maybe368

Has anything changed since it vented?. The original ac label on my truck says 8 ozs of oil, so you're probably good...Mark

ofelas

Absolutely nothing changed. AC still blows cold, everything works, I can't even see any oil spray behind the compressor relief valve, and AC gauges read the same.

maybe368

I'm out of ideas, I suppose that I would just drive it and see what happens. At least it seems that the over pressure valve is working. Maybe someone smarter than me will chime in...Mark

ofelas

I read a bad fan clutch can build up abnormally high pressures at idle.
What beats me is that 1200rpm stops the compressor from continuously cycling - what does airflow or a bad fan clutch have to do with this rpm ?

See post 47 - https://www.dieseltruckresource.com/...-112028/page4/

maybe368

Maybe a shot in the dark here, but, maybe there is an iffy connection or bad wire in the curcuit that activates the ac clutch and at 1200 rpm's, the engine smooths out enough to stay connected. Try running it up to where it doesn't cycle and jiggle the wires around to see if it cycles off...Mark

j_martin

Something does not compute.

When the compressor is off, the pressure should equalize in the system at the temperature of the coldest part. With an expansion valve system, that should be fairly soon, as soon as the condenser reserve tank is empty.

50 psi R12 corresponds to about 50 degrees.

Either something is preventing the freon from getting into the evap, ie plugged expansion valve, or your gauges are lying to you. If the system works, I'm betting on lying gauges.

What is cycling the compressor? You need to probe the high side pressure switch, the low pressure switch, and the cycling switch to find out what's up. Until you do, it's a guessing game. If you probe with your volt meter connected across the switch, it'll read battery when the switch opens.

j_martin

Another item popped up in my mind. You mentioned you pulled a 26 psi vacuum. That doesn't make sense, as 26 psi is not attainable, and 26" of mercury (what is normally measured) is not enough. To charge an open system you need to pull a high vacuum, which will vary with air pressure, but be within a few thousandths of mercury from a perfect vacuum, hold it for some time to evaporate any water out of the system, then without disconnecting the gauges fill that vacuum with the refrigerant. Any moisture in the system will plug the expansion valve, and any air in the system will cause a serious increase in high side pressure.

ofelas

It was 26 hg vacuum (psi was a mistype). I would use different gauges except that the compressor cycles at 1300rpm and the same gauges move down to 240psi on the high side.
This morning while idling in traffic - there was a loud hog squeal from the engine bay and mist came through the vents and hood gaps. I assume it was the high pressure relief valve on the compressor venting.
AC worked normally when I drove the truck again this afternoon.Compressor cycles on and off normally now and my vent temps are 40F with this afternoon being 85F with 69% humidity. Pressures are good as per the FSM.
Maybe the compressor burped an air pocket on the discharge side?
**** Dodges.

ofelas

Update: spraying the condenser with a garden hose causes the compressor to cycle on and off at idle and pressures are 240psi max on the high side.
Air in system? Bad fan clutch?

j_martin

You need to answer the question in the last paragraph of post #36

ofelas

There is no high pressure switch.
The low pressure switch & the probe/micro-relay test out OK as per FSM diagnosis, and I also switched them both out before & after.
Both the lp switch & probe/blackbox/microrelay were pulled from a working system, and I have NOS pieces as well that I used in this install.

j_martin

Let me see. It's 85° and the air out is 40°. Then when you cool the condenser with water, the system cycles.

I'd be willing to bet the probe/relay is cycling, as it should, to prevent evaporator freeze up. If you won't actually probe it during the questioned action, all I can do is guess.

ofelas

Ok. I didn't really understand that. Problem: at idle, the compressor is always ON it doesn't cycle - high side pressure is 450psi during this time.
Spraying the condenser with water makes the compressor cycle normally, and high side pressure drops to about 240psi. Either way, my AC is ice cold.
The fins in my condenser are 95% straight, but the fan doesn't seem to hold a piece of paper against the front of the condenser.
Paper is held against the rad front though.
When cold or hot, the fan barely pushes any breeze from what I can tell. I have never heard the clutch engage. Truck never overheats though.

j_martin

The condenser is not getting enough cooling air. Classic symptoms. The question is "why".

At idle the compressor isn't evacuating the evaporator as fast as it does at speed. The evap then does not run as cold, and it should cycle less, or not at all. The cycling is caused by the controls that keep the evaporator from getting too cold.

ofelas

Fan clutch ?