wiring diagram needed.

[basketcase]

Hey guys. I'm having an odd issue with my tail lights/rear window. I'm certain that I can figure it out with a wiring diagram. What's happening is that the parking light fuse blows a lot. Like almost immediately. If I unplug the rear door harness in the rear quater panel it does not blow. I have removed the harness, inspected and repaired some chaffed wires. What's interesting is that when the lights are on if the fuse doesn't blow, the rear window does not work by switch or key. Turn lights off and window works. My thought is that the window circuit is somehow pulling ground from the light circuit which rests at ground but goes away when the lights are on. If anyone has a wiring diagram of the tailgate it'd be much appreciated. 1987 4runner btw. thanks!

 

[RAD4Runner]

...If I unplug the rear door harness in the rear quater panel it does not blow. thanks!

The license plate light is the only thing in the tailgate that takes power from tail light circuit. Did you inspect that circuit for short to ground?

My thought is that the window circuit is somehow pulling ground from the light circuit

Yes. Possibly combination of:
1) Intermittent short of license plate light power to ground side of window control wiring, AND
2) Ground side of window control wiring has intermittent contact with ground (varies from high resistance to low resistance).

Note:Rear Window really inputs need to be grounded in order to activate.
When ground side of window wiring has resistance to ground, BUT has contact with license light power, that pulls relay inputs high, preventing it from working.
When ground side of window wiring has low or no resistance (intermittent) to ground turning on light will blow the fuse.

I have schematic posted on Photobucket but it is down at this very moment for routine maintenance. I'll post link when back up.

 

[basketcase]

The license plate light is the only thing in the tailgate that takes power from tail light circuit. Did you inspect that circuit for short to ground?


Yes. Possibly combination of:
1) Intermittent short of license plate light power to ground side of window control wiring, AND
2) Ground side of window control wiring has intermittent contact with ground (varies from high resistance to low resistance).

Note:Rear Window really inputs need to be grounded in order to activate.
When ground side of window wiring has resistance to ground, BUT has contact with license light power, that pulls relay inputs high, preventing it from working.
When ground side of window wiring has low or no resistance (intermittent) to ground turning on light will blow the fuse.

I have schematic posted on Photobucket but it is down at this very moment for routine maintenance. I'll post link when back up.

I inspected every inch of the rear harness for shorts.

What you described is exactly what I suspect (although with more technical language). The wiring diagram will help me pin down the locations where the two circuits converge. I suspect I may have found the culprit in my sleep. lol. When I removed the whole interior for cleaning a while ago I noticed a scotch lock on the console window switch but never gave it a second look. This morning I removed the switch to inspect. Someone had "fixed" the window by paralleling the blue and red/blue wires at the switch. I did not have my meter or diagram to verify but I suspect that the red/blue is illumination for the switch (parking light circuit) and blue is ground input to the switch (for relays). I removed the scotch lock and now the window works with lights on. We shall see if the fuse blows but hasn't yet. My assumption is the the ground input to the switch is intermittent and a "mechanic" "fixed" it by seeing another wire that shows ground and using it as the input to the switch. Your diagram will be very helpful in further tracking this down. Thank you for your input!

 

[RAD4Runner]

I inspected every inch of the rear harness for shorts.

What you described is exactly what I suspect (although with more technical language). The wiring diagram will help me pin down the locations where the two circuits converge. I suspect I may have found the culprit in my sleep. lol. When I removed the whole interior for cleaning a while ago I noticed a scotch lock on the console window switch but never gave it a second look. This morning I removed the switch to inspect. Someone had "fixed" the window by paralleling the blue and red/blue wires at the switch. I did not have my meter or diagram to verify but I suspect that the red/blue is illumination for the switch (parking light circuit) and blue is ground input to the switch (for relays). I removed the scotch lock and now the window works with lights on. We shall see if the fuse blows but hasn't yet. My assumption is the the ground input to the switch is intermittent and a "mechanic" "fixed" it by seeing another wire that shows ground and using it as the input to the switch. Your diagram will be very helpful in further tracking this down. Thank you for your input!

Yep, what you're thinking makes sense.

High resistance to ground from control switches.
When tail light AND license plate lights were OFF, someone thought positive wire to it is ground (zero volt because tail light was off). Connected door switch ground to it.

Here's plain schematic (1986-1988):
I suggest you remove any hack-job (Don't you just hate those?)

This could cause your symptoms:

Best of luck.

 

[basketcase]

Thanks! It looks like removing that scotch lock fixed the problem. They used the "ground" of the illumination wire as the input to the switch. Funny thing is everything works fine with it removed. Wonder what problem they thought they were fixing. Maybe the window lock button was on and just didnt know it. Thanks again for the help. Now I can drive at night!

 

[RAD4Runner]

Thanks! It looks like removing that scotch lock fixed the problem. They used the "ground" of the illumination wire as the input to the switch. Funny thing is everything works fine with it removed. Wonder what problem they thought they were fixing. Maybe the window lock button was on and just didnt know it. Thanks again for the help. Now I can drive at night!

Cool! What's a "scotch lock"? Would be nice to understand what happened in your case. The ground of illumination wire should still be constant ground and hard-wired to real ground. There is no switch that interrupts it.

Regards and happy memorial day weekend, guys - OOORAAH!

 

[RAD4Runner]

Cool! What's a "scotch lock"?...

Oh I see.
I've never trusted those. You would not know if your wire strands get cut.

I prefer careful manual stripping, then wire wrapping or crimping AND soldering for my taps.

 

[basketcase]

Oh I see.
I've never trusted those. You would not know if your wire strands get cut.

I prefer careful manual stripping, then wire wrapping or crimping AND soldering for my taps.

Yeah. Those things are nasty. I hate them. I've been doing 12volt electrical since 1999 and everything gets soldered. Its the right way.