No pressure reading on my AC system
 

I went to take pressure reading on the low side and high side of my AC system today and the needle on the gauge didn't budge.

A few months ago I took readings on the high and low side and got about 34 on the low side, but the schraeder valve on the high side was leaking so I couldn't get an accurate reading. I capped that high side schraeder valve as tight as I could. I sprayed the cap with soapy water and nothing happened, so even if it was leaking I think it was very slow. I finally had the valve replaced this past week and the mechanic used that special tool which keeps the refrigerant in the system. Unfortunately the tool didn't work as it should and I heard a 1 second burst of refrigerant come out of the system while he did the job.

The AC blows a little bit of cold air, so I know that there is something in the system. I have a hard time believing that I've lost 34 pounds of pressure between a slow leak and the mechanic's mishap. Is there a failing part in the system that can cause you to lose all pressure? Thanks

If I understand what you are saying - you don't trust the gauge?

I considered that the gauge might be shot, but I haven't used it since I took those readings, so I can't see why it would all of a sudden fail without warning.

I went to Kragen to see if they had a way that I could test my gauge, or one that I could borrow and use on my car, but they didn't. The guy suggested that a bad accumulator or evaporator would keep the system from maintaining pressure.

I would start here, with reading VsTech's legendary a/c thread...
http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Connect your gauge to a known-good vehicle and see what it say's.

I thought about that, but Summer might be over by the time I find a vehicle around here with a working R12 system:mad:

Doesn't have to be an R12 system just to check whether the gauge works or not. Earlier this year I had a bad expansion valve and I had no pressure on the suction side of the compressor. I did however have pressure on the suction side when the ac was off as the high side and low side would equalize.

If the compressor is running and cool air is functional even a little, there is freon in the system.

You can use R12 gauge on your home ac... If it is r22... NOT IF IT'S R410! If it's 410, you can ONLY hook to the low side... The high side can damage R12 gauges.

Use compressed air with a rubber tip to check if the gauges work. Don't forget to open the valves or you won't get a reading. It's more likely you did not connect the gauges properly.

The valves on each side of a AC gauge manifold connect the outer hoses ( blue for low , red for high ) to the center yellow hose. The yellow hose is used to vacuum / charge the system.

Do you have a flare fitting for 3/8" tube? ( Standard flare not inverted / brake flare ) You can rig something up to test the gauge with air.

If there is any cooling going on, there is something wrong with your gauge.

It looks like my gauge is shot because I just got some readings using an R134 gauge with an R12 adapter. I think that there is something missing from my R12 gauge fitting since I don't get the hissing that you usually get when connecting a gauge to a schraeder valve.

So the numbers are as follows:

Car not running: low side 120 psi
Hi Side around 120 psi

Car running with AC on: Low side 25psi
Hi side 125psi

It was about 75 degrees outside when I took these readings. According to the gauge I was reading, 25 on the low side is the minimum for a full system of R134. I have a tapped can of R12 with maybe 10oz. in it. Should I empty that can, or maybe look into cleaning the evaporator and condenser fins? Thanks for all of the replies so far.

25psi seems low. are there bubbles in the sight glass?

I haven't checked the sight glass because according to the numerous threads that I've been reading, the sight glass is not a reliable indicator when diagnosing an MB AC system.

I thought that those static numbers seemed pretty high compared to other numbers I've seen people post.

The end of the R12 hose should have a valve depressor nub, have a look on yours.