CAT 938F ELECTRICAL ISSUES

[kenworth]

Hello to all the Cat experts,
Need some help with some electrical issues on this loader.
Machine S.N. 1KM01776
Hours: 12,0000

Here are the problems:
1. With the key in the off position everything is still energized: warning lights in dash flashing, and the heater fan will run.
2. With the machine running the parking brake light is always flashing.
3. If you are operating the machine it will sometimes kick into neutral by itself without moving the F & R selector lever.
To get it out of neutral when it does this you have move the shifter back to neutral and then back to F or R for the transmission to re-engage and the machine to move.

Owner says that the wiring is in good condition, and the battery's are brand new.
Emergency Brake switch on the handle has been replaced.
He did replace a relay in the junction box where the diagnostic port is located on the right side of the back wall of the engine compartment.
Changing this part did not fix the problem of the warning lights flashing with the key in the off position but it did make all the warning lights go off after the machine was running but this only lasted for 4 hours and then the parking brake light started flashing again.
Transmission display in the dash no longer shows the gear position, it now only shows the machine hours.

Everybody is guessing that it is the controller box behind the seat, but I am not sure that it is at this point so I am asking for your help with figuring out the problem or problems with this machine.

I am looking forward to hearing what you guys think.

 

[Nige]

First thought would be main power relay contacts stuck closed, maybe.......?

I'll send you an electrical schemaitc.

To pull Diagnostic Codes you are going to require a Click Box. If you don't have one there is a work-around. See page 3 of this thread, Posts #51 & 52, but it will pay you to read the whole thread to get the idea of how the CB works.

The famous Cat "Click Box"

It is a regular event on HEF for someone to post a question regarding Cat machine electronics, Diagnostic Codes, etc. For a large number of the questions posted the answer(s) revolve around the famous Cat “Click Box”, otherwise known as the 4C-8195 Control Service Tool. This tool usually hooks...

www.heavyequipmentforums.com

Attached the procedure to use the Monitor Panel to pull Diagnostic Codes. Even though it doesn't say so it still needs the Click Box because it makes mention of using the "Service" & "Clear" switches. TBH I would suggest pulling Codes before anything else and see what they tell you.

 

[kenworth]

Hello Nige, thank you for all the information.
Per your advice the machine is going to the Cat dealer today to have the codes pulled.
I will post the information that they find.

 

[John C.]

It's a 27 year old machine and there were a lot of wiring harness issues in those days. Do you see the wiring looms covered in oil in any places? How about cut marks and splicing?

 

[kenworth]

Hi John, thank you for the response. I have not seen the machine recently, but I do know that it is in excellent shape, kind of like the car that the little old lady only drove to church on Sunday.
The owner takes great care of his machines and spares no expense in taking care of them.
I will try and get a look at the wiring harnesses as you mentioned this weekend.

 

[kenworth]

Update, Problem Fixed.
I talked to the owner today and he took the machine to CAT to have it checked for codes and there were none.
The Mech then started troubleshooting by removing and replacing the fuses one at a time to see if the problem went away. What he found was that when the rotating warning light fuse was removed (which the machine is not equipped with) the problem went away, so the fuse was left out now.
Now the machine is operating fine.
So they suspect that there was some kind of back feed through the harness through the rotating warning light circuit due to a wiring harness issue (Rub, Cut, or Splice).
John C. you were correct, thank you!
The owner said that he was going to replace some or all of the harness with a new factory part as a correct fix, I am not sure which parts of the harness that they suspect might have a problem.

 

[Nige]

The owner said that he was going to replace some or all of the harness with a new factory part as a correct fix, I am not sure which parts of the harness that they suspect might have a problem.

It's right there on the schematic I sent to you.
The branch of the harness that feeds a beacon (if equipped) is part of the Upper Cab Harness. The schematic shows it as Harness "N", Part Number 125-2401.Tell him to be sitting down when he asks the price.

The 125-2401 hanress is shown as part of the 104-1742 Cab Wiring Gp for 1KM machines from S/N 1333 to 2508.