#5 Fuse blowing with ignition ON

[Blazin420SHO]

1994 ATX #5 Fuse (rear defrost, climate unit, turn signals) blows instantly every time you turn the ignition to ON. I suspected the ignition switch so I pulled it out and there was some melting on the B-5 pin so I swapped in another one I had and same thing...swapped in a MF switch and flasher unit, same thing. All wiring for each unit looks good and I checked for melting and shorts on the gauge cluster wiring. This is an everytime issue and I got the car like this. I know it can't be the defrost or signals because it is key ON so I suspect the climate unit circuit or the maybe the wiring from the MF switch to the fuse box.
Any help would be great, thanks.

 

[NoSlo]

You've got a lot of stuff that runs off fuse #5. Here's a 1994 diagram; I've edited out or grayed the non-SHO parts:



Power for the inside fuse box goes through the outside 40A fuse and the ignition switch. If the switch or harness connector is damaged, it is likely from the repeated overcurrent that blew the fuse #5.

If you disconnect the battery, then you can start probing.

If you pull out the fuse, you can then instead insert a spade terminal with wire into each side of the fuse socket, for your multimeter. One side should go to the battery positive through the ignition switch. The other side is what we want to test. With all those devices, but the car not running, it will probably have a resistance to ground about 10-20 ohms. If instead the resistance between the fuse terminal and ground is almost zero, you've got a short somewhere.

Your diagnostic will then be finding and unplugging all those modules and connectors to see if you can fix the fault.

If everything is unplugged (which will take lots of hunting and disassembly) and you've still got a short, then it's the Pink/Orange wire that is shorted to ground somewhere, which is going to be a **** of a lot harder to find, since there are only a few harness connectors you can unplug (denoted with C### in the diagram).

Oxygen sensor wires are a good place to start; the wiring can melt, the insides can short out.

 

[Blazin420SHO]

Alright I will read over that diagram and start hunting it down when I get home from work. Thank you

 

[Blazin420SHO]

You've got a lot of stuff that runs off fuse #5. Here's a 1994 diagram; I've edited out or grayed the non-SHO parts:



Power for the inside fuse box goes through the outside 40A fuse and the ignition switch. If the switch or harness connector is damaged, it is likely from the repeated overcurrent that blew the fuse #5.

If you disconnect the battery, then you can start probing.

If you pull out the fuse, you can then instead insert a spade terminal with wire into each side of the fuse socket, for your multimeter. One side should go to the battery positive through the ignition switch. The other side is what we want to test. With all those devices, but the car not running, it will probably have a resistance to ground about 10-20 ohms. If instead the resistance between the fuse terminal and ground is almost zero, you've got a short somewhere.

Your diagnostic will then be finding and unplugging all those modules and connectors to see if you can fix the fault.

If everything is unplugged (which will take lots of hunting and disassembly) and you've still got a short, then its the Pink/Orange wire that is shorted to ground somewhere, which is going to be a **** of a lot harder to find, since there are only a few harness connectors you can unplug (denoted with C### in the diagram).

Oxygen sensor wires are a good place to start; the wiring can melt, the insides can short out.

So I checked the O2 sensor wiring and unplugged them, unplugged the MF switch, the flasher relay, the EACT and headlight switch and it still blows. I cannot find anywjere or in my Chilton manual the location of the VAPS module, air bag module(i think its above the glovebox) the manual lever position sensor, or the shift lock actuator. The remote keyless entry works and I also noticed the check engine light is not illuminating at any time if that matters.

 

[NoSlo]

So I checked the O2 sensor wiring and unplugged them, unplugged the MF switch, the flasher relay, the EACT and headlight switch and it still blows. I cannot find anywjere or in my Chilton manual the location of the VAPS module, air bag module(i think its above the glovebox) the manual lever position sensor, or the shift lock actuator. The remote keyless entry works and I also noticed the check engine light is not illuminating at any time if that matters.

You shouldn't be checking that a fuse blows, which will consume fuses as well as allow continued overcurrent, but use a multimeter to check the fuse socket side that isn't giving you +12 volts. One probe to the "electrical stuff" side of the fuse socket, and then another connected to a grounding point like the dashboard metal frame will show the resistance, which is likely near 0 if there's a short.

Connector C122 is a 16 pin connector, located under the upper radiator hose where it goes to the thermostat inlet. Unplugging that will eliminate all the stuff on the right side of the wiring diagram.

The shift lock actuator is a solenoid that keeps the transmission from being shifted out of park unless the brake pedal is pushed. It should be a wire connector going to the shift lever position that can be unplugged.

The manual lever position sensor is a multi-wire connector going to the transmission. The #5 fuse is used for the backup lights (and electric trunk popper) power. #5 power is also used by the MLP to tell the keyless entry module if you are in reverse. Inside the lever switch there is small chance of a short, since there is no ground inside, and a backup light wiring fault would only pop the fuse in reverse. You might check inside the trunk wiring lid wires to see that they haven't worn through or become pinched inside the plastic wrap.

The illuminated entry module is located in the trunk, mounted under the "package tray" between the right rear speaker and the trunk opening. It powers wires going through the doors, along with the cabin light. With doors opening and closing, this is a potential place for wiring shorts to occur from wear.

The remote keyless entry module, for the number pad, gets power from three different fuses, along with power signals from the driver seat switch, door handles, etc. It powers the key illumination and lights if you have this option, along with door locks and trunk popper, and many other things. It is a larger module with three connectors, behind the right hand side of the back seat in the trunk.

DRL connector is in front of the air filter, there whether you have this optional module or not.

The airbag diagnostic monitor computer is in the dash behind the right side of the glove box, a small module on top and a big module on the bottom, each with a large wiring connector. Don't mess with this unless you've disconnected the battery and left the lights on for 10+ minutes to discharge airbag backup power.

VAPS module is under the dash above the pedals. Its connector has 6 pins on the clip side, and 8 pins in the second row.

 

[NoSlo]

Don't know how progress is going, but another thing to examine is if any aftermarket accessories, such as a radio, or even gauges, have been installed. The installer may have poorly tapped this fuse's power wire or connected the accessory so its wires short out.