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Hey folks - it's been a while since I have had time to come around, but now necessity drives me back here... I know that finding a short in a circuit is all about determination and hunting, but I'm wondering if there are any useful tricks.

We have a short in our brake light circuit on our 92 Pathfinder. I first noticed it when the console shift lock solenoid stopped working (it is wired to read a (+) signal from the brake switch). I checked the fuse for the brake lights and sure enough, it was blown. Replaced it and it worked for a couple of days, but then yesterday, it blew again. Not sure if it blew just when we pressed the brake, or just while driving.

I do have a after-market remote starter that also uses the brake pedal pulse to cancel the remote's control as well as locking the doors on first pedal press - both of which also do not work once the fuse is blown. FYI, this (+) pulse wire is tied into the brake wire just down-stream from the switch on the brake pedal.

So, all that to say - - I am pretty handy with mechanical repairs, and definitely understand much of the theory of electrical. And I also like playing around with the electrical stuff. But when it comes to this kind of thing, I thought I would see if someone could give some guidance, or tips on "where to start..." ? Or are there "common" problems (i.e. light sockets, etc.)?

Thanks in advance for the help.

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

if it iis an intermittant problem, [sounds that way] 1st disconnect the remote start brake lead.. get that out of the picture... replace your fuse, and try pressing the brake pedal - if the fuse doesn't blow, then the problem is withthe rs brake circuit.... if it continuies to blow, [without the rs hooked in] then pull 1 brake light bulb at a time, do the same.... chances are that the bulb socket is bad [moisture, rust , whatever] go to a bone yard and replace it. if both bulbs are pulled and you still blow fuses, then it's bettween the brake switch and the harness.. this requires you to check all the wireing from point A to point B. look for breaks in the wire corrosion ect along the whole harness... The other thing too check is any trailer light connections... again, look for corrosion scotch locks bad splices, where the connection was made... Basically a short like this should be pretty easy to find, just a bit time consumeing.. if ya need any more advice just hollar!

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Tire Proz Stillwater Mn
High End Restyling and Comlete Repair

Anymore help here?!?!?!?

Just completed some diagnostics today. I used a 12v test-light across the fuse terminals to check for the short. With everything connected, pressing the brake pedal gives me a light - indicating the short. I started to unplug the harnesses at the back of the vehicle looking for when the short went away. I eventually found that simply removing the tail/brake light bulbs, with all the harnesses still connected, made the short go away. If I left either bulb in its socket, the short still exists.

So, I'm kind of lost here. The bulbs are a 2-filament, tail-light/brakelight combination. The sockets have 3 wires running ito them. According to my service manual these wires are: brake light, tail light, and ground - - makes sense to me so far.

But what would make the bulbs short the system? I can see if one bulb was possibly bad with an internal short, but both bulbs????

Any suggestions here?

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

Like Hymer said, check your sockets. Probably something shorting out in there. Also you may want to try some new bulbs just for the heck of it.

Number one... a short is a direct path to ground...that is what causes the fuse to blow... If you ground the and of your test light and probe the brake wire at the pedal the test light should light up.. this indicates that 12v is going all the way back to the bulbs...Now your problem seems to be in the rear harness of the truck.. they are usually harnessed in parrallel.. so if one bulb goes out the other will still work...however where the 2 bulbs are connected to the main harness is probably the same feed wire.. I'm willing to bet this is where the problem is at.. If you unplug the main harness at the rear of the truck, probe the brake wire there.. you should get a light. if you do then the problem is farthere down the line (harness running through the bumper) you can check this with an ohm meter. take the bulbs out of the sockets , and measure the resistance between the brake wire and the ground wire. it should read an open [infanite resistance].. that tells you either the bulbs are the problem... if it reads 0 ohms or a low number like 100 or something, this tells you the wires themselvse have melted corroded or something to that effect [the brake wire and a ground ARE touching somewhere] take the tape off the harness and start looking for probs... or hardwire new wirs from the harness connector to the brake wire at the socket [make sure too cut out the old harness wire if you do this] be sure to check the sockets too.... look for green goop and such... this corrosion causes probs like this... god luck!

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Tire Proz Stillwater Mn
High End Restyling and Comlete Repair

Thanks so much guys. Here's an update...

To start, just so you know, the first thing I did this morning was remove some "suspect" trailer wiring connections from the rear harness - thinking this might be my problem. I then did the diagnostic work and got the results I explained above.

I went out again and did similar checks using a multi-meter across the fuse terminals instead of the test light and got some interesting results. I noticed that where before I would not get a light, the multi-meter still showed 12.2 v across the fuse terminals.

Anyway, here's what I don't quite follow...

With the fuse out (actually using a burned fuse), bulbs in and/or out, and brake pedal pressed, I measure 12.2v across at the fuse. Release the brake and the voltage goes to 0.0v. To me (mr. inexperienced) this says there is a short in the harness, no?

Now I place a 10A fuse in the block, replace the bulbs, press the brake and whala! all is well. Brake lights are ON and fuse is fine. Voltage now across the fuse reads 0.0v.

So, is this normal? What gives with the 12.2v reading across a blown fuse?

Either way, I'm hoping that the issue is solved. If it is, I will continue to think it was the funky trailer wiring that was the problem. I actually had trouble with the connections this summer, that's what made me suspect it in the first place.

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

you should NOT read 12v accross the fuse terminals... with good fuse it should read 0 between the two terminals... with a blown fuse it should still read 0 ... if you probe the fuse [blown or removed] at 1 terminal with (red lead) and put your blacl lead to ground... [metel on the body] then 1 terminal of the fuse will read 12v on 1 terminal and 0v on the other.. with the readings you got, this says you have a ground intrudeing on the rear side of the harness... it may not be a good ground, butt it is blowing tthe correct size fuse.. by puting in a biggger fuse, you will just melt wires and creat a bigger problem... I personally would rewire your brake lights on both sides like I mentioned in the previouse post... lot less trouble shooting... good luck!

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Tire Proz Stillwater Mn
High End Restyling and Comlete Repair

Thanks Hymer,

That's kinda what I was thinking as well - that the blown fuse should read 0v. So I guess I still have an issue. Right now, the fuse is holding and the light are on. But like you say, I think I'll maybe run a new wire. Even if it's just a "temporary" for now until I get a chance to do a complete tracing of the wiring.

One final thing - could the brake switch itself be a problem? I think I'll pull the plug and hardwire across to rule this out.

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

One last thing...

I pulled the brake switch plug and hardwired across the plug and got the same results, so I assume the switch is fine.

I set my multi-meter to ADC which I assume measures amps??? When measuring across a broken fuse, the reading is 3.62 Amps(?) and 12.2v. When using a good fuse, the reading is 0.06 Amps and 0.0v. Does this mean anything?

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

With a blown fuse, you should read 12 volts across it (when power is applied).

If it's not blown, you will not have a reading.

That's how those smart glow fuses work. The small bulb is connected to both terminals of the fuse, or in parallel. We know that electricity follows the path of least resistance right? Well under normal circumstances (fuse is good), the fuse has minimal resistance (basically zero). The bulb, in comparison, has a much higher resistance. It's much easier for current to flow through the fuse than the bulb, so that's what happens. When the fuse blows, the path of least resistance is through the bulb.

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My ex once told me I have a perfect face for radio.

geepherder wrote:

With a blown fuse, you should read 12 volts across it (when power is applied).

If it's not blown, you will not have a reading.

That's how those smart glow fuses work. The small bulb is connected to both terminals of the fuse, or in parallel. We know that electricity follows the path of least resistance right? Well under normal circumstances (fuse is good), the fuse has minimal resistance (basically zero). The bulb, in comparison, has a much higher resistance. It's much easier for current to flow through the fuse than the bulb, so that's what happens. When the fuse blows, the path of least resistance is through the bulb.

So, if I follow what you're saying (and I think I do), my system is functioning the way it should and I do not have a short under this symptoms???

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

Yes, that's right. It sounds like you had a problem with either the trailer wiring, or had a bulb that was shorted across.

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My ex once told me I have a perfect face for radio.

geepherder wrote:

Yes, that's right. It sounds like you had a problem with either the trailer wiring, or had a bulb that was shorted across.

All is still well, so I hope I got it. But just one little further clarification if you don't mind...

While I understand the "path of least resistance through the fuse" thing, I am a bit confused why I get a 12.2v reading when I measure across a broken fuse, with no bulbs in the sockets? To me, the lack of bulbs would mean the circuit was incomplete, and I should get 0.0v - - but I get 12.2v. What am I missing here.

Either way, like I say, things appear to be working fine, so I'm glad about that. Just trying to learn as much as I can here. Thanks.

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

That would be because there is a ground element on the [load] side of the fuse... [sometimes with the bulbs in you'll get this], sometimes not... with the bulbs out, you should get no path to ground at all.... and just get a 0v reading... but on the feed side of the fuse you should always get 12v while grounding your black lead of your meter... Glad ya got it fixed.... good luck!

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Tire Proz Stillwater Mn
High End Restyling and Comlete Repair

Wow, thanks for the quick response.

So now I'm super-confused ;-)) By what you're saying, I should get no voltage with the bulbs out, but I did - - at least I think I did. That's what kinda confused me. I guess I'll take another look and see, maybe I was imagining it...

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

just think of it this way... when you pull or blow the fuse, it's just like cutting the 12v feed wire... one side should have voltage and one side won't... pretty simple.... don't let the glow fuse thing confuse ya... just try to keep it basic....

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Tire Proz Stillwater Mn
High End Restyling and Comlete Repair

Sorry for the confusion, I misunderstood you. I missed the part about the bulbs being out. However, since you have a remote start, the wire connected to the brake wire may have continuity to ground through the unit. That's probably why you got the 12 volt reading.

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My ex once told me I have a perfect face for radio.

geepherder wrote:

Sorry for the confusion, I misunderstood you. I missed the part about the bulbs being out. However, since you have a remote start, the wire connected to the brake wire may have continuity to ground through the unit. That's probably why you got the 12 volt reading.

Actually, the RS should be out of the equation right now, because I disconnected the brake pedal lead to the RS to help with the trouble-shooting.

But now that you mention it, there is one other possibility. The brake pedal is also interlocked with the safety switch that releases the console shifter (to take it out of PARK). I'm guessing that's where the ground is. Not sure why it took me this long to figure that out - - it seems so obvious. I guess it's that "can't see the forest for the trees" thing.

Thanks for ALL the help - - this place is a HUGE help!!!!

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Zilverado,
Backyard Mechanic
1986 Chevy Truck & 1992 Nissan Pathfinder

Finding a Short