Peterbilt AC potential evap problem

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Cole2938
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Peterbilt AC potential evap problem

Post by Cole2938 »

Hello all. Id like to start by saying I wouldn't say im a pro at this but I have had quite a few semis over the last few years, between drivers adding cans of freon blowing compressors and general wear and tear, I work on these truck AC systems a lot...

Current issue is with a 93 peterbilt. AC hasn't worked well this whole season of AC use this year. It would get cool enough at night to bear it but Texas heat during the day it couldn't handle. No leaks, low side pressure is good. So I thought it must be a bad expansion valve. Replaced the expansion valve and the evaporator ( had to be cut to get the expansion valve out which really sucked because the valve was $24 and the evap was $350 but what do you do)
All back together, vacuumed, and refilled. AC worked amazing... for one day.

If I go start the truck up from sitting, evap will get down to about 50-60 degrees for maybe 5 minutes and then its back to 80-90, compressor running the whole time. (using inferred thermometer for readings)
I talked to a few people who thought just maybe the expansion valve I put on there was bad, put another one on, big suprise no difference.

IMPORTANT: since it quit blowing cold, every time I turn it on with either one of the expansion valves you hear a very audible hissing coming from the evap. The system isn't leaking. im assuming its the sound of gas going through it but its much much louder than the occasional time you hear it on a normally operating system AND its constant.

There isn't a piece on this entire system that isn't new as of February. compressor, switches, drier, lines and condenser have been flushed out, etc. I have 40 psi on the low side right now when it's 90 degrees out but the kicker with it all is I do not have a high side reading as there isn't a port on the line. for whatever reason the shop that put the new engine in it put a line without a port on it so is what it is for now.

Is it possible there to much "gas" in the system vs liquid refrigerant? It's a sanden compressor, maybe its bad yet again?
thanks for any help.
dasinc
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Re: Peterbilt AC potential evap problem

Post by dasinc »

I'm not super familiar with the truck but I if it's an expansion valve system I assume there would be a thermostat to read evaporator temperature. When replacing the evap did the temp probe get installed properly. If not the evaporator could be freezing up. Reading high side pressure is important. If it doesn't have an access port one should be installed. It would tell you a lot. Without getting proper pressure readings it's hard to say whats going on and anything would be a guess.
Cole2938
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Re: Peterbilt AC potential evap problem

Post by Cole2938 »

It does have a thermostat on the side of the evaporator but it's in fine working order because the compressor is coming on. When the compressor is running im getting 80-90 degree readings straight off of the coils of the evap.
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JohnHere
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Re: Peterbilt AC potential evap problem

Post by JohnHere »

Cole2938 wrote: Wed May 15, 2024 11:25 am Is it possible there to much "gas" in the system vs liquid refrigerant? It's a sanden compressor, maybe its bad yet again?
thanks for any help.
Was this system converted from R-12 to R-134a?

I doubt that it's the compressor failing. When you hear that hissing sound, it usually means that the system is undercharged. I don't have any specs for a Peterbilt. How did you know how much to charge it?
Cole2938 wrote: Wed May 15, 2024 2:49 pm It does have a thermostat on the side of the evaporator but it's in fine working order because the compressor is coming on. When the compressor is running im getting 80-90 degree readings straight off of the coils of the evap.
The thermostat or "frost switch" in the evaporator causes power to be cut to the compressor's clutch when the evaporator temperature reaches a certain low point so that it doesn't get too cold, freeze up, and block airflow.

So basically, at an evaporator temperature of 80-90°F, it's not cooling at all. Sounds to me like you have only refrigerant vapor entering the evaporator instead of liquid.
Cole2938 wrote: Wed May 15, 2024 11:25 am If I go start the truck up from sitting, evap will get down to about 50-60 degrees for maybe 5 minutes and then its back to 80-90, compressor running the whole time. (using inferred thermometer for readings)
I talked to a few people who thought just maybe the expansion valve I put on there was bad, put another one on, big suprise no difference.
You've ruled out the TXV.

I wouldn't use an IR thermometer, though—especially on shiny surfaces and to measure air temperatures. They can be erratic depending on what you're trying to measure. Instead, I suggest a wired thermocouple-type thermometer for measuring hard surfaces, tubing, and hoses; and a stem-type digital thermometer for measuring air temperatures at the center vents.

What did you mean that the low-side pressures were good?

And I concur with the comment above that both low-side and high-side pressures are needed to make an accurate diagnosis. You'll need to install a high-side service port somewhere.
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Mindlessmechanic
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Re: Peterbilt AC potential evap problem

Post by Mindlessmechanic »

You've probably already tried this, but didn't post it; since you've been around trucks for a long time.

Heater core not shutting off?? Try pinching a heater core line......see if that helps. Not sure if your Pete has a hot water/ heater shut off or if it's through the blend door. Kinda like when you have it on defrost. Defrost turns on AC to dry the air, heater core heats the air to melt the ice.

You really need to get a high side port. If your high side is only going to 150psi or less.......then it will never get cool.
Cole2938
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Re: Peterbilt AC potential evap problem

Post by Cole2938 »

Good morning all, Been busy so I hadn't messed with it but I got the truck fixed yesterday. Ultimately I put a new compressor on it along with a drier and it is working as it should again. I did not add a high side port as I figured id give it a shot before hand and it took the charge fine so I'll mess with that the next time I suppose.

The engine in this truck came out of another truck I had and that truck was an absolute nightmare for the climate control system. It had many hands touch it over its 2 million mile life and nothing worked as it should have. Right before I ended up pulling the engine on the old truck I tried to put a new compressor on it (it went through 6 or 7 in the 2 years I had it in that truck, mostly due to drivers thinking they could just add freon every time it didnt freeze them out and they'd overcharge it and blow the compressor) but anyways When I attempted to charge it, it would not take a charge at all. As I mentioned that truck didn't have anything working properly on the ac system and the high/low switches were bypassed in some sort or fashion so when you turned ac on from inside the cab, the compressor was gonna spin regardless. So it didn't work and I took it off and put that "new" compressor on the shelf which I ended up using when I swapped the engines.

All that to say I believe the initial "dry run" or whatever, must have shortened the life pretty drastically and that's what happened. I intend to tear old compressor apart and see if I can figure out what went wrong in an attempt to learn more about this so I'll post pictures if I can.

Thanks for the help!
Cole2938
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Re: Peterbilt AC potential evap problem

Post by Cole2938 »

Also just to clarify for info purposes, this truck has always been r134-a in the 6 years ive owned it. And in that 6 years at one time or another every single line, evap, condenser, etc. has been replaced at some point.

And yes heater core valves were shut off.
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JohnHere
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Re: Peterbilt AC potential evap problem

Post by JohnHere »

Cole2938 wrote: Wed May 29, 2024 6:46 am As I mentioned that truck didn't have anything working properly on the ac system and the high/low switches were bypassed in some sort or fashion so when you turned ac on from inside the cab, the compressor was gonna spin regardless. So it didn't work and I took it off and put that "new" compressor on the shelf which I ended up using when I swapped the engines.
All that to say I believe the initial "dry run" or whatever, must have shortened the life pretty drastically and that's what happened. I intend to tear old compressor apart and see if I can figure out what went wrong in an attempt to learn more about this so I'll post pictures if I can.
With people dumping refrigerant into the system willy-nilly, probably without replenishing the oil, plus all of the system's fail-safe mechanisms either bypassed or jumpered, it's no surprise that the compressors didn't last very long.

It will be interesting to disassemble the last failed compressor to see what went wrong. I could have been either overcharging, leading to ultra-high pressures that destroyed the inner workings of the compressors, or a lack of lubrication.
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