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cat_engines_only, Other
Category: Medium and Heavy Trucks
Satisfied Customers: 2357
Experience:  Repair CAT machines only
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I have a CAT 938H, working on the AC system, I have voltage

Customer Question

I have a CAT 938H, working on the AC system, I have voltage to thee clutch, but no ground supply.
Submitted: 2 years ago.
Category: Medium and Heavy Trucks
Expert:  cat_engines_only replied 2 years ago.
Hello,Please post the full machine serial number. I need it to research the correct information about your machine.
Customer: replied 2 years ago.
Customer: replied 2 years ago.
So I just started it up, now the compressor is on ALL the time!! WTF?
Customer: replied 2 years ago.
How do I get to thee EVAPORATER Core?
Customer: replied 2 years ago.
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Expert:  cat_engines_only replied 2 years ago.
Hello, Thanks for posting the serial number. I am sorry I could not respond earlier. I work out in the field during the day and answer questions at night.I suspect you are losing ground when the compressor is not working and are getting a short to ground when it runs continuously.The wires to the compressor are as follows, the Yellow wire marked 521 is from you HVAC switch in the cab sending power to the pressure switch. When the freon pressure is correct, the switch is closed and sends power to the clutch on the Orange wire marked 513. The White wire marked 522 is a "floating" ground. (Meaning it does not connect to ground directly). The 522 White wire runs to the thermostat switch in the HVAC box and when the correct temperature is in range, the switch is closed and connects to ground on the Black wire marked 200. I will attach a drawing of the compressor wiring. You will also see an ARC suppressor in the wiring. It is to prevent voltage flyback of the clutch causing arcing in the switches. It could fail and cause a short to ground. If the system works with it unplugged but, not when it is installed, replace the(###) ###-####suppressor. I have also attached a file to show you how to access the evaporator. If you need more help please let me know. I will be out during the day but, should be back earlier tomorrow. Thanks, Donnie