2001 Mack CH613 A/C system

keith

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#1
So I had to replace a hose that rubbed a hole in it. Which lead to replacing both hoses the expansion valve, drier, I flushed out the system then added oil and charged. The system seems to operate ok as far as the pressures on the gauges look to me. Around 30/245 on a 90 degree day. As the high side builds for a while hits about 320 then the radiator cooling fan comes on pressure drops to around 220 and fan goes off. The first question I have is that the compressor cycles at random times and it seems to have nothing to do with the pressures, I was told it was a sensor on the evaporator which turn off the compressor clutch to keep the evaporator from freezing up. I was also told the clutch should not be cycling. Does anyone know if this is how the systems works? Second question the vents never blow out below 58 degrees neither in the afternoon at 90 or in the morning at 80 degrees both times 58 degrees coming out the vents. The system will run for five minutes before the compressor cycles still only 58 degrees. I would expect to see between 45-50 is usually acceptable. The low side lines are sweating alot under the hood and the evap drain is dripping alot of water which make me think things are working some what ok. Any ideas?
 
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#2
You are a little low on 134a add about 4-8 oz high side pressure can go as high as 275 with fans on.
220 is low. 250 to 280 should be fine do not excede 300 too much 134a is also not good. You should feel a big change with a tad more. HTH Greasemonkey :)
 

nickb2

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#3
Days like these are not the best to check A/C pressures, but yeah, 280 and no more 8) Plz use PAG oil when recharging and a dye, helps for future checks.
 

keith

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Ok, I tried to add a little more and I got the following readings 30/250-350. The high side does not hold steady it moves, I am guessing because the expansion valve is opening and closing. Still 58 degrees blowing out of vents. I think this is overcharge so I took out just a little then I get 25-30/250-325 same 58 degrees coming out of vents. Both times with the compressor running and the radiator fan running. Nothing I can do makes a differences. The low side lines are sweating good and so is the expansion valve. I am wondering if the evaporator is not running efficient. But I am getting good flow through the vents. I did use pag oil with dye. Any other ideas?
 

billr

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#5
Are there any "blend doors" in that system, so that you could be getting some air from the heater core as well?
 

keith

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#6
No blend door as far as I can tell. The blend knob (red and blue markings around it) goes to a cable that move a heater control valve in a heater hose line. Also it have a manual valve I closed that shuts off the flow of coolant to the cab.
 
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#7
If the heater hoses are closed off and no coolant is going into the cab then it has to be in the ac system. You say the low side lines are sweating as they should be but are they very cold. As said before if the blend air door does not move in the cab it wont send the air though the ac evap properly which will cause your problem. Also check the condensor for plugging by checking the temp from one end to the other and make sure the fins are clean. If you feel a cold spot on the condensor that usually will indicate a restriction which means you need to change the condensor. Only thing I dont like so far is that you said the high side readings are not steady once the ac compressor is on and at a steady rpm with the fans running the pressure should become steady until the system cycles. HTH Greasemonkey :)
 

nickb2

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#8
I wish I had a diagram for this rig. We/us technicians are very visual.

As greasemonkey said, if moisture on low side to evap, system is doing it's job.

Need to look elsewhere, maybe :idea:

Ok, needed to modify.

I was at a night course and we had discussed about overcharging the ac units. @op/keith Do you know the amount of freon should be in the system? Because I re-read all post on this thread, and I am tending towards a over charge. :idea: $.02
 

keith

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#9
Yes on the side of the radiator it says 3 lb of freon and 9 oz of oil which was what I put in it. I could take a few oz out and see how it performs. There is no blend door as far as I can tell only a coolant valve in line hooked to a cable and knob inside. I pulled a cover back and looked at the evaporator and it looked less the desirable. Kind of crusty and bent fins. But I sure don't want to recommend a new one if that is not the problem. I let the a/c run for a 1/2 hour the other day and the cab cooled down to 55 degrees on a pretty hot day because the recirculation was in action. If I replace the evap I had better have 48 or less coming out of the vents or there is a second problem and I don't know anyway to confirm the evap is solely responsible. For all I know this truck has a poorly designed system and could barely cool the truck when it was brand new. Hard to know where to go from here.

Looking at you post I was before considering how an undercharged system would make the compressor cycle to much. Will and overcharged system make the compressor cycle to much, also will overcharged system freeze up the evaporator? I was told the clutch cycles to prevent the evaporator from freezing up, I do not know if this is true. Also I think the low side reading it too low around 25-30 high side 250-300, if it was overcharged wouldn't the low side be a little higher? Or because this has a expansion valve does that would different than a orifice (GM type) which is what I see most the time?