30a PWR fuse blowing- trying to narrow it down (1 Viewer)

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Fairbanks AK
Hey all - longtime lurker here - looking for some guidance. 97 FZJ80 with 312k - roughed up Interior Alaska rig, but reasonably well maintained (mechanically, maybe not the electronics or the cosmetic stuff)

The 30amp PWR fuse blew a month ago, was travelling for work so couldn't deal with it right away - finally took it to a shop to see if they could diagnose. They found corrosion in the plug at the drivers side master switch. Cleaned it and put contact enhancer on and $400 later it works. Drive it home and that night if blows the same fuse again. Arg. Calling the shop they said the master switch itself could have corroded, they didn't pop the plastic cover to get a look at the circuit board inside. They also said it could have just "gone bad". So I pull the master switch out and open it and see a bit more corrosion inside, spray with electronics cleaner and scrub the corrosion away from the long thin metal pieces that go into the circuit board. Still blows the fuse. So I must have a short somewhere or should I spend a few bucks at switchdoctor to get a new switch? Do these come with the circuit board? Maybe cleaning the switch contacts?

I also pulled the kickpanel off and looked at the 3 plugs and wiring down there - 2 had corrosion but not part of this circuit (the bg fat one was clean but this I blieve is the one that handles the "always on" circuit that is shorted- I sprayed and cleaned all 3 and put back - still blowing the fuse. Some other threads this was the location of the culprit (water leaking in the windshield coming down the A-pillar, or wires rubbing and shorting) no such luck here.

I pull the door card off and peel some of the black mastic and plastic back to see if I can get a better look at the wiring inside the door - maybe something obviously rubbing and frayed? No luck.

I look under the front seats to see if the seat wiring is obviously pinched and rubbed/frayed - no such luck though it's hard to see down there. Not sure if I should unbolt the seats to pull them up and get a better look?

I understand that, since the fuse is blowing without the key in the ignition, it's one of the systems that always gets power? (power door locks, seat controls). I'm just dangerous enough with a multimeter to think I know what I'm doing (evidenced by my partially melted test leads), so I set to continuity/tone and test at the fuse box to see if I can find the short from there by unplugging various things and testing for resistance (internet tells me 0.2 to 0.5 is good, anything below or 0 is a short, infinite means I unplugged something). I dont think this is the correct approach? Should I put the multimeter down and slowly back away? I unplugged the master switch and I still get a short reading, I unplug the big plug down at the kick plate and it's infinite (does this mean it's inside that door?) I unplug the control module inside the door and I get infinite. Unplug the door lock actuator and still get a short reading.

I'm starting to become a regular at NAPA buying all their 30 amp fuses. Don't want to take this to the shop again - they threatened me with over a grand in diagnostics fees to look at it again.

Any ideas or hints would help.
 
May not be the cause in your 80, just a link about someone else who had the 30amp fuse blow repeatedly:

 
If your fuse is failing while operating a specific switch, the circuit supporting that switch is likely at fault. If the fuse fails apparently randomly when no switch is being used, it may be the Power Main Relay (90987-02004). You can test the relay with a power source and multimeter, or just swap a new one in. It's in the driver's side kickpanel relay block:
1686051282549.png


The defogger relay is the same part.

Failing those two approaches, you're looking for a wire that's lost its insulation and is grounding itself against the body. Or, it may be the door control relay; if it is, that'll be readily apparent.

FWIW, cleaning the circuit boards and the switches mounted on them isn't a terrible idea, if you have them open already. Deoxit is the stuff to use. I use CRC plastic safe contact cleaner for all my connectors.
 
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