I am trying to diagnose an intermittent AC problem in my '03 F250.

The AC works but will eventually stop blowing cool air. I have observed/confirmed:

1) fuses OK.
2) I get 12V to the compressor when the switch is on "AC" and 0V when the switch is "off".
3) pulley spins very easily when I remove the belt. no grinding sounds or binding. no play in pulley (in/out, side-to-side, wiggle). clutch plate can be depressed by pushing on it. did not measure air gap.
4) under normal operation (without the failure), with AC on and compressor running low side readings start at ~40 and drop to ~25, then the compressor shuts off. Then it cycles on and off again every 30-60 sec and air is cold in cabin. refrigerant pipes are cold. I do not own a high-side gauge so no readings there.
5) when the problem occurs, the AC compressor just doesn't engage at a random time, even though it had been runnin a few minutes before. If i tap the clutch plate very gently, the clutch will engage and the compressor runs like normal and the air in the cabin gets cold again.

My question:
do clutches fail? and does this seem to be the symptoms of a failing clutch? it is possible it's just the air gap?

Thanks!

 

Sounds like you are low on refrigerant. There are pressure switches on the low and high sides to cut off the compressor before either goes out of spec. This can protect your compressor from starvation or prevent your high side from blowing up.

Without a high side gauge you can't be sure, but consistent cycling like that implies that one of them is going out of spec, and the system kicks on again once pressures normalize. I'm no A/C expert though, so perhaps someone else can provide more specific guidance.

 

Thanks Tom! I was wondering about that too... is the refrigerant borderline low so that it soometimes runs and sometimes doesn't.

Today its ~80F ambient and the AC has been running normally over 4 shorter trips.

The thing that throws me is that when the problem occurs and the compressor stops turning, the refrigerant gets warm and there is no cycling. the compressor stays off unless i give the clutch a tap.

 

If you tap the clutch face and it kicks on, the air gap is too big. There is a shim or shims behind it. Remove the center bolt, slide the plate off, inside the plate the shim usually sticks, sometimes it'll stick to the compressor shaft instead. Remove a or the shim if only one, reinstall the plate and check that it can free spun so it's not too tight. Locktite the little bolt. It's a pretty common problem.

 

Thanks Tim, I think that is a good and quick thing to try!

 

There's nothing in that report to indicate the charge is anything but normal.

It's an excessive clutch gap issue, perform a shimectomy.

 

I agree, the air gap on the clutch is too wide. No big deal. The magnet gets weaker as everything heats up and then it won't reengage unless it cools down a little. Good diagnostics.

 

Thanks for all the help guys. I will perform the shimectomy and hopefully everything will be OK.

 

Just following-up to close this out. I took the clutch plate off and there was only one shim, when I put the assembly back together the clutch plate had no clearance. So I ended up grinding down the thickness of shim by about 1/2 and reinstalling it.

Since then, the AC has worked flawlessly!

 

Reducing the shim works.

For those without that capability, for $2 you can get a hardware pack from AutoZone that has a selection of shims, a bolt and the large snap ring that secures the pulley onto the compressor body. Santech MT0986.

 

this guys got it 🤟