No taillight, good bulb

[schrocketeer]

Would you please help me with a taillight issue, as in no taillight?
I've got a good bulb, tested with a 12 volt battery charger as a power source. Both brake lights work, so that makes me think the shared Ground is good.
When I checked the voltage at the flat black connector inside the taillight assembly, I was only getting 4.0 Volts DC, inconsistently.
Everything else (turn signals, headlights, brake lights) works fine.
New battery (January), kept on a Battery Tender trickle charger.
I've put about 600 miles on it in the last two weeks, after a barn find project starting in January.

1985 K100RT

[Inge K.]

When I checked the voltage at the flat black connector inside the taillight assembly, I was only getting 4.0 Volts DC,

What do you measure at fuse #2, and does the front park lamp function?

[Scott_]

You might have a dirty connection at the BMU plug in the electric box(under the fuel tank).

"Barn find" tells me you should check any/all connections for corrosion.

[schrocketeer]

Second fuse from the top (assuming that's #2) was a 7.5 amp in good shape.
No front parking lights, with the key in either position, or the engine running, or not.
I'll check the Bulb Monitoring Unit next. When I had the tank off, I sprayed de-oxit over ever relay in that box, but I did not pull every component and spray all the pins/blades and receptacles.

[Inge K.]

Second try......

[schrocketeer]

No parking lights (front or rear), no flashers (front or rear), but the turn signals (front and rear) work fine.

Am I right that pushing the flasher hazard button on the four switch pad below the instrument cluster should give me car-like emergency flashers?

I'm not sure that I notice anything happening at the first click of the key.

From the schematics, I see that Fuse #2 should be a good test point. I got sidetracked replacing odometer gears this weekend.

[Laitch]

Am I right that pushing the flasher hazard button on the four switch pad below the instrument cluster should give me car-like emergency flashers?

It will four-way flash if the ignition switch is in the On position. When the flashing starts, the ignition switch can then be turned to Off and the flashing will continue until the hazard switch is turned off.

[schrocketeer]

Hopefully the spreadsheet pastes, but across Fuse #2 (counting from the top) the voltage was 0.11 DC with the key in the On position, but the engine off. Starting the engine, the voltage went up to 0.2 Volts. The voltage was the same (0.2 VDC), with the engine running, whether the Flasher were on or off.
If the Bulb Monitoring Unit was acting up, are these the kind of low voltages you would expect?

[Inge K.]

Does the instrument backlight function?
Yes, the problem is in the ignition switch or its connector.
No, the problem is in the connector for the RHS switch assy (if the headlight functions as normal).

[schrocketeer]

Inside the key switch, I jumpered the gray and gray/blue wire and got my taillight working!
There are some solder globs in there where the wires connect to the connector from a previous owner, so I'll clean that up, and also de-oxit the contacts inside the key switch itself.

Thanks for your help! :-)

[LeMoeur]

Had the same problem: no park light but headlight on and blinkers working. Strange fact was that parking light was working when i turned my key past the steering lock position (that could be a useful test). Long story short, i dropped a little piece of copper while disassembling my ignition switch plastic case that was needed to close the contact when the key was in "ON" position  :2thumbup: