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discharge pressure doesn't change but intake pressure does

surbans on Tue May 31, 2011 11:39 AM User is offline

Year: 1997
Make: honda
Model: civic
Engine Size: 1.6l
Refrigerant Type: r134a
Ambient Temp: ~80
Pressure Low: 70
Pressure High: 20
Country of Origin: United States

short version: discharge pressure is constant at ~ 20psi no matter engine rpm while the suction pressure drops from 70psi and then the compressor cycles between ~20-40psi

more details: it doesn't make sense what i'm seeing and i'm close to losing it. some background; i'm pretty handy with cars, girlfriends car had a accident last year that pinched and caused a crack in the low pressure/return aluminum tube that i fixed last year (put on a new dryer also), i thought ac wasn't that good but she said it was never "kick ass" so i left it alone. forward to this year; go to use ac a couple weeks back and nothing. no r134a in system. i locate the leak in the condenser and replace the condenser with a used one and refill system with a tad over the spec'd amount of r134a (evacuated system for ~ 1 hour at ~-28-3/4"hg and it held for several days, added ~3oz correct pag oil and 24oz refrigerant). ac works but not very cold (in garage at ~ 72* it was blowing ~56*). here's the strange stuff; after sitting overnight, the low side is ~70psi while the high side is ~20psi (gauges are good, i switched them and still get same readings)... when the compressor engages, the low pressure drops from 70 to between ~20 to 40 where it cycles on & off but the high side doesn't change! how can it be sucking from the low side and not expelling it out the high side? i considered that the high pressure hose (very thick and ~ 16" long) could be clogged/obstructed but it seems unlikely. at the fittings, the return line is cold/kinda frosty and the pressure line is very hot/too hot to hold onto there's a bit of short cycling at times at idle but if i increase the rpms, the low pressure will stay ~20psi and the ac will stay on/run continuously for prolonged periods. last year after fixing the cracked return tube i was getting similar readings also. i just can't figure what's going on; if the compressor is bad could it be sucking but not expelling? i could put a bit more r134a in to get it up to ~100psi on the low side but i still dont have much high pressure and the high pressure side doesn't stays at ~20psi at idle and when revving. clutch engages fine
anyone have any ideas for i'm at a loss and don't want to replace the compressor if it won't solve the problem; i've googled a lot and haven't found anyone else with this issue sanden trs090 compressor?

thanks in advance for any brainstorming; experts please weigh in with insight

Edited: Thu June 02, 2011 at 9:18 AM by surbans

surbans on Thu June 02, 2011 9:22 AM User is offline

still hoping for some help, does anyone have any ideas; i'm tempted to just replace the compressor but i still just don't understand how it can be pulling suction but not discharging. it's weird and doesn't make sense to me

mk378 on Thu June 02, 2011 11:44 AM User is offline

The gauges must be reading wrong. There's no way discharge pressure can ever be lower than suction. It would not get cold at all with pressures like that. Do pressures equalize to the same after compressor has been off for a long time?

Compressor should not cycle off until the evaporator gets cold. The only other way it cycles off is the pressure switch, located in the high side after the condenser, will trip off if 450 psi, neither of which ever happen in normal operation. Or if the engine rpm drops so low it is about to stall the compressor will drop to prevent stalling.

surbans on Mon June 06, 2011 5:16 PM User is offline

i played with it this weekend and 1. the blend door was bleeding through some hot (i adjusted and it's good now) and there is high side pressure! i worked on the blend door first and then since it was blowing cold i still just couldn't understand the readings; they just didn't make sense i was switching the lines from one side of the gauge body to the other and the lines got all twisted up and rotated the guage faces so that i couldn't see them, when i was twisting everything back around i saw the high side "jump up" to ~ 100psi. i started messing with the hoses/connections/everything and determined that the port is bad. it won't read accurate unless i press down the port connector down onto the port. well, it seems to be reading ~150psi while at ~2k+ rpm. success and understanding at last! the pressure numbers didn't make sense because they were not "real".

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