1990 Montero M/T fuse #4 blows when put in reverse.

[Joe2Slow]

My 1990 Montero started blowing fuse #4 (labeled Back/Gauges) every time I put it in reverse. Truck still runs, but the gauge cluster is on the same circuit and so my gauges die every time it happens. I thought it might be the tail light circuit shorting out, as my reverse light sockets on both tail lights were corroded and rusty. I replaced the tail lights, but it didn't solve the issue. I can't find a service manual that includes these circuit diagrams. I don't see anything on mitsubishilinks.com for the 1990, and'89 files don't have anything I can find that is relevant. I tried to find the reverse switch on the transmission and disconnect it, but the switch I unplugged did not make a difference (the connector was under the access panel behind the shifter, and the switch connected to it was on the driver's side (LHD) kind of near the front diff, I think.)

I inspected the wiring, following the harness from the tail lights to the front by the ECU, and didn't find anything that looked suspicious.

Looking for some advice on what my next steps should be. Does the reverse switch do anything else other than trigger the reverse lights? Or is there something other circuit it energizes that I can check for a short? Or is the reverse switch somewhere else on my transmission? The manual I have said the switch was in different positions on different transmissions, so that wasn't very helpful. There are two switches right next to each other on the right side of the transmission, just back behind and below the shifter, but I don't see any connectors for them that I can disconnect. Can the switch itself short out? I didn't think there was a ground wire connected to the switch.

I have had an aftermarket stereo in the truck for several years, but the wiring harness does not list a reverse signal wire, and I'm not sure the '90 Montero has a reverse signal wire (but I don't have a wiring diagram so...) I did have the stereo out while I was trying to find a water leak, but I'm not sure what to test there if that might be a potential cause. I inspected the stereo wiring harness too and didn't see any obvious bare wires that could short.


Any insight on identifying where this short might be is appreciated.

Thanks,

Alex.

 

[Joe2Slow]

Update:
The "switch" I tested earlier was not the correct one. I found the switch on top right side of the transmission just in front of the shifter. The connector was lower down on the frame on the left side of the transmission. I disconnected the connector, and had continuity between switch-side connector wires when the Montero was in reverse, and no continuity when it was not in reverse. And when I replaced the fuse and put it in reverse with the switch disconnected, the fuse didn't blow. I also noticed that one wire on the switch had continuity to ground. So is my switch bad and grounding somehow creating the short?
I checked the switch wires as closely as I could and didn't see any obvious breaks or places it could be grounding. It's a bit difficult as the wires that loop over the transmission and there isn't much room in the transmission tunnel.

Will know more maybe when I get the new switch and put it in.

Still looking for a full wiring diagrams for the '90 3.0 V6 Montero. If anyone has any idea where these might be available, I'd love to know where I can get them.

Thanks,


Alex.

 

[Michael Brown]

The switch could have an internal failure to ground through the housing.
Hopefully a new switch solves your problem.

 

[Joe2Slow]

Update II. Yeah, it was the switch. I was checking for wiring issues as I had been working on some wiring when it started happening, and wanted to avoid pointing the parts cannon at it, but in this case I should have started with replacing the switch.

Alex.

 

[WOODY2]

As it should be... problem described, discussion, problem solved, final disposition posted. I like it, many posts leave us hanging.