Totally stumped by a brake light

Deschodt · 06-03-2015, 05:56 AM

I usually don't cross post but I got nothing on the 912 bbs, I'm going crazy, and this is not at all 912 specific...

My passenger side brake light doesn't work at all (double filament, works for lights but not brakes). Bulb swapped fine on the other side, and I have 4 new ones anyway, not the bulb. I looked at the wiring diagram, I do have continuity between the driver side brake light (working) and the passenger side (not), good. I seem to have power on the bad side brake connector: 9.9V or so - yes that's less than optimal 12 but I should at least get a glow, shouldn't I? (I don't recall Voltage on driver side, did too many things to recall)

I have ground as well, at least I think I do... I checked ground continuity between the car (taillight screw hole) and the bulb socket on the taillight assembly - (beeeep) good. Besides tail lights work, same socket. I checked continuity also on each side of the tail light assembly (front and backside) to rule out a break in the contacts, good on both pins (tail light and brake light), and ground around the socket hole.

In frustration I pulled the ground connector and the brake wire from the tail light assembly altogether, and directly measured Voltage between the 2 with brakes on: 9.9V. But get this: if connect a good bulb to those ground and live wire directly (wife still braking) -> nothing! Huh ??? How on earth? I inserted an extension wire in those connectors (in case of oxidation on the outside of them) -> nothing.

Same bulb, grounded on battery (-) via its body and getting power to either underside contact = light ! So I know I'm testing it right (bayonet body is ground, connectors are power).... I don't get this - at all !! How is it possible to read 9.9V power between a ground and + wire, and not get a light when touching them to a bulb, but do the same at the battery and get a light ? Would 9.9V not light up a brake filament ? I suspect the ground but if I can get some voltage reading and continuity between the ground cable and the car, and enough ground for a tail light, how can it be ? I'm stumped...

T77911S · 06-03-2015, 06:28 AM

get a test light.

check the pin of the socket on both sides. do both so you know what to look for. ground the light to bare metal.
then connect the test light (TL) to 12v and probe the ground side of the socket, both sides.

work your way back to the brake light switch. if it is like the 911 there is a round connector on the side of the engine bay for the rear lights. check it there next.
you dont really need to know which pin is the brake light. check the working side first, then the bad side. again check both power and ground seperately. (unless the ground worked from the socket, then no need to check it).

MM's really are not good for this. thye can measure 12v and still not have a good enough connection to provide the current to light the light. your goal is to light the brake light so so use light to troubleshoot

Deschodt · 07:03 AM

Quote:

Originally Posted by T77911S

get a test light. MM's really are not good for this. They can measure 12v and still not have a good enough connection to provide the current to light the light.

OK, that's the part I wasn't getting I guess... Because otherwise it makes no sense ;-)

I did all the pin testing on both sides, I ruled out the M/C because I have ONE good brake light and the bad one piggy-backs on the good one. I'm kinda ruling out ground because I have ground for the tail light to work (same bulb even)! So it should be power to the second filament, brake power. Despite the 9.9V reading (which really should be 12 so that's a hint)

I'm going to re-measure power at the working brake light to confirm, and run a spare wire from it to my bad one. "If" that is the issue, how is that wire routed from one brake light side to the other normally? The wire harness for tail lights just dissapears in the body, it's hard to track !

timmy2 · timmy2's Garage

I believe for the '68 the black/yellow brake wire runs to the drivers side tail light first and then doubles back through the harness over to the passenger side.
Problem may reside in the connection where they are spliced together in one connector on the drivers side.

Deschodt · 06-03-2015, 07:35 AM

Quote:

Originally Posted by timmy2

I believe for the '68 the black/yellow brake wire runs to the drivers side tail light first and then doubles back through the harness over to the passenger side.
Problem may reside in the connection where they are spliced together in one connector on the drivers side.

Yes, that is my hope. Now that MM were explained to me, that's the only logical explanation. Hopefully just the contact at the driver side, because routing another wire sounds like it would not be fun ! Thanks guys, that really helped !

DRACO A5OG · DRACO A5OG's Garage

If like the 3.2, the assembly at the sockets are put together by PCB type plastic that may have deteriorated and or fractured causing a short or poor continuity. All the years of pushing in new light bulbs takes it toll on the plastic especially if PO pulled on the contacts thinking that will re-fresh the contacts. This actually makes thing worse, liken to bending a brittle piece of tin, it will eventually break.

If I may suggest, disassemble the light fixture and carefully look behind each socket and check the ground connection behind the back cover. I bet this is where you will find your issue.

To confirm my theory, check the voltage going to the assembly from the rear of the light fixture. As stated above, by timmy2. If anything you may have a shorted wire in between the two fixtures.

Jim

rick-l · 06-03-2015, 09:15 AM

You must be using an analog voltmeter (the kind with a needle pointer) with a low input impedance.

Here is the physics behind why it reads 9 Volts

Deschodt · 06-03-2015, 09:27 AM

No it's digital, but I believe you are right on the concept... Shorted/damaged wire between the 2 fixtures is indeed the most likely. I hope I can replace that easily...

I think I was troubleshooting OK all in all, I even checked the driver side, all the continuity at the conenctors, but I got foiled by my multimeter reading proper continuity and 9.9V - I'd have expected no continuity and no Volts for an easy find. Now I know to use a test light instead for this !

rick-l · 06-03-2015, 09:43 AM

With a digital multimeter it could be like this but I am having a hard time coming up with an element that would read less than 10 Ohms when the meter passes current through it and 2.5 Meg at very low current. Must be something chemical in nature.

Deschodt · 07:57 PM

Fixed ! As you guys said, apparently a wire *can* show 9 or 10V per a multimeter but fail to carry any load with a light bulb at the end of it... Also that same wire *can* show continuity (presumably via one last good strand of copper) despite being absolute garbage at lighting up a bulb... Lesson learned ! I spliced in a new wire between the plugs that go from the engine compartment to the tail lights. I'm not moving and opening a 46y old brittle harness. For now I routed the new wire under the closing panel as a proof of concept. Since I used new connectors, this w/e I'll take the time to reroute a longer wire next to the existing harness path in a less visible color and connect that - 5 minutes job...

Yeah, 2 bright brake lights ! Lesson learned: I'm gonna get me a nice test light, the multimeter CAN lie!

DRACO A5OG · DRACO A5OG's Garage

Outstanding!

T77911S · 06-04-2015, 02:41 AM

Quote:

Originally Posted by Deschodt

Fixed ! As you guys said, apparently a wire *can* show 9 or 10V per a multimeter but fail to carry any load with a light bulb at the end of it... Also that same wire *can* show continuity (presumably via one last good strand of copper) despite being absolute garbage at lighting up a bulb... Lesson learned ! I spliced in a new wire between the plugs that go from the engine compartment to the tail lights. I'm not moving and opening a 46y old brittle harness. For now I routed the new wire under the closing panel as a proof of concept. Since I used new connectors, this w/e I'll take the time to reroute a longer wire next to the existing harness path in a less visible color and connect that - 5 minutes job...

Yeah, 2 bright brake lights ! Lesson learned: I'm gonna get me a nice test light, the multimeter CAN lie!

i use to do auto electric work and i have tried to encourage people here to use a test light instead of a MM. i have had great resistance from some. there are those that think they HAVE to know what the voltage is and as you have found out, 9.9 volts can lead you down the wrong road, and i have even seen others read 12v and dont know why the light does not work.

save the MM for working on the charging system.

DRACO A5OG · DRACO A5OG's Garage

So what is a good test light? I am not looking to break the bank but something decent.

TIA,

Jim

universeman · universeman's Garage

I use this one. $7

Electricity Tester 6V 12V Voltage Tester Pen Light Probe Driver Handle Repair | eBay

DRACO A5OG · DRACO A5OG's Garage

Oh, I have that. I thought we were supposed to use a LED type, no?

DaveE · 06-04-2015, 03:01 PM

No. There again an LED pulls such little current that it could fool you on a troubleshooting exercise like this.

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MBAtarga · 06-04-2015, 05:46 PM

You got a Horrible Freight store local? 12ft cable, $6

Extra Long Circuit Tester

rick-l · 06-05-2015, 09:03 AM

Quote:

Originally Posted by DaveE

No. There again an LED pulls such little current that it could fool you on a troubleshooting exercise like this.

We don't really know. The original poster was using a meter that draws micro Amps to test a circuit that typically draws 2 Amps. Is 10 milli Amps good enough? Is 100 better?

I think you have to look at what you are testing and interpret the results of what you see in each case. That is there was an anomaly in his test results