Mechanics

WEIRD A/C PRESSURE READINGS

2000 Isuzu NPR

Air Conditioning problem
2000 Isuzu NPR V8 Two Wheel Drive Automatic 160000 miles

VIN #4KLC4B1R6YJ805937

Isuzu NPR with a mounted box food delivery body. Driver said A/C just stopped working Monday when he got in the truck; I test drove the truck Saturday after replacing the rear brakes and the ac worked fine. I hooked up the gauges and had no low side pressure and 50 psi high side. I checked the low pressure switch on the dryer and it had power so I installed a paper clip so the compressor would engage. I put a can of 134a leak detector in the system and the pressures started going up slowly but then started back down; the low pressure ended up going in a vacuum. I checked for leaks and couldn't find any so I started adding 134a a little at a time to see if the pressures would go back up. After a couple minutes they did and the vent temp went from approx. 100 deg to around 75 deg; I reconnected the low pressure switch plug and the compressor worked on system pressures. I could feel the system cooling but then the compressor popoff valve started venting even though the high pressure was only around 100-120psi. I stopped adding freon and checked the system; the compressor started cycling, the low pressure was in vacuum and the high pressure was between 50-70 psi. I'm guessing expansion valve but not sure, have never had a system go into vacuum before. Need an answer asap, thanks.
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Mtrdude
July 29, 2009.




Hi
Expansion valve faulty. Does not allow freon to return to comp. Low side in vac. Will not let freon to flow so backs up to comp droping pressure at port but big time at comp. Relief valve blows.
Hope this helps
Thanks for donate

Docfixit
Jul 29, 2009.
Thanks for the help & the quick reply, that makes sense. Never replaced an exp. Valve before, can you give me an idea or a diagram/schematic that shows where it is?

Tiny
Mtrdude
Jul 29, 2009.
Don't have info on NPR but can provide some help. Expansion valve will be located on tail pipe of evaporator in evap box. Dis charge system remove lines going into evap. Remove blower motor then the evap box studs through fire wall remove nuts. Once box out should see valve useing two wrenches remove valve.
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Docfixit
Jul 29, 2009.
Got the valve & drier in, valve was definitely clogged. Vacuumed system 30 min, all good no leaks. Started charging system & freon started blowing out pressure valve in back of compressor less than 2 cans in. Compressor pressure valve had adjustable set screw inside it (never seen one like that before) and I slowly had to turn it all the way in before it stopped leaking. Put approx. 36-40 oz. In system and had good pressures (low 25-30 & high 225-250) and vent temp at 52 degrees. Is the pressure valve bad, is that what was causing it to vent?

Tiny
Mtrdude
Aug 10, 2009.
Pressures look good is valve still venting? My view is once they unseat I will replace. Not sure they are designed foe repeated service. Can you ID compressor cause Isuzu uses many different types?

Docfixit
Aug 10, 2009.

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