Hello: I hooked up the hoses of my a/c recovery/recharge unit to my 1990 300SE and the gauges show zero pressure. So, unless there's something wrong with my a/c service unit, there's no gas in my car's a/c system. Is it common for it all to leak out? I have a bad pressure sensor switch that I'm going to replace along with the drier and the auxiliary fan switch. Would a bad pressure sensor switch normally allow ALL of the gas to leak out of the system?
Thank you.

I had a set of gauges where the "open" and "close" labels on the valves where backwards. Harbor Freight of course.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Well, this is an actual a/c recovery/recharge unit this is supposed to be fully automatic, whatever that means. I opened the valves on the recovery tank and the hoses and there was no change, just zero readings on both gauges.

Franc, sure it's possible to have Zero pressure. All leaks will eventually result in that if given long enough.
However, since you have retrofit connectors, they may just not be interfacing properly with your machine, and depressing the valve core pin properly. A quick check is to just momentarily press one of the valve pins on the car and see if you have any pressure on the system. If you have any freon at all, you should have about 90 psi at an ambient temp of 80 degrees F on a non-running system. That should be eassy to tetect with a quick punch on the valve core.

I take it you found that sneaky little high side port.

DG

Yeah, I found the high side port. It's on the OUTSIDE of the engine compartment just medial of the bottom of the left hand headlamp assembly and lateral of the left hand fan. I didn't think to look there until I got desperate.

I just depressed both of the valve pins as you suggested and there's no pressure there. Not even a hiss. I guess I have to go to the next step now. I haven't taken the parts out that I want to change yet. I'm following the manual. I think I have to pull a vacuum next.

Thank you.

Franco, yeah, I sent you a personal msg about the high-side fitting location.
Hope you got it, didn't want you to think I forgot you.

Seems like the time to do a vacuum check. I have a feeling you'll not hold vacuum well. Next step would be to get a can of 134 with UV dye in it, and shoot it into the system and see what creeps out.
I'm doing just such a test on my other car right now. You can get a little UV flashlight on ebay for under $10, works good.

DG