1999 Chevy Van Express A-C Relay Compressor Wiring Diagram
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Discussion Starter · #1 ·
Hey Guys i have been searching for a while and cant seem to find exactly where my issue is.
2005 Chevy Express 2500 4.8L
AC Compressor will not engage
I have bypassed low pressure switch. Nothing. Relay is good
used TECH II tool to engage relay and compressor turns on (it happens to be shot as well but i don't want to change it till i figure out my wiring issue)
I looked over the wire diagram i found on google but i can't see what wire is the signal wire to test from the hvac control panel, i changed that out just in case it was faulty and that did not correct the issue.
I have traced the harness down as best as i can, and for the life of me i don't see a short. since the compressor is controlled by the pcm i figured my potential short would be in the dash area since i can active the compressor by triggering the relay and that tells me that my pcm is good because i can control it with tech II
if anyone knows these harnesses or has any help as to what could be my issue i would be very grateful for the assistance
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Discussion Starter · #2 ·
nevermind i found it 
it was a bad high pressure switch or at least an activate high pressure switch. bypassing the high pressure switch which is behind the ac compressor allowed it to kick on with the control. will check that before hours of chasing wires in the future. lol
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If you have very low refrigerant, the AC compressor will not go on also.
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Discussion Starter · #4 ·
That's controlled by the low pressure switch which was bypassed.
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First step is to check system pressure. If above a certain pressure the low pressure switch will activate.
Bypassing any switch could damage system components like the compressor. There should also be a "pressure switch" on the back of the AC compressor depending on design.
Get at least one can of R-134a into the system if pressures are below 35lbs.
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Discussion Starter · #6 ·
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You get "burned out" when people post the same problem over and over with the same generic statements. Since I post a lot, I just speed read the question, then give a reply. My failure.
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Hey guys. I'm fighting a similar problem, but with less knowledge.
My system shows a good charge, fuses are good, relay is good, but my compressor won't engage. I probed the plug and found I have no power coming to the compressor. My guess is:
1. Bad dash controls,
2. Broken wire
3. Bad high or low pressure switch.
Any guidance on how to trouble shoot this further before I relinquish the beast to the local shop?
Sorry to pirate your thread.
dave
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What is the system pressure at rest???
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According to my gage I'm just barely in the good.
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Check "ALL" the fuses in the power distribution center and dash panel fuse for "opens".
Please check both sides of the fuse. Have ignition on, motor not running.
Use this chart to give a general idea what is normal operation. DO NOT ASSUME any other opinions or standards to normal AC operation.
R-134a TEMPERATURE PRESSURE CHART
Ambient Temperature °F / °C Ambient temp is the outside atmospheric temperature.
Low-Pressure Gauge *** High-Pressure Gauge
65°F (18°C) 25-35 psi / 172-241 kPa *** 135-155 psi / 931-1069 kPa
70°F (21°C) 35-40 psi / 241-276 kPa *** 145-160 psi / 1000-1103 kPa
75°F (24°C) 35-40 psi / 241-310 kPa *** 150-170 psi / 1034-1172 kPa
80°F (27°C) 40-50 psi / 276-345 kPa *** 175-210 psi / 1207-1448 kPa
85°F (29°C) 45-55 psi / 310-379 kPa *** 225-250 psi / 1551-1724 kPa
90°F (32°C) 45-55 psi / 310-379 kPa *** 250-270 psi / 1724-1862 kPa
95°F (35°C) 50-55 psi / 345-379 kPa *** 275-300 psi / 1896-2068 kPa
100°F (38°C) 50-55 psi / 345-379 kPa *** 315-325 psi / 2172-2241 kPa
105°F (41°C) 50-55 psi / 345-379 kPa *** 330-335 psi / 2275-2310 kPa
110°F (43°C) 50-55 psi / 345-379 kPa *** 340-345 psi / 2344-2379 kPa
INTERPRETING PRESSURE READINGS
Low-Pressure Gauge --- High-Pressure Gauge Action
IN RANGE IN RANGE - A/C is working properly.
LOW LOW ------- Add Refrigerant.
LOW HIGH ------- Need service, possible blockage of the expansion valve or orifice tube.
HIGH LOW -------- Needs service, possibly faulty compressor.
HIGH HIGH ------- System is overcharged. Slowly remove refrigerant. Venting is illegal in USA.
It is illegal to vent 134a refrigerant into the atmosphere.
Next step is to apply battery voltage the AC compressor coil directly. Does the AC compressor engage???
What are the high and low side pressure readings??? Add 1 can of R134a to raise this pressure if goes lower than 25 psi.
If the compressor clutch does not engage, the AC compressor clutch coil has a open and has to be replaced with the "PROPER TOOLS".
There is no need to replace the whole compressor. Cheaper and simpler to replace the clutch coil.
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Thanks Rick. I have checked fuses in the cab and under the hood, but only those designated for AC. Likewise with the relay.
I'll see what i can do with the information above. My primary problem now is that I don't have power in the plug at the compressor. Something is keeping if from energizing. I have no idea if the compressor is good yet. I need to sort out the energy first.
I appreciate your detailed information above.
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Source: https://www.gmtruckclub.com/threads/ac-compressor-not-engaging.1452428/
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