matt13

If not, how would I totally discharge it, then force the compressor on while I recharge it?

Right now my A/C compressor is locking up (i.e. not spinning and creating a huge drag on the belt) when it is turned on. Any idea what could cause this? The R134a pressure might be on the high side, would that create cause it?

TIA

Tom Serrago

Without knowing the charge/operating pressures, it's all guess work.
Even under slight overcharge conditions, a compressor/system in otherwise good condition would not act the way you describe. Without any other information, I would suspect your system is very low on compressor oil, and/or the compressor is going bad.
Maybe a VERY high overcharge might cause the compressor to bind, but I would expect the HP switch to kick out the clutch before that happens.
You mentioned you think the system may be overcharged, did somebody add R134A to your system without gauging?

matt13

Last week, the AC started taking 10+ mins to cool down, and it had been making a whining noise that varied in pitch with the engine RPM in week prior. It was low on refrigerant and I'm guessing low on oil. I recharged the AC (with a gauge).

However, I did something stupid. I inverted the can when I charged it, not knowing any better. So I "slugged" the compressor by making it attempt to compress liquid R134a. After I recharged it with this incorrect method, the compressor started to bind. Later, when I added the oil, I inverted the can as well, which probably didn't help things and certainly didn't fix the problem.

So basically, I'm guessing I need a new AC compressor at this point. Thanks for the reply, though and if you have any suggestions I'd be glad to hear them.

Tom Serrago

by inverting a can of 134A. I do it and never heard the compressor slow down or even moan. Where you are adding refrigerant on these cars (after the evap and before the dryer) you're looking at about 25 psi, and there is plenty of room for expansion before the suction port at the compressor. A weak compressor may bind, but I don't feel you caused any more damage. Also remember that at start up and depending upon outside temps, the suction port can easily "see" 100 psi and better. A new can probably delivers that much pressure at the service port, but the gas will expand and pressure will be less at the suction port.
Best bet is to check system base pressure in the morning before starting the car. Note outside temps., a properly charged A4 cc system will show little better than 100 psi @ about 90 deg F.
Let me know

matt13

Here's something else that I remembered last night: I recently did my timing and accessory belts. For the AC compressor belt, I didn't find any info on how much tension to put on the belt, so I made it as tight as I physically could. I'm not sure if this could be a contributing factor or not.

In any event, I really appreciate the help.

Tom Serrago

Bentley says with a cold engine @ 77 deg F, base charge should be 81.2 psi.

There is a tensioner for the compressor belt (at least on my 99.5 A4 1.8TQ) If yours is straight to the crank pulley, it may be to tight, there should be some deflection in the belt mid way between the pulleys.

matt13

It kicks out when I rev and hold the engine beyond idle. It also feels like it's putting quite a load on the engine when spinning, but at least it's half working now. So, basically if you know of a way to partially discharge the system since it's overcharged I'm all ears, but this is beyond my capability to fix so it'll be going into a shop friday if it isn't working by then. (I spent 1.5 hours in stop and go traffic today in the heat, so I've got to get it fixed...)

Once again, you've been a great help, but I know when the car has me beat ;-)

Tom Serrago

You can partially discharge the AC by one of the service ports. Just depress the center of the port (similar to a schraeder valve on a tire), you may want to put a rag over it so R134A and oil don't scatter all over. The AC manifold gauges are handy for this. I would suspect the HP cutoff switch is what is shutting you down when revving; just to much pressure. Just let out a bit at a time ( a few seconds at a time). after each time, start up and see if AC stays on. If you get it to where it stays running and cooling, get a base pressure reading first thing the next morning, just to see how close you are to a correct charge.

I know what you mean about letting a shop do things you can't. I still have to do that too, but I will do all I can first. To many years of owning Audi's and experiencing "the experts" do a worse job than me. There is nothing I hate more than that plus having to pay for it.

Let me know.

Tom