Odd A/C behavior

[86 Wonder Wedge]

Calling A/C techs...

 

I had a theory about the problem, but I'd like to confer before proceeding.

 

A/C system was working last 2 years, but now it only cycles for ~10 seconds, then shuts off until I cycle the A/C (or defrost) off/on. Does this every time, no matter how long I wait between A/C cycles.

 

Belts are tight, no clutch slippage (that I can observe) and the engine DOES idle up when running (pulser is gapped to spec)

 

Hooked up the R-12 gauge and there were the readings:

 

Engine at EOT, system off:

 

Low side: ~50 psi

HIgh side: 55 psi

 

System cycling:

 

Low: drops to ~0 psi quickly

High: raises to ~75-85 and holds

 

After cycle:

 

Low: raises to ~55-60 psi

High: drops to ~60 psi

 

I added less than 5oz of R12, but same result, only the high side raises slightly, but the low raises when the system is not cycling. Before I add (or maybe waste if the receiver/drier needs replaced) any more, I just wanted to confirm that it just might be low on R12 and the trinary switch is deactiviating due to low pressure.

[czny]

Rule of thumb when testing high/low side pressures & charging is to raise engine RPM to 1500-1800.

 

Block the airflow to the condenser & see if the high side pressure rises. Remove block & let system stabilize. Your pressures are too low.

 

Read this, it may help:

http://www.vintageair.com/DownloadsSection/Documents/Troubleshooting%20guide.pdf

[skishop69]

Yep. Way too low. You've lost freon somewhere. I suppose it's possible that somehow your TXV has stuck open and is not allowing the system to build pressure, but I have only seen one do that in 20 years. They almost always fail closed.

[czny]

IIRC, normal R12 pressures (low/high) about 20/150-180 @ 85*F. With a correct charge that is.

Edited May 29, 2014 by czny

[86 Wonder Wedge]

short update, I've pumped more R12 into the system, and the low side stays a bit higher and the high side will climb and if I keep cycling the compressor manually (A/C on and off via the button) the pressures will stay in the appropriate zones.

 

However, upon inspection, I noticed there was damage to the pulser in front of the clutch (some of the pick-up coil is exposed) and I NOW believe that the "Belt saving system" is in effect. The car can't or doesn't see the compressor is spinning at the same RPM and will deactivate the clutch to save the belt (the A/C module is assuming the compressor is locked).

 

I unplugged the pulser to observe how it acted and it cycled for the exact same time as if the pulser is plugged in. I also verified the trinary switch is NOT causing the problem by backprobing the connector and the circuit was still complete when the compressor disengages.

 

So the question is, does anyone have a spare pulser I can buy/try?

[SmashedGlass]

IIRC, you can bypass the pulser system by jumping the wires together to the pulser control unit, located behind the glove box, and system operates normally.

 

Check post #3 here:

 

http://www.ultimatesubaru.org/forum/topic/119625-ac-help/

[l75eya]

Agree with above. You can bypass. Check out that post linked in. Very easy to do.

Doesn't sound like pulse to me. Sounds like low pressure or maybe your ....I can't remember the name of it. Front of car, pass side under the hood. There's a sight glass on the top of it. Clear it off and take a peak, if you see bubbles in it while the a/c is on, you're pressure is low.

[Loyale 2.7 Turbo]

Are you completely Sure that the Three A/C Relays are in good working order?

 

I've seen similar A/C Behaviour due to a failing Relay.

 

Kind Regards.

[czny]

Agree with above. You can bypass. Check out that post linked in. Very easy to do.

 

Doesn't sound like pulse to me. Sounds like low pressure or maybe your ....I can't remember the name of it. Front of car, pass side under the hood. There's a sight glass on the top of it. Clear it off and take a peak, if you see bubbles in it while the a/c is on, you're pressure is low.

Filter/dryer.