Brake Light Problem

[mirion]

I have 1997 Standard. I was told my brake light wasn't working so I picked up a bulb. It isn't the bulb. (Tail light filament does work) Previous owner wired the rear turn signals into the brake circuit so those also light up when the brake is applied and those do work so it isn't either brake switch. When I put a voltmeter on the bulb socket it shows 5 volts when either brake is pressed, zero volts when released. (The tail light filament shows 10 volts.) Not sure how many volts are supposed to show at the brake light. Where would you start looking for a crossed or broken wire?

[Valker]

At the socket.

[RonW]

Previous owner wired the rear turn signals into the brake circuit so those also light up when the brake is applied and those do work so it isn't either brake switch.

• Where would you start looking for a crossed or broken wire?

If you're able to locate the crimp/solder where the rear signal wires tap into the brake light wire, I would inspect that joint. Possibly the brake light wire downstream from the crimp is severed at the crimp and not able to send electricity to the hi-filament in the taillight bulb.

Anyways since the rear signals do function as brake lights, but the taillight doesn't function as brake lights, the culprit is on the length of brake light wire somewhere between the socket and the rear signals' crimp.

Are the rear signals 1157 (dual filaments)?

[Jims99]

I could possibly be the ground wire. If you’re getting some voltage (back feed) goes away with brakes, sounds like a bad ground wire. Check all connections, should be a simple fix.

[RonW]

(Paraphrasing) "voltmeter on the bulb socket shows 5 volts when either brake is pressed. Not sure how many volts [should be]. The tail light filament shows 10 volts. Tail light filament does work."

The bulbs are 12V. I don't know how low is the minimum voltage threshold for the hi-filament to still light up. For example, 10V for the tail light (low-filament) apparently works despite the low-filament rated at 12V.

5V is probably too low to light up the hi-filament. 5V indicates there's something wired inadvertedly in series with the brake light consuming 7V. (12V minus 7V leaving the brake light with 5V, if that makes any sense.)

Even if the brake light wire is partially severed, it shouldn't lower the voltage, amperage maybe.

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Previous owner wired the rear turn signals into the brake circuit so those also light up when the brake is applied .....

Did the previous owner use a Run/Turn/Brake electronic module?

[CamTom12]

Corrosion in the wires can cause a voltage loss…

[mirion]

(Paraphrasing) "voltmeter on the bulb socket shows 5 volts when either brake is pressed. Not sure how many volts [should be]. The tail light filament shows 10 volts. Tail light filament does work."
5V is probably too low to light up the hi-filament. 5V indicates there's something wired inadvertedly in series with the brake light consuming 7V. (12V minus 7V leaving the brake light with 5V, if that makes any sense.)
Did the previous owner use a Run/Turn/Brake electronic module?

Yes, that is the thing that stumps me. Some voltage (5V) is getting through but not enough to light the brake light. There is not a Run/Turn/Brake electronic module.

I have been away but will report back when I get to the bottom of it, thanks for all the suggestions.

[da prez]

  My usual rant is to verify the ground ,all of them. Dis-connect the plugs , clean both ends and make sure the connections are tight. I (my preference) use silicone dia-electric on the plug ends. It makes an air / water tight connection. If you have 12V at the switch , then you need 12V at the light.

                                            da prez

[RonW]

There is not a Run/Turn/Brake electronic module.

If a RTB electronic module wasn't used, then the PO tapped into the brake light wire usually with a vampire tap (Scotchlok™). Scotchloks aren't suited for vibrations as the vibrations will cause the Scotchlok teeth to chew into the strands including the donor wire. The brake light wire. Perhaps inspect the Scotchlok tap if present. A problem is you might damage the wire in the process of prying open the Scotchlok as some of the strands are barely hanging on.

[CamTom12]

The good news is that it’s a simple fix. Buy some “solder sleeves” (can order from Amazon or find in some auto parts stores).

With those in hand, cut the wires where the scotch-loks are installed, put a sleeve over one side (like heat shrink tubing) twist the junctions together, slide the sleeve and center over the bare twisted wires, hit it with a heat gun until you see the solder inside the sleeve flow.

I’ve used a lighter (carefully) in places I couldn’t squeeze my heat gun.

You could do the same repair with crimp connectors, just do a “Y” out one side. Not as nice as solder sleeves though.

[Chrisj CMA]

Check the connection in the wiring harness under the fender where the two halves of the rear fender join. It could be loose.

[h13man]

With the different voltages I'd be looking at a ground and corroded sockets.