Automotive Air Conditioning Information Forum (Archives)

john510 on Sun May 06, 2012 7:52 PM

Year: 1991
Make: Mack
Model: RD686
Engine Size: 300
Refrigerant Type: R134a
Ambient Temp: 75

I have a 91 RD686 Mack Dump and developing a problem with the Air.. It was changed to the 134a system while back by Mack and worked so so ever since but want to get prepared to maybe make it work little better .. seems to cycle a lot and not as cold. I did a vac of the system and reloaded it and seems better but it almost as if its not at it full potential I've included some pics of the compressor , dryer, switch on the dryer but where is the orifice tube at? is it in this block going into the firewall?

How often should the compressor turn on for then off for?

I do have the gauges , vac and tools to fix it..

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mk378 on Sun May 06, 2012 8:47 PM

This is a TXV system, block type TXV. It should be cycling on evaporator temperature-- if not good and cold it should run constantly.

The condenser appears to be plumbed backwards, it's supposed to go in at the top of the core and out at the bottom. That oil around the top compressor fitting could be a leak.

Edited: Sun May 06, 2012 at 8:51 PM by mk378

john510 on Mon May 07, 2012 12:11 PM

Quote
Originally posted by: mk378
This is a TXV system, block type TXV. It should be cycling on evaporator temperature-- if not good and cold it should run constantly.



The condenser appears to be plumbed backwards, it's supposed to go in at the top of the core and out at the bottom. That oil around the top compressor fitting could be a leak.

I think you may be mistaken on the direction of the flow I added a pic of how it flows it looks like the line goes from the high side to the bottom but that's actually the TOP of the condenser the other line comes form the bottom of the condenser and runs up top of the radiator then back down so it looks like that.

All so if its a TXV system where do they put the temp control coil , i dont seem to have that?

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Edited: Mon May 07, 2012 at 12:32 PM by john510

mk378 on Mon May 07, 2012 3:17 PM

A block type TXV has internal temperature sensing (since the cold low-side line flows through it), it doesn't need a probe like the ones mounted only in the high side line have.

You have the flow arrows right. The gas from the compressor should be going into the top of the condenser coil, and then the condensed liquid comes out the bottom and goes to the dryer. Your picture isn't clear on whether it's really doing that.

bohica2xo on Mon May 07, 2012 4:03 PM

You need to get the gauges on it, and get us some pressures to work with.

It could be cycling on the HPCO, and that is never good.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

john510 on Mon May 07, 2012 5:29 PM

Quote
Originally posted by: bohica2xo
You need to get the gauges on it, and get us some pressures to work with.



It could be cycling on the HPCO, and that is never good.



B.

I put gauges on it when I recharged it..

the low side would go to 25psi and the high side would be if I remember correctly about 110psi then the unit would engage and run to
45psi on the low side and 150 on the high side then disengage.



Edited: Mon May 07, 2012 at 5:30 PM by john510

bohica2xo on Mon May 07, 2012 8:26 PM

So how much refrigerant is in the system now?

Were those pressures at idle?

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

rfrank22 on Mon May 07, 2012 11:17 PM

Replace the Dryer and Thermostatic Expansion Valve. Use new seals. Replace 2 ozs. of PAG oil into dryer. Vacuum down for 30-45 minutes. Leak Test and charge with correct amount of refrigerant. Watch pressuires. Turn fan on inside the truck to manual while charging.. Thank You..Richard rautoac@gmail.com

ice-n-tropics on Tue May 08, 2012 3:26 PM

john510 (funny handle),
Guess you adopted the name of the Sanden SD510 five cylinder A/C compressor on your Allentown built truck.
Looks to be a genuine OEM retrofit w/ label and drier. Hopefully the Bulldog truck tells you the recommended OEM R-134a retrofit quantity.
The compressor appears to be the original equipment, although Sanden did not continue using the SD510 for new R-134a applications because the aluminum front compressor housing cracks above 500 psig discharge pressure.
Maybe your system is overloaded w/ R-12 lube plus R-134a lube and suffers from excessive lube. The SD510 COMPRESSOR HAS OPTIMUM PERFORMANCE ON A SYSTEM WITH UNDER 3 LBS OF REFRIGERANT USING a total of 6 oz of lube.
We're unable to help further w/o pressures vs rpm and ambient.
hotrodac

-------------------------
Isentropic Efficiency=Ratio of Theoretical Compression Energy/Actual Energy.
AMAZON.com: How To Air Condition Your Hot Rod

john510 on Tue May 08, 2012 8:21 PM

Quote
Originally posted by: ice-n-tropics
john510 (funny handle),

Guess you adopted the name of the Sanden SD510 five cylinder A/C compressor on your Allentown built truck.

Looks to be a genuine OEM retrofit w/ label and drier. Hopefully the Bulldog truck tells you the recommended OEM R-134a retrofit quantity.

The compressor appears to be the original equipment, although Sanden did not continue using the SD510 for new R-134a applications because the aluminum front compressor housing cracks above 500 psig discharge pressure.

Maybe your system is overloaded w/ R-12 lube plus R-134a lube and suffers from excessive lube. The SD510 COMPRESSOR HAS OPTIMUM PERFORMANCE ON A SYSTEM WITH UNDER 3 LBS OF REFRIGERANT USING a total of 6 oz of lube.

We're unable to help further w/o pressures vs rpm and ambient.

hotrodac

pressure readings? look up.. I added a total of 3 cans to the system after vacing it out

bohica2xo on Thu May 10, 2012 3:37 AM

Converted system, could be overfilled with a mix of oils. Could be undercharged. Could just be the low ambient temp of 75f

Never saw a cycling switch cut OUT @ 45 psi then back IN at 25. Could be a bad switch.

We can make random guesses for a long time. Or, you can get some actual performance data & post it.

Both doors open. Cabin fan on highest speed. Run engine @1500 rpm. Run at that speed for 5 minutes to stabilize the system. Record the ambient temp, vent temp, high & low side pressures If it is cycling, record cut in & cut out pressures. All while maintaining 1500 engine rpm.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.