electrical and ignition Burning Fuse #8

[Paulo]

I need some help diagnosing and fixing an electrical problem on my 1976 2002.

 

After an evening drive last week, I noticed my dash lights were not working.  When I got home, I also noticed that my right rear taillight and right front parking light were also out.  I checked the fuse panel, and sure enough, fuse #8 (15a) was burnt out.  I replaced it, and it popped as soon as I turned on the headlights (with the ignition off, btw).  As a test, I tried a red (16a) fuse, and it popped.  Then I tried a blue (24a)  fuse, and watched it smoke and burn before I shut off the lights.  

 

Any ideas on what could be causing this, or how to diagnose and fix it?  My knowledge of automobile electronics extends to changing bulbs and fuses!

 

Thanks,

 

Paulo

[Hans]

Not a good idea to add more amps. The tii page has wiring diagrams; find out what is on that circuit. Then check each of those items to see if something obvious is shorting. You may need to use a continuity tester on each hot wire to see if it is grounding.

[Mike Self]

How to diagnose:

 

First of all, you can rule out the headlights, as they don't function with the ignition off...

 

Second, it sounds like a dead short somewhere to pop a 25 amp fuse, much less a 16 amp.  Somewhere you have a hot wire that's touching a ground, and from your description it's in one of the lights you mentioned.

 

Take a look at your owners manual or the fusebox sticker and see what's on that circuit #8.  Then (with an ample supply of fuses!) disconnect one load at  time, then try a new fuse.  When the fuses stop popping, you've found your culprit.  You might be able to short circuit (so to speak) this procedure by checking the visible wires in the circuit--the tail light, license plate light, parking light etc and look for wires touching.  A short like that will be pretty obvious--burned spots, melted wires etc.  Bet that's gonna be the problem.  

 

happy troubleshooting.

 

mike

[Jakeismartin]

I found that using a headlight and running a wire from each post to each side of the fuse ears help greatly and it also keep me from having to buy many fuses.

I found all of my shorts this way

 

Tip from other FAQ'ers

 

Jake

[AustrianVespaGuy]

Ok, so fuse 8 only GETS 12V from one place, a grey/green wire from the headlight switch, and only PROVIDES 12V to two places, the front right headlight and right rear tail light, via grey/white wires.  If you put in a new fuse and it is blowing only after your turn the light switch on, then there is a short to ground somewhere in one of those grey/white wires.  Find them and trace along them until you find the spot where the insulation is all melted and the wire inside is touching against the chassis or engine metal somewhere.

[Simeon]

Ok, so fuse 8 only GETS 12V from one place, a grey/green wire from the headlight switch, and only PROVIDES 12V to two places, the front right headlight and right rear tail light, via grey/white wires.  If you put in a new fuse and it is blowing only after your turn the light switch on, then there is a short to ground somewhere in one of those grey/white wires.  Find them and trace along them until you find the spot where the insulation is all melted and the wire inside is touching against the chassis or engine metal somewhere.

True that.

[Paulo]

Thanks all for your help. Unfortunately the problem remains.

The circuits fed through the #8 fuse (as stated in my owners manual) are the right-side rear and side parking lights, the right side rear license plate light, and the instrument lighting. Per the advice above, I individually isolated each of these exterior lights by removing the bulbs and turning on the light switch (with the ignition off). I blew a fuse each time. I also visually inspected the wiring from each of these lights wherever I could access it.

The only circuit I did not isolate was the instrument panel lights. Per Austrian Vespa Guy, it sounds like the instrument lights don't get enough voltage to blow a fuse, correct?

Any suggestions? Did I follow the tips you guys gave me properly?

Thanks!

[Mike Self]

You were doing things in sequence, as suggested, only you weren't performing the correct test.  Removing the bulbs probably wouldn't isolate the problem, as bulbs rarely develop internal shorts.  You'll need to disconnect the wires that feed each item at the fuse (or at a terminal point near the fuse box) for your test to work properly.  

 

I would carefully examine the bulb sockets on the appropriate parking, tail and side marker lights.  The tail light, parking and license plate light assemblies have hot terminals and bulb clips that can easily come loose enough to pivot until they touch a ground, thus blowing the fuse.  And if you've had loose stuff in the trunk, it could have hit the back side of a tail light housing (where the wires are) or the side marker and caused a short.  I'd do that before disconnecting the wires--it's much easier and you have a better chance of spotting the problem.

 

Keep looking; you'll find the problem!

 

mike

[Paulo]

 I think I found a problem.  Per Mike, I pulled the fuse panel this morning, and it was very evident that the grey/green wire's insulation was melted as far as i could see it (only about 4").  Sounds like this is the connection between the light switch and the #8 fuse terminal.  

 

Is this the problem, or a symptom?  If I understand AVG correctly, there is likely still a short in the grey/white wire between the right rear taillight and the light switch - is that correct?  Or is the melted grey/green wire the sole culprit? Remember that both of my headlights still work fine.

 

Also, the plastic socket for the taillight shows some evidence of melting.  The terminals in the socket both look fine, however.

 

How to fix it?  Do I need to pull a new grey/green wire from the light switch to the fuse terminal?  How do i remove the plastic connectors that plug into the fuse panel? Should I still be looking for a short in the grey/white wire from the rr taillight to the light switch?

 

Thanks all for your help

 

Paulo

[Paulo]

So I decided to replace the taillight bulb while I had everything apart.  Went to my local NAPA with the old bulb, and discovered I had been running a 13.2V bulb, not a 12v.  Could this be the source of the problem?  Might using a too-high voltage taillight bulb have resulted in too much current to the wire between the light switch and the fuse panel, resulting in the melted grey/green wire?

[Simeon]

You are confusing volts and amps. A car electrical system is a nominal 12v if you measure the voltage at the battery when the engine is running you will get closer to 13.2v than 12v. The bulb is fine.

The reason why the wire is burnt is most likely because you kept putting bigger and bigger fuses in place. The fuse is there to protect the wiring of any circuits connected to it. It is specifically designed to blow before enough current can flow through the wire to heat it to the point of melting itself. You will have a dead short to ground somewhere on that circuit which will cause a potentially massive current to flow. By upping the fuse size you have allowed more of that current to flow through the wire that the fuse can stand but the wire cannot.

Keep tracing that circuit, you will find part of the copper wire touching either another bare wire or terminal, or alternately the body of the car since this is all at ground potential (connected to the battery negative via the battery strap to the body / engine.

[Paulo]

Thanks for the information.  This has been an education!

[FunkyLaneO]

For future people pulling up this thread trying to find why their fuse is blowing DO NOT PUT A BIGGER FUSE IN BECAUSE THE ONE IN THERE IS BLOWING. Find out why it is blowing first or you will damage your wiring. Each circuit is designed to accommodate the fuse specified in the users manual and nothing else.

 

A great way to test for shorts without going through a box of fuses is to take an old sealed beam headlamp (low beam) and connect jumpers from the terminals on the lamp to the terminals of the fuse holder (without the fuse in place)  If there is a short the lamp will shine brightly, if no short it will be barely visible.  You can look at the lamp while you unplug, wiggle, etc, and see if you can narrow down where the short is, when the light goes out you found it (without melting wires).

 

Hope this helps.

[AustrianVespaGuy]

I think I found a problem. Per Mike, I pulled the fuse panel this morning, and it was very evident that the grey/green wire's insulation was melted as far as i could see it (only about 4"). Sounds like this is the connection between the light switch and the #8 fuse terminal. Is this the problem, or a symptom? If I understand AVG correctly, there is likely still a short in the grey/white wire between the right rear taillight and the light switch - is that correct? Or is the melted grey/green wire the sole culprit? Remember that both of my headlights still work fine.

 

Hi Paulo, this will indeed need to be fixed, but I do still believe that this is a symptom and not necessarily the root of the problem.  This insulation melted due to the extra high current running through the wire to wherever you have the short to ground, but I think I know how we can find out for sure:

 

1.) Unplug the grey/white wires from the back of fuse 8 but leave the grey/green plugged in.  Then replace the fuse (low amperage, 5 or 10 amp fuse will be fine), and turn then turn on the lights and see if the fuse blows.  If it does, then your problem indeed likes upstream in the grey/green wire and/or the light switch.

 

2.) If the fuse does NOT blow, then the short is still downstream, aka somewhere in the grey/white wires.

 

Let me know what you find out!

[SirJojo]

i am having the same issue, but can't wait to see what you find out.