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spankyjr1, Journeyman
Category: Medium and Heavy Trucks
Satisfied Customers: 2602
Experience:  I worked on all types of equipment semis,forklifts,farm,ect.
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2001 Peterbilt Model 357 with a Cat C10 engine. I have

Customer Question

2001 Peterbilt Model 357 with a Cat C10 engine. I have replaced every single item on my AC and it still continues to engage erratic all to the point where I have a bypass relay with on/off toggle switch and I kick it on and off all day while leaving the
fan fully engaged. Well if it worked perfectly that way then I wouldn't be so frustrated with it. After around mid day, the performance of it withers the hotter it gets and I can tell you it's not from freezing up. The interesting thing is faster I drive the
hotter it gets to and the recirc door is locked in place and the heaters valves are turned off. I don't get it...
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  spankyjr1 replied 1 year ago.

Hi, my name is ***** ***** you for your question, I will try and help you out.

Take a pair of needle nose vise grips and crimp off one of the heater hoses just in case one is leaking by internally when engine gets hot after running half day.

If you have a set of gages we can diagnose other issue so you don't need to bypass.

When you say you replaced all ac parts please list parts changed?

I think you are saying ac clutch cycles to much without system bypassed ,how much freon do you have in system?

Sleeper ac or no?

Let me know either way please

Customer: replied 1 year ago.
I have replaced everything as in all sensors, hoses, evap, condenser, blower motor, blower motor resistor, control head, orifice tube, and accumulator. The pressure in the system is exactly 4.3LBS as requested on tag on the firewall. Today I was talking to another mechanic and he said that if I bypass the low pressure switch and have it revved at 1500rpm and it stays on, then the low pressure switch is bad. I did exactly that and the fan kicked on at 300psi and kicked off at 200psi and low side was crazy cold but not freezing. I thought FINALLY we found the problem. Whipped into Pererbilt and bought another brand new low pressure sensor and pulled the bypass out and it is doing the same exact thing!! Idle will throws ice cubes out of vents (not literally) just awesome cold and the at 1500rpm high idle, it cycles every 1-3 seconds and then off for 4-5 and then 1-3 seconds again and pressures are perfect! As soon a so bypass the switch, it works like its suppose to work and freeze me out on highway.
Expert:  spankyjr1 replied 1 year ago.

Thanks for the info, let me see if I can find a wiring diagram, I think this system uses one more sensor that can kick clutch in/out but I want to be sure. I need my coffee first too!!

Talk soon

Expert:  spankyjr1 replied 1 year ago.

I can't find any diagrams for this are you having any engine low power or overheating issues,, maybe the engine ecm might be turning it off to try and cool engine.

Any engine codes?

Customer: replied 1 year ago.
No codes and cooling fan cycles properly and it does it when the fan manual switch is engaged too.
Customer: replied 1 year ago.
Not sure if you can read this schematic or not
Expert:  spankyjr1 replied 1 year ago.

Just got out of shower let me take a look and see If I can blow it up.

Expert:  spankyjr1 replied 1 year ago.

Okay this is showing 3, ac pressure switches, I blew up a copy for you,,,check the cab through wall (pass thru) connector on firewall,

it's possible you are losing a good contact when you rev engine (possible broken mounts allowing wires to be pulled)

I attached the picture of plug I want you to check but I'm not sure it's attached but I will check after I reply,,if it does not go through I will do over.

Expert:  spankyjr1 replied 1 year ago.

Lets try again

Expert:  spankyjr1 replied 1 year ago.

Did you ever check amp draw on ac clutch?

If ac clutch uses a breaker and most Pete's do it may be tripping breaker and not be a ac issue.

Let me know

Customer: replied 1 year ago.
I don't think the clutch uses a circuit breaker. Here is the picture
Customer: replied 1 year ago.
Also you can go to and find zoomable schematics for my truck. It under the same family as 379 model.
Customer: replied 1 year ago.
Try again:
Expert:  spankyjr1 replied 1 year ago.

After thinking about this some more I think you got a defective low pressure switch ,I have seen this happen before with new parts,,a customer or service tech returns part so they don't have to pay or warranty won't pay and the part was actually bad.

Bypassing ac low pressure switch the system works fine so that confirms everything else in the system including wiring and all is good, so you either have low pressure there or the switch is defective.

If you want to test to confirm back probe the sensor wires with meter reading on ohms and see if its dropping out when ac clutch does.

Customer: replied 1 year ago.
I bought a brand new switch and installed it and I'm still having the same issue, so I bought another brand new one in a bag sealed up from the factory. I even went back in the back with the parts guy and picked the part out of the drawer myself...same problem. These switches are the house brand switches and non-returnable if opened and installed. I can dive in and spend 3 times as much for Peterbilt OEM switch if that will fix it. One other thought, could there be trash in the system at the schrader valve on the low pressure side causing it to malfunction?
Expert:  spankyjr1 replied 1 year ago.

Yes there can be a issue with schrader valve, if its not fully unseating when switch is installed,that would explain a sudden drop in pressure and when clutch kicks off allows time for freon to refill and turn switch back on.

If you have a laser temp gun I would check the temp of low side of system including evaporator and look for a hot spot,that would indicate a partial blockage,

I would do that first then evac system ,change schrader valve,remove and check new orifice tube for any contamination or excessive oil trapped in screen,,that may be partially blocking flow.

Customer: replied 1 year ago.
I bought the OEM low pressure switch today and it did the exact same thing. I finally got a schematic from the Peterbilt parts guy so we're going to try and see if we can't trace down some wires and find any problems there.
Expert:  spankyjr1 replied 1 year ago.

Okay sounds good let me know if you need me to look at it

Expert:  spankyjr1 replied 1 year ago.

When you changed pressure switch did you also replace schrader valve?

Customer: replied 1 year ago.
No sir I didn't. The truck runs daily and I didn't have access to machine to vac system so I just swapped sensors to see if quick fix. Almost never is the fix but worth a try.
Expert:  spankyjr1 replied 1 year ago.

I know what you mean about not having truck.Always worth a shot to avoid downing truck!

One place I worked for the drivers where supposed to swap out their inventory (jays potato chips) for when I needed their truck to service or repair it,,,,it was like pulling teeth!

Expert:  spankyjr1 replied 1 year ago.

Would you please rate positively or accept my answer,,, if you don't the website does not share the fee you paid with me for assisting you.

Thank you

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