Automotive Air Conditioning Information Forum (Archives)

elwood911 on Mon July 16, 2012 3:20 PM

Year: 1966
Make: Mercedes
Model: 250s
Refrigerant Type: R134a
Ambient Temp: 85
Pressure Low: -20
Pressure High: 100
Country of Origin: Germany

I'm trying to diagnose this and haven't been able to figure it out, so I'm hoping someone here has some advice.

My car is a 66 Mercedes with a part-original and part-aftermarket A/C system that was nonfunctional when I bought it. Unfortunately, I don't know how well the system worked (when and if it did) before I got it. Have to assume it hadn't worked in a while, since it was empty. To start with, I replaced the drier/accumulator, in-line filter, and expansion valve, flushed, added oil and evacuated. The system holds a vacuum. It also holds freon (pressures constant and no evidence of dye leakage), but what it doesn't do is work.

Static pressures are currently low side 107psi/high side 100psi and it's about 85 degrees in the garage. When I turn it on and the compressor starts to cycle, the low side drops into a vacuum, but the high side remains unchanged. No frosting on the lines or the TX valve. When I turn it back off, the low side pressure rises back to the static level at about 1psi every 10 seconds... again with no change on the high pressure side.

If I understand correctly, a blockage would result in a spike in high side pressure readings. All the same, I assumed it must be a blockage, had the freon recovered, blew out the lines and the TX to confirm that they are not blocked and started over... pulled a vacuum, refilled. Same result.

Unfortunately since the system is not factory spec I can't be certain how much freon is correct, but I've got 36oz in there now which would be right if it were all original. The low side vacuum has been consistent throughout filling the system both times (except when a can was connected to it), so I don't believe it can be an overcharge. I would think it's an undercharge, but those static readings seem high enough, don't they?

Any advice?

Thanks.

mk378 on Mon July 16, 2012 5:52 PM

Something is blocked. The high side does not rise out of control because all the refrigerant just pools in the condenser and receiver and stays there. Make sure the TXV flow direction is correct. Installing the valve backwards or connecting the compensator line (if not an internally compensated type) to the high side will make it slam shut under pressure. Is there any sort of suction throttling valve between the evaporator and compressor?

elwood911 on Tue July 17, 2012 12:39 AM

Thanks mk378. The TXV has different size fittings on each end, so it can't be backwards (otherwise I wouldn't put it past me... I've done more boneheaded things before). No compensator line on the valve and no suction throttling between the evaporator and compressor, unless the inline filter installed there handles that function. The service ports are both actually mounted on the compressor itself, rather than in lines, so even if the refrigerant was pooling as you describe due to a blockage, shouldn't there still be a pressure spike on the high side?

I have a fairly nonsensical area of concern that I'd like to get out there in case it might be the issue. Nonsense because the act of pulling a vacuum on the low side seems to imply that the compressor is doing its job correctly. In working on the system I noticed that the connections on the compressor for the high and low side hoses appear to be interchangeable. I know I didn't reverse them, but the guy I bought the car from seemed to think he was quite the shade tree mechanic for a guy who was driving around with air in the clutch and two pistons not firing for lack of a proper tune-up. He told me ethanol in modern gas gums up the fuel line and you occasionally have to push it out with compressed air, when what was actually happening was extreme rust in the fuel tank clogging the fuel filter... that kind of thing. In other words, if A and B can be mixed up, I have to assume there's at least a small chance the guy did so. Please tell me this is as crazy a notion as it seems. On the other hand, if that's the issue at least it would be easy to fix.

Thanks.

bohica2xo on Tue July 17, 2012 1:21 AM

All it takes is a blocked suction.

In another thread I posted a picture of a compressor with the discharge line open to atmosphere - with a 1000 micron vacuum holding on the low side fitting at the compressor with a blocked suction line.

Chances are the "inline filter" in the suction line is completely blocked.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Tue July 17, 2012 9:07 AM

Are there isolation valves on the compressor fittings? Having the low side one closed and the high side open would do what you describe (as the service ports are on the compressor side of the valve).

bohica2xo on Tue July 17, 2012 10:41 AM

Good catch 378

A 1966 MB would have a York compressor. Could be a closed isolation valve.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

elwood911 on Tue July 17, 2012 1:29 PM

mk378, you are my hero. THANK YOU so much for taking the time to mull this over. Sure enough, the isolation valve on the low side was closed (looks like someone went after it with channel locks and a vengeance, too). I opened it and retested and now I'm seeing around 20psi low side and 220psi high side with air blowing cool.

Oddly there is no isolation valve on the high side... which is too bad since I'm hearing a lot of knocking from the compressor at idle now that the system is actually flowing. I'm hoping the noise is going to turn out to be the clutch, but will have to investigate that further later. I was so excited by your feedback I just left work to check and see if that solved it and I don't have any more time to look into it right now.

Seriously, thanks very much to both of you for trying to help. I was at my wits' end and turning a valve solved the problem. Advice doesn't get any better than that!

I suppose this means my evacuation was somewhat compromised since I was only pulling an actual vacuum on the high pressure side, but I let it run for a good long time, so hopefully I still got the moisture out.

bohica2xo on Tue July 17, 2012 3:15 PM

Compressor could be low on oil, or perhaps the oil was never changed when it was converted to 134a.

Since the "mechanic" was so clumsy, you should check the oil level (or replace it!) before you run it too much. That compressor has a sump & is splash lubricated.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

elwood911 on Tue July 17, 2012 10:52 PM

Thanks bohica2xo. I actually drained and changed the oil myself. That's how I learned the high and low pressure fittings look interchangeable. I shouldn't really say "drained" since there was hardly any oil in there, but I'm pretty sure it's to spec now, at least according to the Mercedes service manual which should be accurate for the compressor, if not the rest of the system. I did the r134a conversion myself. Apparently all the car's previous "mechanic" did to the A/C was chew up the isolation valve with the wrong tool when closing it for some inexplicable reason. I should probably count my lucky stars that's all he did.