Automotive Air Conditioning Information Forum (Archives)

Rob990 on Sat July 16, 2011 9:46 PM

Year: 2002
Make: Freightliner
Model: MT45 Tool Truck
Engine Size: 5.9
Refrigerant Type: R134A
Ambient Temp: 90
Pressure Low: 20
Pressure High: 125
Country of Origin: United States

Hope someone can help.
I have a mobile tool truck that has a Thermo King A/C unit. It uses a very common looking automotive compressor. The evaporator/ condenser unit is mounted on the roof. The condenser has two electric cooling fans. I am unable to find any mfg. numbers, so troubleshooting info is hard to come by. The unit was cooling effectively after recharging recently. The other day it began to get warm in the truck towards the end of the day. Upon inspection, some air bubbles were noted in the sight glass. Additional R134A was added until the sight glass was clear. Again, the air was comfortable until the afternoon. As of today, the sight glass is clear but the air is warm. As noted, the pressures are approx 125 H. and 20 L. The compressor is hot enough to burn your hand after running for only a few minutes. This is not new, the compressor has always run this hot even when the air was working. I should note, I just purchased this truck in May. The compressor was replaced by the previous owner last summer. He states that oil was added to the new compressor. I also have had a belt slip issue. Again, this would occur at the end of the day. I was thinking the slip could be caused by high head pressures after running all day, but am beginning to wonder. I have checked that the condenser is clear, and both cooling fans are working. What would cause the compressor to be so hot? Is this my problem, or just a symptom of something else?
Sorry for the long winded post.Thanks for your help.

mk378 on Sat July 16, 2011 10:02 PM

In 90 degree weather I'd expect the high side to be higher. Seems undercharged. That will cause the compressor to run hot because it is the flowing refrigerant that cools it.

Sight glass will go clear when there is no refrigerant left too. Having to recharge every few days obviously there's a leak.

bromodragonfly on Sun July 17, 2011 1:34 AM

I agree with MK on the possibility of a large leak. And yes, pressures seem way too low for a load of that ambient.

If you're low on charge, the suction gas becomes highly superheated in the evaporator. Compressor will be much hotter, system cooling will decrease.

Another possibility may be a bad compressor discharge valve. Head pressure will not rise to what it should be, and hot gas will push backwards into the compressor body, making it extremely hot. But that wouldn't account for the system operating normally again after pumping in more R134.

What are pressures when the system is actually cooling?

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There is no knowledge that is not power

yorled on Sun July 17, 2011 7:17 AM

Motor not having sufficient oil to lubricate moving parts can cause motor to be extremely hot.

Rob990 on Sun July 17, 2011 8:07 PM

Update
Spent some time today repairing some leaks at the back of the compressor. I recovered about 4.25 lbs of134a. Put the system in a vacuum for 30 mins. and added 6.0 lbs back in until the sight glass was clear. High side reading is 175 low side is 22. No cool air. No clutch cycling. As far as the hot compressor, it was just as hot after driving for 30 min. with the A/C off as having the A/C on. I think its the nature of the beast. I can see some small particles of debris moving through the sight glass. Not to much, but there is some. Could I have some debris plugging the orifice? Maybe the dryer?
Thanks for the help.