucks... it works but has bad static (I'll screw with that later)

The system is most likely overcharged. The system cycles when low-side pressures reach the point where the "cycling switch" more properly known as the low-pressure cut-off switch kills the A/C compressor clutch. When the pressures creep back up the system kicks back on. An overcharged system will never cycle because low-side pressures never reach that point, and an undercharged system will cycle constantly as the pressures are always dropping below the kill-switch level.

R134a systems also have high-pressure cut-offs, but your trucks likely won't have them as they were desgined for R-12 (a proper retrofit includes a HP switch, but few actually do it!).

What kind of low-side pressures are we seeing on these trucks @ 2000RPM, engine fully warmed up, A/C cranked on a hot day?

Doc

For the life of me I cannot find the link but this seems to be a common problem with the AC flashing. I know the book says it indicated low pressure/ the compressor cycles for 10 consecutive times for less than 1.5 seconds each set the fault. I have seen but cannot locate the information online about fixing the control head. Basically you cut one wire from the control head and splice it to another (I believe ground). I was navigating this site and its links when I saw it but cannot find it again. good Luck

As for the static in the radio ck the antenna where it enters the amp behind the ashtray. Mine was allowing water to seep down and corroded the end on the antenna at the amp. I managed to salvage it, but eventually replaced the radio with a head unit from Crutchfield. Best thing I ever did.