1980 300SD - Another AC question - PeachParts Mercedes-Benz Forum

Ether · #1 05-19-2011, 08:05 PM

Seems to be a lot of climate control issues with these cars on the forum lately. Well, now it's my turn.

I have a few issues to address but the immediate one is as follows. Car has been in dry dock for many months waiting for a new ACC Servo. Got that installed today (thanks Robert!) and after doing so decided to fire up the AC (which worked fine last summer) Well, apparently my system has a leak because a quick check of the gauges showed only minimal pressure in the system.

Expensive option though it is, I decided to add a little R12 to confirm compressor function. Good news is the compressor fired up right away once enough gas was added to satisfy the low pressure switch. Bad news is that I'm only showing pressure on the low side. About 1/2 can I would guess was added, and this resulted in a low side pressure of 90 psi and nuttin on the high side. Shut the system of and pressure will equalize on both sides.

Is it a safe bet that my expansion valve is toast and the pressure equalization is through the compressor?

79Mercy · #2 05-19-2011, 08:23 PM

Yupp, sounds like a bad expansion valve.

1980sd · #3 05-19-2011, 08:24 PM

I'm learning so slap me if I'm off...

Isn't the high pressure coming FROM the compressor to the evaporator and expansion valve? Don't you add freon to the low pressure side?

So wouldn't the freon have to pass through the compressor before being constricted by the expansion valve?

Wouldn't a bad expansion valve be either open/closed which would cause high pressure on the hi side (Closed) or a lack of significant pressure differential between hi and low sides (Open)?

Just respectfully wondering why your data leads to your conclusion?

Hopefully it isn't your expansion valve

vstech · #4 05-19-2011, 08:42 PM

Quote:

Originally Posted by 1980sd

I'm learning so slap me if I'm off...

Isn't the high pressure coming FROM the compressor to the evaporator and expansion valve? Don't you add freon to the low pressure side?

So wouldn't the freon have to pass through the compressor before being constricted by the expansion valve?

Wouldn't a bad expansion valve be either open/closed which would cause high pressure on the hi side (Closed) or a lack of significant pressure differential between hi and low sides (Open)?

Just respectfully wondering why your data leads to your conclusion?

Hopefully it isn't your expansion valve

you are correct.
no pressure on high side while the compressor is running and 90 on low side is an impossible situation.
your hoses are reversed on your manifold...

1980sd · #5 05-19-2011, 08:44 PM

Quote:

Originally Posted by vstech

you are correct.
no pressure on high side while the compressor is running and 90 on low side is an impossible situation.
your hoses are reversed on your manifold...

Who is correct?

79Mercy · #6 05-19-2011, 08:48 PM

Quote:

Originally Posted by vstech

you are correct.
no pressure on high side while the compressor is running and 90 on low side is an impossible situation.
your hoses are reversed on your manifold...

Sounds like his system was working fine before storage. So idk how his hoses would have gotten switched...

vstech · #7 05-19-2011, 08:50 PM

better?
sorry, I thought my answer would show the problem.
1/2 a can in a flat system is not enough gas to show much besides a compressor that runs.
but even so, pressure on low, and nothing on high while the compressor is running is impossible. the hoses on the charging manifold are reversed... you have to be reading the high side.

79Mercy · #8 05-19-2011, 08:54 PM

I always thought that a stuck wide open expansion valve would read very little pressure on the high side, and high on the low side.

A stuck closed valve would read high on the high side, and very low on the low side.

Am i correct?

1980sd · #9 05-19-2011, 08:55 PM

I thought hooking up the gauges was pretty much fool-proof? Just asking because I'm thinking about seeing how much damage I can do with a set of gauges...

Probably a lot

vstech · #10 05-19-2011, 09:00 PM

nope. original R12 fittings are same high and low on the car.
also, he could have the hoses on the wrong ports on his manifold, he could be color blind, he could have other things going on....
anyway, if he has the high pressure hose hooked up to the highest port on the car, and the low pressure gauge hooked up to the lowest port on the car... he's got em on backwards...
the high pressure hose has to be hooked up to the port on the small line, and the low pressure gets hooked up to the large line.

vstech · #11 05-19-2011, 09:08 PM

Quote:

Originally Posted by 79Mercy

I always thought that a stuck wide open expansion valve would read very little pressure on the high side, and high on the low side.

A stuck closed valve would read high on the high side, and very low on the low side.

Am i correct?

a totally open txv with a low charge would show little pressure on either side, but the low side would always be lower. remember, the txv is after the condenser, and the reciever in the circuit. so, to get 0 on the high side, the txv would have to be gathering the freon, sending it forth, and a valve/obstruction would have to prevent the freon from passing the compressor... but allow it to pass the low port.... crazy... and
what you forcast is likely IF someone keeps adding freon trying to get pressures up. but 1/2 can isn't going to show that.

Ether · #12 05-19-2011, 09:26 PM

Quote:

Originally Posted by vstech

you are correct.
no pressure on high side while the compressor is running and 90 on low side is an impossible situation.
your hoses are reversed on your manifold...

Aw crap! Of course you are correct, I have them reversed. What a dummy. There's no color on the fittings so you just have to be careful which hose you're on and I obviously was not!!

Silly mistake on my part. I'll do a proper check tomorrow. Back to my other issue of where did the gas go over the winter????

vstech · #13 05-19-2011, 09:34 PM

likely the system has old hardened o-rings. you may want to order a set from phil, and go through the system while it's nearly flat.

Ether · #14 05-19-2011, 09:42 PM

I replaced the o-rings last spring when I converted back to R12. Possible I got one pinched or a fitting not tight enough I guess. Need a sniffer.

vstech · #15 05-19-2011, 09:50 PM

before you go to the expense of a sniffer, I'd get a cylinder of co2 or N2 and pump the system up to 200 psi and see if you can bubble the compressor body, the manifold, and all the fittings for signs of leakage. if it's not found, a sniffer would find the remaining R12 and show you the leak.