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Low side high side same pressure?

fastshivy on Tue July 13, 2010 12:10 PM User is offline

Year: 1988
Make: Mercedes
Model: 560SL
Engine Size: 5.6
Refrigerant Type: R-12
Ambient Temp: 88
Pressure Low: 100
Pressure High: 100
Country of Origin: Germany

Ok this is weird! What did I do wrong? Here is the story. I replaced the compressor, expansion valve, and drier on my 88SL. Per this forum I kept it R-12. The system was flushed and I bought a dedicated set of R12 gauges, and a dual stage vacuum pump from ac kits. System held vacuum for 2 days as I wanted to make sure there were no leaks. I hooked up the gauge set, bled the supply line, opened the low side gauge and let about 8oz of refrigerant in to the system (engine off high side closed) I then opened the high side and let it take a few more OZs. I closed the high side valve (the reading at this point was about 40psi) and cranked the engine up the high and low side immediately jumped to 100PSI. I should not have gotten a high side reading with the high side valve on the gauge closed?? Should I ? I shut the engine down and here I am looking for an answer. I just left everything hooked up. I am positive the low side and high side are hooked up correctly i:e there is a big S on the compressor for the low side suction.

HVargas on Tue July 13, 2010 12:53 PM User is offlineView users profile

Are you sure the compressor is engaging? 100psi on both sides sounds like a static system pressure. Did they fluctuate at all of just remain steady?


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piperflyer on Tue July 13, 2010 1:00 PM User is offline

I'm a DIYer, not an expert, but regarding getting a high side reading with the valve closed, that's normal. The valve closes off the high side line from the middle of the manifold (where you connected the vacuum pump or freon) it does not isolate the gauge from the high side hose / system.

fastshivy on Tue July 13, 2010 1:07 PM User is offline

Yes it is engaging and running quietly. I was actually probing fuses when it kicked on so I wasn't at the gauge set to see how fast it spiked. Found a bad ignition switch that feeds the compressor and blower. I also had the low pressure switch by passed. Should I bleed some pressure off the high side gauge and see what happens? Don't want my learning curve to screw anything up!

fastshivy on Tue July 13, 2010 1:26 PM User is offline

Thanks for both answers...i will go back and make sure the compressor is engaging. Wasn't sure how these gauges took their readings either.

fastshivy on Tue July 13, 2010 5:15 PM User is offline

OK, not sure what happened, but i went back a few hours later cranked it up and it started acting normal...finished charging it have 40F vent temperatures low side is 35PSI high side is 250. Maybe i should not have dumped oil in the condenser? Well it is fixed....very glad to know the gauges work and static pressure. Thank you all

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