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Need some help - custom system

steveclv on Fri March 05, 2010 2:50 PM User is offline

Year: 1984
Make: Monaco
Model: Motorhome
Engine Size: 454
Refrigerant Type: R134
Ambient Temp: 55F
Pressure Low: 35
Pressure High: 120

I'm having some problems with a custom built system that we have installed on my 1984 motorhome - when I purchased it, the only part left in it was the evaporator!

I purchased a brand new compressor, new hoses, a high efficiency condenser and pair of fans from a Ford Focus, a new receiver/dryer and a new GM control valve (which has the bulb sensor strapped to the evap outlet(suction) line and a pressure line)

As it's a custom system, I didn't really know how much R134 to put in there, so I started with 60cc of PAG100 oil and a 2 cans of R134. Pulled a vacuum for about 40 minutes before starting.

The low side pressure sits at 35psi and the high side at around 120psi with an ambient temperature of about 55F but the vent temperatures inside are around 60F after 15 minutes of fast idle.
If I slow the blower fan down to slow, the vent temperatures decrease to 55F - still not as cold as the 40F I get from my Jeep.

I've tried to get my head around what's happening and I did test one thing - with the manifold gauges connected and the low side open, if I very slowly opened the high pressure side, it filled the sight glass immediately with liquid R134 - by reading the various web sites I found on the Internet, it appeared that the compressor is compressing high pressure GAS and it should not be liquid until it gets to the bottom of the condenser - could this mean that I have too much R134 in the system? - it had 24oz yesterday and I added another can today so it's now at 36oz - how do you determine the capacity of R134 in a custom made system?

Any help or thoughts would be welcome as I'm fumbling in the dark here

iceman2555 on Fri March 05, 2010 9:24 PM User is offlineView users profile

First, with a ambient temp of 55, you are going to have a great problem making the AC system function. There simply is insufficient ambient heat. The temp needs to be mid 70's. Another issue with evacuating a system with this ambient temp is complete removal of moisture. There is simply not sufficient ambient heat to 'boil' moisture in the system. Moisture beings to 'boil' at 70degrees @ 29.15 in/hg. Most service vacuum pumps are unable to evacuate to this level. Do not simply rely of what your gauges indicate.
Try recovery of the refrigerant and then evacuate the system once more...this time operate the engine and use this heat to aid in the removal of moisture. Once this is completed, charge the system as before, then await a day with a sufficient amount of heat level to 'load' the system.
One method to determine charge rate is to operate the system. Add refrigerant, app 10-12 oz to the system. Operate the system and measure the temperature of the inlet and outlet of the condenser. Watch for a drastic temperature drop. This is the point where the system is charged. Temp drop should be app 25-35 degrees. Probably around 27-29 degrees. Insure that the fans (engine cooling) are operational during this process. Engage and operate at maximum speed.
Opps forgot..max air, high blower, engine 1200-1500 rpm, doors closed. With the size of your cabin, it may take several minutes for the system to stabilize.
Also, 2 oz of lubricant is drastically insufficient. The system should contain about 6-8 oz. What type compressor are you using? Is PAG 100 the correct lube for this unit?
One possible downside of this 'experiment' is that the 2 oz of lube already in the system maybe moisture contaminate already. When you recover the refrigerant, why not flush the not use mineral spirits.....and start all over. I realize this takes time and seems like a wasted effort...but it may pay dividends in the future. Add 4-5 oz to the evap....2-3 to the compressor suction side. Evacuate the system for app 30-45 minutes with the engine operational. Remove all the moisture from the system and then begin the recharge process.
Do you have the original spec's for the vehicle. If so, use them for a guide. With the adding cooling of this new condenser, you maybe able to add 134a to match the OE 12 specs and will not suffer any damage.
Good luck!!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

steveclv on Sat March 06, 2010 12:03 PM User is offline

Thanks SO much for that - extremely useful.

As I'm in Arizona, moisture should not be a problem as we are at less than 15% rH. Also recovering the refrigerant isn't an option as Autozone only loan out a pump and manifold gauges and I hate to dump it to atmosphere, even if it is R134.

Thanks for the idea of testing the inlet/outlet temps of the condenser - I have an IR gun and can do that easily so will wait for a warmer day next week to do some testing

JACK ADAMS on Mon March 08, 2010 10:42 AM User is offline

Just to add to Iceman post, with the rain we been having in Az, you should pull a double vacuum. Pull your first vacuum for a good hour and then shut your gauges for a half hour (with pump off), then restart you pump for another half hour with both sides open. This will help boil off any moisture that was not boiled out the first time.

Hope this helps and good luck.

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