AC questions.. continued (Page 1/1)
cam-a-lot OCT 05, 06:04 AM
Hello folks

I am still having some AC issues with my 3800 Fiero (3800 compressor)

I have vacuumed the system, replaced orifice tube and accumulator/dryer. The system held vacuum. Replaced with 1 kg of R134a and 3 oz of PAG oil

AC works great... until it doesn't

For no specific reason, it just stops working- compressor does not kick in. It is not "cycling" when this happens. Just no compressor . I have changed the pressure switch in the high side, did not help

Refrigerant level I believe is fine, since when the AC does work it works well. I could park the car, come back an hour or two later, and I have good strong AC again. Vehicle speed or engine speed do not seem to make a difference

I was told to try to jump the AC relay to see if it is defective

Was also told to try to run power directly to the AC compressor when car is running , AC is turned on, but is not working, to see if the compressor will kick in.

Any suggestions ?

thanks

[This message has been edited by cam-a-lot (edited 10-05-2025).]

olejoedad OCT 05, 07:29 AM
Do you know how the AC control system is wired?
Is it under PCM control ?
cam-a-lot OCT 05, 07:55 AM
Yes, it is wired into my 3800 PCM. So when it works, it does work properly . I can have 2 minutes to 30 minutes of continuous cooling. I guess it is possible that the wiring or connection may be defective to the PCM. I did have properly functioning AC in the past which worked for months until minor leak lowered the refrigerant level. But this issue is different. I have enough refrigerant for sure. AC just intermittently stops working. Next day, it works again
olejoedad OCT 05, 08:01 AM
Others here know more about the AC than I do, perhaps they will chime in.

It is important to know how the system is wired, I assume the PCM is monitoring pressure via the three wire plug on the manifold?
Brian A OCT 05, 10:09 AM
As you’ve already noted, it looks like it needs just plain old electrical troubleshooting. Yeah, try unpluging/jumpering switches to see if one of them is not working. Watch the high-side pressure if you directly jumper the compressor / clutch so that you don’t blow something up should the system be plugged (unlikely).
Dennis LaGrua OCT 05, 11:13 AM
The 3800SC A/C operation is under control of the PCM and is dependent on a high side line pressure sensor signal to operate. If you did a R-134a conversion I would have used Ester oil as it will mix with any residual A/C oil that might have remained in the system. The guessing game is entertaining but you really need test data to pinpoint the issue.
I would recommend investing in a R-134 pressure gauge set to read how everything is operating,. If the high side pressure is escalating over the required range it will shut down the system. Overcharging could cause this but you wont know unless you take readings. Do you have any idea what they are?

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 10-06-2025).]

cam-a-lot OCT 06, 11:06 PM
thanks guys\

I do have pressure gages. When compressor is off, the Low side is very high , over 100 PSI. High side is around 110-120 PSI

Will take the gauges with me and check pressure when AC is working and when AC is not working.

I agree, likely something electrical, need to play with it some more..
Dennis LaGrua OCT 06, 11:51 PM
First measure the static reading on the high side to determine level of fill'
Static Pressure Test (engine off) to Outside Temperature
R-134a A/C system
Ambient Temp Full System Pressure Readings (approx)
70*F 71 psi
75* F 78 psi
80*F 86 psi
85*F 95 psi
90*F 104 psi
95*F 114 psi
------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 10-07-2025).]

cam-a-lot OCT 08, 09:25 PM
Thanks Dennis

Will do

what will this tell me? If I am too high or too low, is this just sign of incorrect amount of refrigerant?
Dennis LaGrua OCT 12, 10:03 PM

quote
Originally posted by cam-a-lot:

Thanks Dennis

Will do

what will this tell me? If I am too high or too low, is this just sign of incorrect amount of refrigerant?



Those are the FULL A/C system static pressure readings with the engine off. Lower readings will indicate low refrigerent.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "