I've got a unique issue for y'all. I haven't seen anything like it before and couldnt find anything similar with a search. I have my 94 4runer, 3.0 5 speed. When I got it everything worked fine except the the speedo and reverse lights didn't work. They plainly were inop. I found the vss and reverse switch simply weren't getting any power. I checked wiring diagrams and they were pretty basic. I couldn't find an immediate problem and figured there was just a break in the wire somewhere. I spliced a new wire into the sensors and ran it to the battery with a 5 amp fuse. Everything worked perfect for about 4 months till a couple days ago.

First issue I had I noticed in the day. Speedo was inop again. Checked fuse it was blown. Replaced worked again. Later that night, while reversing the lights went out. Checked fuse. Blown. Replaced. Couple days later, same thing. Confirmed it was reverse causing this.

Fast forward to tonight I reversed and again the lights went out. Checked fuse of course it was blown again. Replaced, no lights. Well damn. Went forward and noticed I was bright behind me. Lights were on. Put it in reverse lights go out. Out of reverse lights come on. Noticed the 4wd indicator dimly lights in reverse and then goes out in other gears. Gets brighter but not full brightness when 4wd is actually engaged. I pull the fuse just to make sure I get home without burning down another vehicle. 4wd indicator is now fully lit at all times key on. (Keep in mind, I broke the outer driver CV about 20 minutes before this all started happening. Probably not related.)

What do you guys think? Anybody else had a similar experience?I've never seen anything like it. Wiring diagrams (Mitchell) are straight to the point and show no stray circuits related to the reverse light circuits. 4wd indicator while could be related to the broken CV but its not likely. This runner has ADD and it's always been functional and still is functioning correctly. Had to drive home in 4wd just to keep the busted CV from grinding (kept the shaft spinning with the hub).

Tell me what you think and give me some ideas. I'll look into anything as soon as I get the chance.

 

Any physical damage to the switch that you can see??

Also where the harness plugs into the switch

You have a short someplace in the circuit that causes the fuse to open

unless it is physical damage to the wires it comes down to the switch being shorted

For what ever reason either the switch or the transmission is causing the switch to be closed when it should be open

 

There's no visual damage anywhere. Im gunna disconnect the switch after work and that should confirm if it's a switch problem or a wiring problem. I'm betting on a wiring problem. I think what's going on is on most of Toyotas switches and relays for everything, when in the off position, the switch is in a normally ground state rather a normally open state. If that's the case with this switch then I could have a short to power on the reverse ground circuit (which the 4wd indicator is also grounded on). This would cause power to the lights while supposed to be off. When put into reverse both sides of the circuit will see + and cause the lights to turn off. This is my current guess however, pulling the fuse I added which is connected to the hot side of the circuit caused them to turn off. The only way I could see that intruducing power to the ground side of the circuit could be through the reverse switch so maybe it is that.

 

Yes, check your backup switch. Also the wires leading to it. That harness may vibrate, rub against tranny and wear out the insulation, causing a short.
Check that splice you did to the sensors. You should find out the real problem with the original wires and fix it, not band-aid it.

 

edit: I was looking at a different wiring diagram. Your truck is different.

I'll get diagrams in a sec...

 

Check the combination meter for any issues. It looks like there's a wire leading there (for the reverse circuit), but no function.

Back-up:


4WD:

 

I figured I'd conclude this thread today in case somebody has a similar issue. I always seem to fail at remembering to comeback and reply to my own threads as I forget them. Anyhow I digress

I solved the issue with the lights. What happened was the reverse switch went out and caused it to short to ground when in reverse. Because of it doing that it causes the power feed to the switch and vss to burn and essentially weld itself to the output wire to the reverse lights and the switched wire for 4wd. This caused the reverse lights lights to stay on normally and turn off with a voltage drop. The 4wd light would light dimmly because the reverse switch would introduce a heavily loaded ground to its circuit. Engaging 4wd would lighten the load making it a tad brighter. The 4wd indicator then stayed on because the switch burned up and shorted to ground.

It was a simple problem that caused some really seemingly odd issues and took way to much thinking to understand. It's difficult to describe it but once understood it makes perfect sense.

I eventually found the shorted wires and have repaired them and replaced the switches but I've still yet to find the break in the original feed wire. I think it might be pretty high in the dash. I'm about to do a 3.4 swap. When I tear into I'll likely dig deeper into the dash.

 

Thanks for the update. Glad you got it working. Some of the weirdest electrical problems boil down to a simple ground issue.