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Low pressure too low

steve48348 on Mon September 20, 2010 9:43 AM User is offlineView users profile

Year: 1983
Make: Chevy
Model: Van G30 (Motorhome)
Engine Size: 6.2
Refrigerant Type: R134
Ambient Temp: 82
Pressure Low: 20
Pressure High: 165
Country of Origin: United States

I decided to post this again with a new title as the title of the previous continued posting had nothing to do with my current problem anymore.
So here is what I currently have.

The vehicle is a 1983 Chevy Van Motorhome with a 6.2L diesel engine.
It had an aftermarket A/C system that looked in fact very similar to the one AMA offers for this vehicle.
Originally it was equipped with a Sanden SD508 compressor and ran with R12.
I replaced the compressor with a Sanden SD7H15. The original tube and fin condenser I replaced with a larger 15x27 parallel flow condenser.
I also installed a new expansion valve, all new hoses and a new dryer.
Only the evaporator unit is the original one.

The SD7H15 compressor comes with 9oz of oil pre-filled, which seems high for just 24oz of refrigerant.
A poster in my previous thread recommended to go with that oil amount since most of the remaining system was dry. So that's what I did.
The initial fill quantity for the AMA system is 24oz, according to Adam from AMA.
Since I don't have any documentation for my system I attempted to put 24oz of R134 in but ended up with 32oz as my electronic scale screwed me up.
I lost some refrigerant when the high pressure hose came loose on the gauge set in the process of removing the gauges.
I would guess at this point I have about 30oz of refrigerant in the system.

Here are the numbers I took a couple of days after I filled the system.

Vehicle condition:
Doors open, fan on high, temp. knob turned to max A/C, this A/C system is always in recirculation mode - it pulls the air from under the dash.

Ambient temperature 82*
Static pressure with engine cold 95psi/95psi

Started engine and let it idle for a few minutes before taking readings.

Idle readings
Pressure 20psi/165psi
Temp in front of condenser 75*/behind radiator 92* (radiator was still cold as thermostat hadn't opened yet - can't measure behind condenser as it sits right in front of the radiator).
Temp before evaporator 80*/after evaporator 32* (probably surface temp of evap as I pointed the thermometer right at it)

1500 rpm readings
Pressure 8psi/155psi
Temp in front of condenser 75*/behind radiator 107* (but radiator was getting warm so this temp is probably not accurate).
Temp before evaporator 80*/after evaporator 30* (evap surface temp again)

I am satisfied with the way the system works at this point, however I'm a little concerned about the 8psi low pressure reading at 1500rpm.
My A/C repair manual lists specs for a lot of vehicles, and some are as low as 20psi for the low side but nothing below that.
Is there any reason for concern?
Thanks everybody.

dasinc on Mon September 20, 2010 11:19 AM User is offlineView users profile

You should check to see at what temperatures your thermostat is cycling the compressor at. Depending on the manufacturer it could be running an electronic thermostat or a bi-metal thermostat. Close the doors and windows, set the blower to low and put a thermometer in your center vent. If the thermostat is cycling at too low of a temperature it could affect your low side pressure. My rule of thumb has always been that it should cycle out at about 38 degrees and cycle back in about 42 degrees vent temp.

Take care,


steve48348 on Mon September 20, 2010 5:36 PM User is offlineView users profile

It has a bi-metal thermostat, which I had cranked all the way to maximum so the compressor never disengaged or cycled while I was taking my readings.

dasinc on Tue September 21, 2010 1:14 PM User is offlineView users profile

I would be a good idea to check the cycle temperatures. If it gets too cold the evaporator could freeze up. Being as it's a class C motorhome you have alot of air to cool. To check the cycle temps you may have to hang a blanket or something behind the seats so you're not trying to cool such a large area then do the procedure I described above.

Take care,


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