dead short on brake light circuit - i need help.

[johnceggleston]

any one else had this problem?????

 

97 GT w/ej22, auto trans, 165k miles, red wagon, needs tires, clear coat peeling

 

i have a pretty good understanding of electricity, more 120v than 12v,

but i really do not have the patience to track down electrical problems.

but i can't afford to send the car to the crusher either.

i have been putting this off for a few weeks while driving my son's 98 obw.

 

i blew the fuse for my brake light circuit, partially melted it.

this fuse also controls the trans shift lock out and the horn.

(i experienced this a few years ago but it corrected it self, go figure. i'm thinking loose wire?????)

 

my first thought was my trailer lights connection, but no. still there after removal.

i pulled up the harness diagram and started with the center brake light at the top of the hatch.

undo the connector at the light and check continuity on the harness between the black  and the white/blk, dead short.

it acts just like i touched the probes to each other.

(i'm ASSUMING the black is the ground and the white/blk is the power???)

 

i disconnect the harness at each brake light fixture and have the same results.

so far the problem is up stream of the actual bulbs, i think.

i then put the hatch back together and re-connect all the fixtures.

oops, i didn't check the connectors at the chassis brake lights.

 

next i disconnect the chassis harness from the ''hatch'' harness. (under the right rear tail light )

hatch harness has black and white/blk wires,

(there is no other connector between there and the connectors at the hatch bulbs.)

chassis wire colors may be different, i'll have to re-check and edit.

i looked for a broken wire where the harness jumps from the chassis to the hatch, it all looked good.

my rear wiper motor has never worked but the motor is good,

i expected to find a broken wire at the crossover, jump point.

 

but here is where it gets squirrelly.

BOTH sides of the connector, both harnesses show the white/blk wire direct to ground.

the hatch harness shows white/blk as ground,

and the pin it connects to on the chassis shows the same.

 

i must be doing something wrong.

or my logic is wrong,

i know i didn't let the white smoke out of the meter.

 

do you see any obvious mistakes?

is there a chance the white/blk is actually the ground?

every other black wire shows ground.

the brake light bulbs SHOULD show some resistance, right? or wrong?

i guess i should remove all of the brake light bulbs and start over.

i'm lost.

 

any suggestions?

thanks.

 

ps: i ordered wipers from rockauto last month and threw in a brake light switch for $2.45,

i guess i could install that as well?

Edited February 2, 2016 by johnceggleston

[lmdew]

Get the Factory wiring, sometime you can find them online.

Load,

Ground

Power

 

Once you have all three located you can test the circuit.

I've stripped a few cars of your vintage, if my memory serves me correctly, all the wiring runs down the passenger side door frame, lower rail.

I believe there is a main connector up at the passenger side front kick panel as well.

 

Find a good diagram, otherwise your just chasing your tail.  You have done some good testing so far.

Good Luck, Larry

[porcupine73]

I don't know if this will be useful at all but just brainstorming a couple thoughts... if you remove one brake light bulb and check the resistance between the wires on the harness I am going to guess that is in parallel with all the other brake light bulbs and anything else that comes on with the brake lights. What resistance do you see with your meter if you check just the brake light bulb itself when it's out of the holder?

 

I don't know how many watts the brake lamps are but if they're say like 12 watts each and there's three of them in parallel that's only going to be like 1/3 ohm total, so on a typical meter I think it's going to be hard to differentiate between that and pretty much a dead short. If using the beepy continuity check feature on the meter it's definitely going to beep for that.

 

I know over the years there's been posts of people with bad bulbs like where the filament brakes and then shorts out in the bulb causing issues or issues with the bulb holders themselves causing a short like stray/frayed wire inside where it makes contact with the bulb. Does it have power mirrors and do those work? I know there was an issue on some of those years with something getting pinched in the drivers door that could cause a fuse to blow but I don't know if that was the same that feeds the brake lamps.

[Fairtax4me]

Most of the time, the hatch wiring gets broke somewhere near the flex boot where the hinges are. Not always within the boot, it could be a few inches either side. 

 

It could also be due to a shorted socket. That is harder to determine because you have several sockets wired in parallel. 

 

Disconnect the hatch wiring from the body wiring. Remove and unplug all of the brake lamp bulbs. Then check each wire for the brake lights in the hatch for continuity to the hatch itself. (scrape some paint off one of the bolt heads to get a clean spot if you need to). 

If you get continuity to the hatch, its a short to the hatch.  

 

If the wires only show continuity to each other, then they're either shorted to each other, or one of the sockets is shorted. Some sockets have connectors and the socket itself can be unplugged from the harness. If they do, unplug them one at a time and test each socket individually. Sometimes you can pop the sockets open and pull the pins out, but usually not.

If they don't have connectors on them, and no way to pull the pins out, the easiest way to check them is to clip one wire at each socket, and test just that socket for continuity. After testing the sockets, if you don't find one offending, retest your harness wires for continuity to each other, and you'll likely find the harness is damaged somewhere. Again here, damage to the harness is usually somewhere near where the harness flexes. If the harness is excessively difficult to remove, there's always the option to run new wires from the harness connector to the sockets. 

[johnceggleston]

thanks guys.

it makes more sense to me now.

isolate the harness from the sockets and test it.

add back sockets one at a time until i find the fault.

 

as much as i may have been fumbling yesterday,

i feel like i now have a direction.

time to buy a six pack and turn on some music while i work.

 

thanks.

[Cougar]

I think Fairtax4me has the right idea also. The story you are about to read is true.

 

My son had an Outback that would blow the fuse for the brake light anytime the pedal was pressed. The short turned out to be due to the wire insulation melting in the back of one of the light sockets due to heat from the bulb. It was a good opportunity to show him how to find the trouble using an ohmmeter connected between the fuse socket and ground and watch the resistance change as light sockets were disconnected. When the resistance went higher, to a reasonable reading, you knew you had disconnected the socket with the trouble on it. 

[Bushwick]

If someone inserted an incorrect bulb or forced an incorrect bulb in, it can cause the wrong filament to light i.e. turn running lights on and it lights the brake filament, or both filaments act like a single bulb due to a shorting across the bulb's lead contact points. 

 

I've seen that often where someone might have 2 physical bulbs per side, 4 will light like running/parking lights, and outer most bulbs will be dual filament and also light as brake lights, and while they just have running lights on, one of their brake lights will be illuminated as well. Over hours and hours of constant use, that can melt plastic or even deform the housing on some cars. In some cases I've seen single filament forced into a dual filament holder, so any time I buy a car and a rear bulb is out, I'll pull all bulb connectors out and inspect/replace everything in pairs, making sure to adhere to what the car calls for. When I got my Legacy, it had 2 lights out and one of the bulbs was incorrect for the car. Worth ruling that out.

 

Also, does the fuse blow when lights are on? Or is it blowing once the brakes are applied? I've seen the actual switch that's connected the brake pedal fail on a handful of cars over the years. Normally they fail in the ON position, causing the brake lights to remain lit even after car is shut off. Suppose it's possible the switch could break/crack, and might short out to the actual pedal or whatever the switch is attached to. If that happened at the switch, I imagine it'd be hard on the fuse. Worth a check.

[Fairtax4me]

Any luck John?