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90 bmw - low side pressure is reading minus 20

NY450ES on Tue June 28, 2011 7:26 AM User is offline

Year: 90
Make: bmw
Model: 325i
Engine Size: 2.5
Refrigerant Type: r12
Ambient Temp: 75
Pressure Low: -20
Pressure High: 145

my brother's bimmer that we put in a reman compressor 2 yrs ago. we flushed out the condensor and installed a new evap. added the right amount of oil, pulled a vacuum and added r12 by weight. system worked great for a year and then slowly stopped working. the car sat for most of last year. put the gauges on this year with the car off and had a static reading of a little over 100 on high and low side. turned engine on and a/c on max, fan on high. the high side read 145 and the low side went into vacuum to -20. then every so often the low side would spike to 30 then instantly go back to -20 and stay there. the compressor is engaged the hole time. do i have an obstruction or is the compressor shot. i don't remember where my brother bought the compressor but i remember it's not new. any suggestions?

NY450ES on Tue June 28, 2011 7:29 AM User is offline

also while the system was running the low side line was cold but not sweating.

mk378 on Tue June 28, 2011 9:26 AM User is offline

"Slowly stopped working" would suggest a leak-out. But as the pressure sometimes jumps to normal levels it could be the expansion valve sticking closed or other obstruction. If the line leaving the receiver gets cold, the receiver is obstructed. Everything on the high side should be warm all the way up to the expansion valve. You should recover and check the charge weight that was present, if it was proper, replace TXV. Compressor seems OK.

NY450ES on Tue June 28, 2011 4:53 PM User is offline

NY450ES on Tue June 28, 2011 4:55 PM User is offline


bromodragonfly on Tue June 28, 2011 7:19 PM User is offlineView users profile

Surprised the low pressure switch didn't cut power to the clutch. Either it is defective, and if not, could be explained by an obstruction somewhere between it and the point at which you are connecting your low side gauge.

Low side restrictions can be kind of tricky because if the TXV is working properly, the liquid will flash there, and if most of it boils, there will be no obvious temperature difference afterwards indicating a low side restriction. It's hard to judge if something is a bit colder than cold.

Follow MK's advice, start feeling the high side lines to ensure there's no premature flashing (coldness) indicating a high side restriction, and check the refrigerant charge.

If you did not change the liquid line drier/receiver (or suction side filter/accumulator, if applicable) when you changed out the compressor, it would be a good time to do so, if you're going to try to repair the system. If original compressor failure was the reason you switched it with a reman, there could still be a of gunk floating around and logging itself in your lines, even if you did flush the condenser and install a new evaporator.

Hopefully your low pressure switch has been jumped or isn't working properly, and the entire low side is at the same pressure.

There is no knowledge that is not power

NY450ES on Tue June 28, 2011 8:38 PM User is offline

we changed the reciever/dryer and flushed the condensor, installed a new evap but kept old txv. system worked great for a year then went dead at the start up of the following summer. couldn't find anyone that sold a new compressor. i think we have to drop the dash to get to the evap and txv.

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