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Peterbilt A/C

thejudges69 on Fri June 18, 2010 12:18 PM User is offline

Year: 1992
Make: Peterbilt
Model: 379
Engine Size: 450
Refrigerant Type: 134A
Country of Origin: United States

Ok, since i don't know a whole lot about A/C i'm going to ask people on here. My truck was factory with R-12 and was converted in the early 2000's when the r-12 pump finally took a dump. For a while i worked great then it all started, it seems every year now theres problems with it. I converted from the sanden rotary type compressor to the old style tube-o-ring compressor for the simple fact that i wasn't getting any life out of the rotary style. Ok, so a little over a year ago i put a custom sleeper on the truck had to make custom a/c lines to the bunk, which in turn ended up about the same length as the original factory lines. Well, last year the a/c wouldn't work, i had it checked and the pressures were off the chart, in a search to find out what happened, both of my expansion valves were plugged with little black beads, rubber type stuff, it all originated from the drier. I changed the drier, took all the lines apart and blew them clean, and changed the expansion valves. The only thing i couldn't clean was the pump, but it didn't appear that anything had gotten back to the pump it seems the expansion valves stopped it all. But i still cleaned all the lines, both evap cores and the condenser as well. Now this year it was working fine a few weeks ago and now it won't work at all. you turn on the switch and the compressor engages and the fan kicks on (auto fan) and cycles 1 time for about a minute and then it won't cycle anymore. It don't get cold in the cab, my wife said it was cold in the sleeper though, 2 separate systems with separate expansion valves and evap cores, but all ties into 1 in the engine compartment. I haven't had a chance to put gauges on it yet but i'm going to borrow a set today and check it, somethings telling me that these are going to be off the chart again. I have 1 line on the whole system that has not been replaced, which i went ahead and bought and intend on replacing now when i vac it down.

So what i'm trying to figure out, is why would the drier come apart? is it a flow issue or to much pressure? could the compressor be coming apart and puncturing the liner or the drier?

Also, is there a way to properly figure out how much freon you should put in a system, i believe factory it called for 3.5 lbs of r-12 which they say you should put in like 2/3 or that in 134, well we tried that and it just don't do the job in a cab that big, last year we had 3.5 lbs in it and it didn't do to bad, a friend of mine had 4 lbs in his and it worked great. I know you can over charge but how do you know when enough is enough?

Any info will be a help i appreciate the info in advance.

ice-n-tropics on Fri June 18, 2010 12:51 PM User is offline

Tough to haul w/o A/C!
A PB dealer install item was a after market protection device that sensed A/C problems and shut off the A/C to prevent dumping the compressor if there was a flow stoppage or low refrigerant charge. It was a little 4" x 4" electronic box with trouble code lights and often mounted in the engine compartment near the steering column. After it tripped the clutch disengaged. It would automatically reset the next time the ignition was switched on.
Desiccant beads (molecular sieve) in the receiver drier were XH5 with R-12 which had poor compatibility with R-134a. XH7 (automotive) or XH9 (automotive or commercial stationary application) is recommended for R-134a.
The monster R/D from PB stores about 2 lb of refrigerant, therefore the factory R-12 charge is close to the R-134a amount. The add on auxiliary sleeper cab system adds refrigerant quantity to the single evaporator cab spec. More oil is also recommended for sleeper A/C.
Some PBs had the R/D mounted to the truck frame. Wrong thing to do because high "G" forces wound scramble the guts of the R/D. The Truck R/D should mount to a cushioned surface like the radiator mount or the cab firewall to prevent loose descciant.

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

Edited: Fri June 18, 2010 at 6:39 PM by ice-n-tropics

thejudges69 on Fri June 18, 2010 6:33 PM User is offline

i spoke with the dealer just a little bit ago, I was able to put gauges onto the compressor and the high side was around 210, and the low side was below zero (0). According to the dealer, the high side should be going from 250-350 lbs in cycling, and the low side should be staying right around 65 lbs. Were in assumption that there is a blockage in the expansion valve in the cab, the bunk seems to be cooling fine, were thinking that the only line left original on the truck is starting to come apart internally. My truck does not have any type of electronic box on it, it was built in 1991, So that would be before its time. The cab and sleeper both have individual expansion valves and evap cores, it splits off in the engine compartment and goes to both the cab and sleeper. The dealers saying that he would vac it down and check for leaks and then proceed with 4 lbs of freon. Apparently the new truck have 5.5 lbs of freon from the factory but they have a large condensor thats roughly 40 inches by 40 inches where mine is 40 inches by 9 inches. So were gonna vac it down, change the expansion valves, blow the lines out, change the drier and i think the compressor cause it has an oversized O-ring in it due to a casting problem. and then proceed with a recharge.

Chick on Fri June 18, 2010 8:59 PM User is offlineView users profile

When the low side is going "below" zero, you have a clogged system, most likely the desiccant let go, you'll need to change the drier and expansion valves and flush the system. no need to recharge it as the dealer wants, your high side is pumping it out, but the suction side is going into a vacuum, it's clogged...

Email: Chick


Freedoms just another word for nothing left to lose

chris142 on Fri June 18, 2010 11:49 PM User is offline

Your going to have to go with pressures when you charge it. Start with 2x the ambient temp with the engine @ 1200-1400 rpm. Don't forget to turn your hot water off and your engine fooling fan on.

I usually end up somewhere between 2.2 and 2.5x ambient temp. As a guess I would start with 2.5 lbs. The small condensor Peterbilts use 1.88 lbs of R134a w/o a sleeper. Figure at least another .75 lb or so for the sleeper. This should get you into the ball park then you can work with up to 2.2-2.5 x ambient temp pressure.

So if it's say 80F outside start with 160 on your high side, then 175 and 200 max.

I never did understand Peterbilts thinking here. They use a huge 36x36 inch condensor on day cabs and your 9 inch tall one on sleeper trucks.

thejudges69 on Sat June 19, 2010 9:48 AM User is offline

The peterbilt condensor didn't matter whether it was a day cab or if it had a sleeper. In the late 80's and early 90's the air to air for the engine went over top of the radiator and stuck out to far for a condensor so they mounted the condensor under the air to air, in 96, they redesigned it and mae the air to air so it wrapped around the sides of the radiator and the moved it closer to the rad leaving room for a big condensor.

Like i mentioned, when we charge the system to factory specs it don't cool any better the having a fan blowing on you in a hot summer day. with 3.5 lbs it worked ok but i think a little more would have done wonders. Also the high pressure switch in the system is set to bring the fan on around 300 psi so can you still charge it with a max of 200 on the high side? won't the fan not cycle when your going down the road or sitting?

Chick, whats the desciccant? last year the drier let go, clogged both expansion valves, we changed both expansion valves, blew out the lines and recharged and it worked ok with 3.5 lbs of freon. This year were going to change the compressor, expansion valves, drier and this one original line, then all the components will be new as well as all new o-rings again.

mk378 on Sat June 19, 2010 5:40 PM User is offline

The desiccant is the stuff in the drier that will come out if the wrong refrigerant is used. Make sure your new drier is labeled as OK for R-134a.

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