Blowing tail light fuse, possibly short in headlight switch?

[tarheel40]

I’ve had my 76 FJ40 for about 6 months, no electrical issues, and as I’m driving down the road last Saturday night my dash lights suddenly go out (after hitting a medium bump). After getting back home I realized it wasn’t just the dash lights but the whole circuit - parking lights, tail lights, etc. The 15 amp fuse at the top of my fuse box was blown, I replaced it and was immediately back in business. Figured the fuse had gone bad, no big deal.

Drove the 40 some the next day (headlights not on) and then checked the lights that night just to make sure everything was working - fuse blew as soon as I pulled the headlight knob out and lost the whole circuit again. I took all the bulbs out of every light on that circuit and the fuse blew again as soon as I pulled the knob out (even before headlights came on). Checked all wiring, harnesses, etc and no obvious exposed wire, shorts, etc. I took the headlight switch partially out, unplugged the harness, adjusted the dimmer a few times and then viola - everything worked again. My dimmer isn’t dimming for what it’s worth, only an on/off switch and I’ve haven’t gone to LED dash bulbs (yet!).

I checked the lights almost every night this week and everything worked fine, even after a short test drive. Took it out for a spin tonight and after about 10 minutes the same fuse blew. It seems like I’ve got an issue in my switch the best I can tell? Does this sound familiar to anyone? I’ve got the dash out and the headlight switch completely out, still don’t see any obvious shorts with the wiring except for two exposed wires coming from the middle of the dash that I’ve now covered up (don’t think they’re even on that circuit?). Is there anything I can do to take this switch apart and diagnose further (not an electrical guru)? It was a bit dirty around the connectors so got that cleaned up. Hate to just buy a new switch since I’m not positive that’s the problem and honestly not even sure if they’re available. Here are a few pics. Any and all help is appreciated, this is driving me crazy!

 

[1911]

Another common cause for that is previous owner added trailer lighting, you might check for that also.
Good luck in any event.

 

[Godwin]

I had something similar happen with the fuse for running lights blowing when the headlight switch was pulled. In my case it seemed to be related to the rheostat and only when the dash lights were cranked to maximum. At lower levels for the dash lights there was no problem but once the rheostat was turned up to full the fuse would blow. I've never tried to fix it, I just adjust and live with it.

 

[tarheel40]

Another common cause for that is previous owner added trailer lighting, you might check for that also.
Good luck in any event.

Thanks for the suggestion. I had seen seen that mentioned in several threads and checked, don’t see any signs of additional wiring added for trailer lights. I also unplugged the rear light harness near the rear axle and was still blowing that fuse, so I eliminated everything in the rear of the truck.

It’s either parking lights, gauge lights, the dash pad light or the switch mechanism. Being that the short seems intermittent and my dim function isn’t properly working I really think it’s in the switch.

 

[tarheel40]

I had something similar happen with the fuse for running lights blowing when the headlight switch was pulled. In my case it seemed to be related to the rheostat and only when the dash lights were cranked to maximum. At lower levels for the dash lights there was no problem but once the rheostat was turned up to full the fuse would blow. I've never tried to fix it, I just adjust and live with it.

This sounds very similar to what I’ve got going on. My dim function isn’t working, if I crank the rheostat all the up it just turns my gauge and dash pad lights on. It’s basically an on/off switch which I’ve seen can happen when the dimmer is “tired”. Earlier in the week if just left it full open and gauge/dash lights on everything was working so I was planning to live with it too, and just never try to dim. I was pretty disappointed last night when the fuse blew with the rheostat in that position - maybe a bump on the road caused that position to temporarily change and that’s when it blew.

I have no idea how to refurbish the dimmer function or what could be causing the short - @ToyotaMatt any ideas? Or do you have a switch I can buy to see if that solves my problem?

 

[ToyotaMatt]

This sounds very similar to what I’ve got going on. My dim function isn’t working, if I crank the rheostat all the up it just turns my gauge and dash pad lights on. It’s basically an on/off switch which I’ve seen can happen when the dimmer is “tired”. Earlier in the week if just left it full open and gauge/dash lights on everything was working so I was planning to live with it too, and just never try to dim. I was pretty disappointed last night when the fuse blew with the rheostat in that position - maybe a bump on the road caused that position to temporarily change and that’s when it blew.

I have no idea how to refurbish the dimmer function or what could be causing the short - @ToyotaMatt any ideas? Or do you have a switch I can buy to see if that solves my problem?

YES i can help here i have Fully Restored Headlight dimmer switches ready for immediate shipping , no delay or wait period ,....

will need to find out if you have a 60010 or 60011 ?

 

[mrldcty]

I have similar behavior where my tail fuse keeps blowing. I went after it with a meter tonight. When I measure Amps across the fuse socket (key off), I get 24 amps, so something is pulling a load even though nothing is actually on. The weird part is that I get zero continuity between the negative side of the fuse socket and the frame, but when I take a tail light connector off, I get continuity between the hot wire and the frame. When I replace the fuse, it works for a while, then pops... haven't tested just how long it lasts or if the light switch/rheostat effects it. Looked all over for any sign of grounding along the harness, and found none. Can anyone confirm that a bad light switch could behave this way before I go to the effort of replacing it?

 

[charliemeyer007]

You should start with the correct wiring diagram for your rig. Use a VOM meter and a jumper wire to test each wire on the fuesd circuit that blows for both continuity and a dead short. Take out the bulbs and un-plug the head light switch to test the wires. Clean and inspect the bulb sockets, then lube them lightly with dielectric grease. Didn't find anything, then pull the head light switch. You can test it for function according to the wiring diagram. I think the rheostat only functions on the instrument cluster.

I have at one time had to take apart about every switch on my 72. Cleaned them, repaired the damage and put them back together. I sort of recall the head light switch "burnt up" was replaced the first time like 40 years ago. Maybe 25 years I had to service the head light switch. I think I put in a new foot powered hi/low head light switch maybe 30 years ago.

I found your post hard to follow. You didn't provide the year model market, always a good idea when asking for help.
"The weird part is that I get zero continuity between the negative side of the fuse socket and the frame" - the fuse clips should always be isolated from the ground.
"but when I take a tail light connector off, I get continuity between the hot wire and the frame" which tail light connector or both if referring to the bulb sockets connections - is the head light switch on?

 

[mrldcty]

You should start with the correct wiring diagram for your rig. Use a VOM meter and a jumper wire to test each wire on the fuesd circuit that blows for both continuity and a dead short. Take out the bulbs and un-plug the head light switch to test the wires. Clean and inspect the bulb sockets, then lube them lightly with dielectric grease. Didn't find anything, then pull the head light switch. You can test it for function according to the wiring diagram. I think the rheostat only functions on the instrument cluster.

I have at one time had to take apart about every switch on my 72. Cleaned them, repaired the damage and put them back together. I sort of recall the head light switch "burnt up" was replaced the first time like 40 years ago. Maybe 25 years I had to service the head light switch. I think I put in a new foot powered hi/low head light switch maybe 30 years ago.

I found your post hard to follow. You didn't provide the year model market, always a good idea when asking for help.
"The weird part is that I get zero continuity between the negative side of the fuse socket and the frame" - the fuse clips should always be isolated from the ground.
"but when I take a tail light connector off, I get continuity between the hot wire and the frame" which tail light connector or both if referring to the bulb sockets connections - is the head light switch on?

OK, sure. It's a 1980 North America market with 60K original miles. I've been driving it with no electrical issues for 4 years now. The weird part to me is that the neg fuse clip is isolated from ground even though the hot wires of the tail lights are not.

 

[charliemeyer007]

Sorry I don't understand "neg fuse clip". My glass tube fuses have +12 V DC power coming in on one clip, going threw the fuse and out the other clip still as +12 DC. Some circuits are hot with the key off.

Maybe watch some youtube vids on turn signal/marker/brake light fault finding. There might be one LC's but about any toyota 1980 model will be close