Compressor skipping at idle
 

Well I have my ac system working now on the 79 240D. I replaced the expansion valve and filter dryer. Charged with r420a and getting some good cold air at the vents.
The problem is when I come to a stop light and the engine is idling the compressor starts locking up. Like it's under so much strain to push the fron out it starts skipping and making the belt squeel. If I speed up the engine it's fine. Tightened the belt and it's still the same. I watched and it will stall and spin and stall over and over until I accellerate the engine.

I think I may need to get a compressor and redo this thing to get it done right.

Btw what is a proper pressure reading? I have 40 low side, 150 high side.
Does this seem normal if I'm running it at pressure that are supposed to be close to r12?

Refrigerant
 

"Choice" is a Registered Trademark of RMS Georgie ,LLC.
R420a is a 85/15 blend of R125/R134a and a proprietary lubricant.

"They" suggest 90% of R12 values/FILL for R420A.

"Capice"??? (I.E. Too Much Refrigerant is likely your nemesis.)

Also,you're in the "Off the Charts" zone as far as Temps and Pressures.
[A.K.A UNKNOWN]

Suggest you contact the Refrigerant MANUFACTURER.

At what temperature?

Is the clutch shot? Low voltage to the clutch? Or is it the belt slipping?

I am reading that your clutch stays engaged, but the belt slips on it as the compressor locks up at idle... not a good thing.
sounds to me like the compressor is light on oil, and you are flashing liquid to it. these things can handle a 200psi pressure differential no issues, so if it's locking up, something mechanical is hitting it.
dump your charge (legally of course) and weigh it in according to the refrigerant makers recommendations.
either that or you have a damaged compressor, and it's warning you of impending doom...

Ok. Let's see where to start.

Temp outside when checking pressures is around 85F, give or take a few degrees. It's hot out there.

The freon is under charged by about a pound. I know the car holds 2.6 pounds. Still getting good and cold at the vents but needs a little bit more to be totally right. I quit adding because the compressor starts acting crazy as I add more to it. Like it get worse and has a harder time turning the more freon that's in there. Not just while adding but all the time.

I am also aware that with r420 it takes a little less than r12 to have a full system.

When I say low side/high side I mean, on my set of gauges one side is low pressure one side high pressure.
And with those I am reading 40/150 at idle at 85F.

The clutch seems fine and th compressor works quietly and smoothly while driving. It only starts to stutter and lock at idle. I did refill the oil I added 7 oz. Of ester oil which I thought to be compatible. The bottle says it is compatible with r12 oil and residual chlorine, since r420 is made up of mostly 134a I am assuming this is the correct oil for what I'm doing.

I chose this freon and oil specifically for this purpose because it was once running straight r12 with mineral kit and I felt this would be a good middle of the road setup for somethig that isn't completely one way or the other.

Btw I belelieve the percentages are 88% 134a and 12% isobutane.