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Curtis B.
Curtis B., Technician
Category: Construction and Road Equipment
Satisfied Customers: 1019
Experience:  Technican turned service manager with multiple lines of equipment and rental units.
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2002 Bobcat 753, fuse F4 (computer) blowing. Blows without

Customer Question

2002 Bobcat 753, fuse F4 (computer) blowing. Blows without turning key.
Submitted: 1 year ago.
Category: Construction and Road Equipment
Expert:  Fieldtech59 replied 1 year ago.

Hello, is this a G Series machine with triangle shaped headlights?

Is it a standard control machine, sticks control drive and foot pedal controls loader functions? Or is it a machine with ACS or SJC option, sticks control both drive and loader functions and you can select pedals to control loader functions as well?

Customer: replied 1 year ago.
G, standard controls.Serial ####-##-####
Expert:  Fieldtech59 replied 1 year ago.

I'm working on some pics to post, give me a few minutes to see if I can accomplish this.

Customer: replied 1 year ago.
Great.
Expert:  Fieldtech59 replied 1 year ago.

F4 circuit is an unswitched circuit, comes straight from battery, hence the reason if there was a short to ground in that circuit it would blow F4 fuse even with key off. You're going to need a multitester that will test both voltage and ohms (or continuity). Pull F4 fuse, test the two terminals in the fuse box receptacle with your volt meter to determine which terminal is the battery terminal. Once you determine that, the other receptacle will be the circuit that goes to the main controller (computer). Test that receptacle with ohm meter (or continuity tester) with one lead connected to receptacle and the other lead connected to ground. If it's blowing F4 you should see continuity to ground on your tester.

Once you've verified that, disconnect J1 connector from the main controller. Check receptacle at F4 again, see if you still have continuity to ground. If you do, you have a short circuit to ground in the harness that runs from fuse box to J1 at main controller. If you don't have continuity to ground, you need to test the connections at main controller.

Once power leaves F4 the circuit splits off into 3 wires, wire 1110, wire 1120, and wire 1130 and they go to pins 13, 28, and 33 at J1 connector to main controller. With J1 disconnected, test pins 13, 28, and 33 at J1 of main controller to see if one of those three show continuity to ground.

Customer: replied 1 year ago.
Will go to the jobsite in a few hours and test it out. So, a constantly blowing F4 fuse doesn't necessarily mean a fried controller?
Expert:  Fieldtech59 replied 1 year ago.

I'm hoping it's just a spot in the harness that has rubbed and now making a short circuit. Bobcat controllers are fairly durable, they don't go bad that often, they are very self protective in not allowing things to fry them, but that's always a possibility.

To access the main controller, if you're sitting in the seat, just to the left of your left foot will be two cover panels, you have to remove those two panels to access the main controller.

Customer: replied 1 year ago.
According to the service manual, page 60-70-4, controller is located in the main body, left side to the seat and not in the footwell.
Expert:  Fieldtech59 replied 1 year ago.

Ok, it may be my manual is a little different by the serial numbers. If you have the service manual that's specific to your machine it should have the diagrams in it so you can check that the info I posted matches what's on your machine. Hopefully you'll find what's causing your problem. Let me know what you find.

Customer: replied 1 year ago.
Will do
Expert:  Fieldtech59 replied 1 year ago.

Ah, I just viewed 60-70-4, sure enough, seems it's mounted to the left frame side just below the seat, and it sure looks like there's enough crap in that area to rub a spot in the harness. One thing that's typically required to work on Bobcat machines is a choice list of swear words.

Customer: replied 1 year ago.
Tell me about it... I will report back, once I arrive
Expert:  Fieldtech59 replied 1 year ago.

Yes, please do.

Customer: replied 1 year ago.
13, 28 and 33 all have continuity
Expert:  Fieldtech59 replied 1 year ago.

All three have continuity to ground (earth) when J1 is unplugged?

Customer: replied 1 year ago.
they do
Expert:  Fieldtech59 replied 1 year ago.

That don't sound good. I can't confirm with 100% certainty because I've never seen this before, but logically pins 13 28 and 33 on the main controller should not show continuity to ground since they're dedicated battery supply circuits. A Bobcat service manager should be able to confirm this, but it sounds like you have a problem with the main controller.

What about your harness testing, did the circuit test good, no short circuit to ground with J1 disconnected?

Customer: replied 1 year ago.
No short to J1 when disconnected. Where do 13,28 and 33 go to?
Expert:  Fieldtech59 replied 1 year ago.

All those pin connections go internal, inside the main controller. I've never been inside one of them, don't know if a person can even get into them. I think the next step for you is to contact Bobcat, I wish we could have fixed your machine, but it looks like it's getting beyond what I can help with. Don't bother giving me a rating and I won't get paid anything for your question as I feel I haven't been able to help fix your machine, maybe narrowed it down, but came up short with fixing.

Expert:  Fieldtech59 replied 1 year ago.

The thing about the Bobcat controller is, to my knowledge, Bobcat doesn't provide diagrams of the internal circuits of the controller, they keep that a secret, most likely to keep someone building one aftermarket. Again, I've not seen one do what yours is doing so I don't know what the cause and effect would be, but I'm curious if the problem extends beyond the controller and is instead a short circuit in the protected power or computer circuits that go to the cab controls. I don't know what will happen, again, I don't know the internal circuits of the controller, but I wonder if we disconnect J2 connector from controller, re-connect J1, then go back and test the terminal at the fuse box for F4 that goes to the controller, see if we still get continuity to ground on that circuit.

Customer: replied 1 year ago.
That's an interesting tryout. Will do tomorrow morning and let you know what the result is.
Expert:  Fieldtech59 replied 1 year ago.

Well, again, you're going into something I haven't seen before with these machines, but I do hope it tells us something. As for the main controller, since we don't know the internal circuits, we don't know how the main controller connects to ground (earth). Does it only connect to ground via pin connections? Or does it ground internally via it's own frame? According to the diagrams there are two pin connection grounds, pin 38 on J1 and pin 38 on J2. By disconnecting J2, we take away one of those grounds. Nevertheless, since we don't know the internals of the main controller, it's still a test worth doing and see what result we get.

Customer: replied 1 year ago.
I removed the controller and checked the unswitched battery pins for continuity. No continuity with J1 and J2 removed
Customer: replied 1 year ago.
However, continuity between any unswitched battery pins and ground pins on controller (pin 23 and 38).

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