High low side pressure, normal high side

[ScottNC]

New to me 2003 Tahoe with dual climate controls. I fear the AC compressor may be dying. R12 was $1 a can last time I did any real AC work so, realizing how far out of the loop I am I'm hoping for informed advice.

Long story: AC worked fine during the test drive except at idle, I figured it needed a new fan clutch. Over the next weeks the AC seemed picky about how the controls were applied (auto then switch to manual w/recycle on) before cold duct air appeared, the low side lines were frosty (indicating the AC was working) but the duct air was hot. Thinking it a blend door actuator or control problem the climate control settings were reset to the factory baseline settings after which all appeared normal again. Then Friday it seemed picky to control settings again. Had a new GM fan clutch by then so it went on and the condenser and radiator were verified to be clear of road debris. No change.

Short story: Yesterday, no amount of tinkering with the controls produced cold air. I found the low side, post orifice tube, to be barely cool to the touch - so I put gauges on it. Low side 70, high side 200, ambient mid 90's with extreme humidity. Adding a can of R134a made a big improvement but the AC only got really cold if the engine speed was increased. After the refrigerant addition the low side remained 70 (should be what, 55?) while the high side raised to 250. Also noticed while the hood was open, AC on, that the compressor has a wee bit of rattle. The climate controls seem to have been fine all along, it was probably that screwing with them while driving gave time for the system to cycle enough to make cold air.

Is the high low side pressure combined with compressor noise a sign the compressor is struggling and should be replaced near term?

[tbirdtbird]

" Low side 70, high side 200"
Please give RPM when giving pressures. BTW Idle pressures have little meaning

Low S/B about half of what you are getting now that you added a can (about 35 ish)
If your hi is 250 it would seem the compressor is pumping.

We need to have the others chime in here at this point

WoW a buck a can for R12 I don't recall it ever that low. Time to turn back the clock

[bohica2xo]

2003 model year, how many miles? Front A/C only, or do you have rear A/C?


Those trucks are known for idler / belt / compressor issues. Look under it with the system running and see if the belt / idler is jumping and bouncing around

Do a full load test. Doors open, cabin fan(s) on highest speed. Run engine rpm to 1500 - 2000 rpm and hold it there for at least 90 seconds. Record the high side / Low side / vent temp and ambient while the engine RPM is above 1500. Post the results here.

[ScottNC]

264K. It has dual climate control in front with separate fan and temp controls for the rear. The rear temp control is via a roof mounted blend door NOT a dedicated evaporator and heater core.

@2000 rpm, doors open fan on high; low 37-40, high 147-150, using a Thermopen duct temp 63.8°on both sides, ambient 83°, humidity 80% and cloudy. Driving down the road @2000 rpm both sides temps hit upper 40°'s.

AC was in "auto" for the above where the lowest temp setting available is 60°. Tried using "manual" mode (the snow flake button and recirculating) to see if the temps would go lower, nope - lost AC as soon as the climate control was switched. Maybe there really is a climate control problem? I sure miss click switches and cable door controls.

[bohica2xo]

Looks undercharged. You can't just add refrigerant and hope to hit the correct charge. You should recover it and weigh in the correct charge for further testing.

While it is evacuated remove and inspect the orifice tube. If it is filthy replace it.

You may have some ATC calibration issues, failed steppers etc. keeping the system from delivering cold air to the vents as well. Let's get the refrigeration loop working well first.

I believe you can read the ATC codes with the right scanner. GM Tech here is the pro on that stuff.

.

[ScottNC]

Thanks for the help! Like I mentioned in the first post, my experience is dated... very dated. I overlooked something as simple as raising the rpm, something I used to do back when a screwdriver wedged in the throttle linkage sufficed.

The 12oz. of refrigerant added was a roll of the dice. With 70 psi showing on the low side (erroneous due to low engine speed) I feared other issues but gambled the low high side pressure meant a low charge. Out of curiosity, was the low side reading high at low engine speed due to some amount of high side pressure making its way back to the low side?

The climate control "module" in these trucks is known to have issues, some have recommended picking up a spare if a suitable candidate is spotted in a salvage yard. The unit in the truck seemed to pass being returned to base settings. The blend door actuators seem to work correctly but something is going haywire to both left and right zones in manual mode. I don't understand to need to make controlling the heater/AC so complicated that a self learning computer is necessary.

I have access to a refrigerant recycling machine so I'll make arrangements to have the system discharged. The question of compressor condition is still up in the air... I'll have the fellow removing the refrigerant listen to it. I have an electric AC vacuum pump so can verify if there's a leak before recharging.

Thanks again.

[tbirdtbird]

" I don't understand to need to make controlling the heater/AC so complicated that a self learning computer is necessary."

Neither do I

but the bowden cable days are over, sadly

[DetroitAC]

I used to have exact same Tahoe, every once in a while the rear unit controls would screw up and blow hot air. I never figured it out, I think I would shut the truck off, get out, run around the truck, get back in, start up, back to normal.

it had a separate control the rear passengers could use, could also be controlled by the driver, rear passengers not able to change, could also be set to just follow the front unit. It is a normal rear HVAC, mounted behind hard trim behind the passenger side rear wheel, you'll see the air inlet grille there.

[bohica2xo]

The low side is high when only a small amount of liquid refrigerant gets to the hot evaporator. That 70 psi accurately reflects the refrigerant vapor temperature.