74 Burning those tail light fuses (1 Viewer)

[1chukar]

Excuse me all if I get to rambling here – but this problems needs to be sorted out, and maybe writing about all the issues will help, or maybe someone has super skills to help troubleshoot from a distance. I’m staring at it and can’t get it. The experts on this forum are very impressive. I’ll admit to using info here for reference or entertainment at times, and up until now that’s been enough to get me through. But now I am stuck.

My first FJ40, its a 6/74 that’s mostly stock – with few nice upgrades. Owned it about 6 years, In the past someone has tried wiring that could be called either creative, lazy, or crazy. Like they used house wire nuts, lots of splices, blue painter tape, etc. You get the picture. Also its been ‘desmogged’ so there are occasional dead end connectors or wires. I’m trying to tape, or heat shrink these I I come across them.

This rig starts and runs great. I really like it. It just doesn’t get enough runtime and that’s mostly because of lacking confidence with present electrical systems. Within a few weeks its got to work and will drive it from Seattle to Oregon (getting outta this now to crowded city) so I’m under a little pressure here.

Everything works – except that when the headlight switch is pulled the 15 amp tail light fuse (top one) burns. This leaves the headlights working, but takes out tail and indicator lights. If I don’t pull on the headlight switch, then it can daylight drive and all brake and signal lights work just fine.

And lately when daylight driving the alternator gauge consistently reads a bit to the negative. Not sure if that’s related.


Trying to sort this out, here’s a list of electrical changes I’ve made this last off-season -

1. Replaced both rear taillights – bought the cheaper Taiwan units, so needed to replace rear plugs. That was pretty straightforward. And took out a real hack job wad of trailer light wiring at the same time.
   
   
2. Took out cheap radio and big ugly speakers .
   
   
3. Electrical fuel pump replaced to new with a switch added (nice theft security since it’ll only go about 200’ when I forget to turn it on) New because existing one just crapped out
   
   
4. Pulled and refinished the steering wheel so cleaned up horn contacts and turn switch. No major disassembly.
   
   
5. Added power steering pump, so this moved the alternator over to the PS of engine. Put on a new VR on firewall at that time with extra attention to its ground.
   
   
6. Had an automotive tech guy add relays to the headlights.
   
   
7. New dome light, with fresh wires all the way back to the fuse box (during this latest diagnosis it’s been unplugged)
   
   
8. Recently replaced emergency brakes with a new cable – while doing this I fussed with the ebrake dash light and switch on the pull. Everything ohms out but I never did get the dash indicator light working. Would like to though
   
   
9. Removed disassembled and cleaned and reinstalled fuse block with all new fuses.
   
   
10. Removed bumper winch and all associated power wires along with dash mount switches and control wires. This will be a project for later on – once current issues are sorted out.
    

Whats been done to diagnose / fix the short

1. Ran a 4/0 gage direct from battery negative to rear bumper. All tail lights connect direct to this.

2 Disconnected the two harness connectors on PS of firewall – completely disconnecting all harness connections to rear lights

3. Just pulled the steering wheel and visually inspected for shorts around turn switch, horn or ignition

4. Replaced headlight switch with a new one from Ragingmatt. He’s a good guy with plenty of knowledge so I totally trust him and his products.



So folks, any tips, suggestions or snarkey remarks are welcome.

 

[1chukar]

Something is really screwy

I just unplugged the two front signals, apron indicator, headlight connectors, horn, and the headlight relays from the fuse block... and still burning fuses...

Waaaaah

 

[Solace in Solitude]

I would look for some place where you are inadvertently energizing the frame/body... E.g. a live wire shorting to ground.

You first said that the fuses pop when you turn in the headlight switch...

Then you said they still blow after you disconnect the headlight connectors, etc.

I presume you have the headlight switch on, in both scenarios... If so, the headlight switch/circuit could have a short.

If you are inadvertently energizing your grounds and also grounding to them, the fuses will pop.

I know you cleaned up your fuse block, but look closely to the connections around it. The fuse block is grounded to the body... Maybe it's also energizing the body.

 

[thebigredrocker]

Could be an unused loose wire under the dash goes hot when the switch is pulled. Old radio light wire?

Or an underdash splice that isn't covered and is contacting metal(ground).

Make a test wire that attaches to battery + and has a male spade. Don't let it touch metal or you'll have big sparks. Pull your headlight switch connector, energize each wire separately via the harness plug to see which one pops the tail fuse. Then try to follow it.

It will probably be something simple that's just hard to see under the dash.

 

[1chukar]

Thank you both, pngunme and thebigredrocker. I think that you are each onto something and I just am not seeing it.

Here's where it's at. Both the headlight switch and fuse block pulled out and carefully inspected. Tried the (+) jumper idea, ( thanks thebigredrocker) which seems like a really good concept and woke me early. Probing each of the headlight switch connector five wires it didn't ever blow the twilight fuse. Did get a small spark at the center connector and can' t figure that cause at all

With the fuse block and switch just dangling, I reconnected everything ( rear harness, headlights, etc.). Turned it on and every light worked perfectly!!! including brakes and had arms. Very exciting and feeling real good now.

Reinstalled the switch and the fuse block -- and poof - blew fuses again. Now I've removed the headlight switch and block , and it's working perfectly one more time.

So I don't know and can't find where the short to ground is - but its definitely a hot one.

 

[Solace in Solitude]

I believe your fuse block grounds to the body... I could be wrong, I've never looked at this schematic before.

But, assuming it grounds to the body, that could indicate the body is energized somewhere else...

Try dangling the fuseblock and mounting the switch... Does the fuse blow?

Try dangling the switch and mounting the fuse block... Does the fuse blow?

You could have a live wire, with missing insulation, shorting to the body anywhere... Most likely is the holes in the firewall and the clips and tight spaces under the dash... Anywhere a live wire passes thru or against metal.

 

[1chukar]

You are spot on...
Installed the switch , turned it on, and all lights perform well.

Looking over the fuse block it doesn't look like there are any pads where it's supposed to ground. And there are some connectors showing burned evidence of excess heat. So still without finding a specific culprit or issue, I thought to isolate the block. So cut two 1/4" long pieces of fuel hose as standoffs. Installed block with these using longer screws . And all is working not - But on very little faith. Durn electrical buggs anyway

 

[Solace in Solitude]

You really need to find your short... where it's energizing the body.

That's not a good thing... could cause all sorts of problems.

The scorched areas in the fuse block could be the result of the current coming from the body, via what would normally be a ground.

It seems like my fuse block is grounded with a wire under one of the two screws.

 

[1chukar]

Totally agree with what you're saying. And thank you for your help

Just took it for a quick run around the neighborhood and with lights on the ampmeter is pegged at -30. So it's not,good
I'm barely qualified to take this on, but somebody's got to d something

Thanks again

 

[Solace in Solitude]

It's not generally easy to find... But, an organized approach, using the starting points I suggested, will usually make shorter (pun intended) work if it.

 

[1chukar]

Didn't Yogi Berra say something like " you can see a lot by just looking ?"

Getting back at this problem , and not really certain where I'm at

Found tha PO has put a rocker switch inside the "pocket" where the outlet used to be. With switch on there was a loose hot wire just flopping around under the dash. This has been corrected

Current question is, Should Amp Gauge load to negative with each circuit turned on. And never rcame cover no matter how long I drive.

Last night left a charger was on the battery.

Today I removed gauge cluster, disconnected one side of amp meter and connect in a DVM for measurements

Measurements are a follows. Please note, this rig has an electric fuel pump. Lots else is stock.

Engine off, parking lights on, 6.15amp
Engine off, headlights full on, 14.28amp
Engine on, headlights on, fuel pump in 17.28amp

So this additive amps continues, and measurements never go to positive.

replaced the following within last 100 miles

Voltage regulator
Alternator
Head light switch
All fuses

Anyone have a guess or educated poke at where I can go next?

Thanks. Jeff

 

[Splangy]

Unless your frame has a gaping hole in it, there's no need for a ground cable running all the way down to the back of the truck. Just bolt it to the frame at the nearest location to the battery if you're worried about a bad ground to your frame. Your frame rails have virtually zero electrical resistance to them and will carry electricity just fine to the rear crossmember and lights.

 

[1chukar]

Thanks Splangy
Appreciate your comments
One can see that I've been throwing parts at this problem, hoping for a fix
And with this wire here I shouldn't hear anything about needing better grounds

But I'm at a loss now