Puzzling electrical behavior

[guitardad]

The bike in question is a '96 R1100RS, that I helped a friend with this past weekend. It had been sitting for about a year, and we cleaned up wiring. It had several electrical accessories installed - a Kisan Signalminder, LED taillight and brake light, extra LEDs on the luggage rack, and a Centech fuse panel with power switched thru a relay, triggered off the taillight wire. Here's the weird part......

The LED taillight did not work. When we pulled the wire off the taillight, we could measure +12V on the wire. But plug it back in, and the voltage went to 0V. The Centech relay would work fine with the LED disconnected, but would not energize if the LED was plugged in. I plugged in the standard incandescent taillight from my '94, and it worked fine. Seems like just a bad LED, probably shorted out, right? But then we tried this sequence:
1. Plug in LED - no light, no power at the connector, Centech relay does not energize.
2. Power off, plug in incandescent light, power on - no light, no power, no relay.
3. Power off, nothing on the end of the taillight wire, power on - got +12V, relay energizes OK.
4. Power off, plug in incandescent light, power on - light on, go +12V, relay energizes OK.

We repeated the sequence several times. Trouble always started with plugging in the LED. If we went straight from LED to incandescent, it didn't work. If we powered up with nothing on the wire in between, it worked fine. It acted like the LED triggered some sort of circuit breaker, that would reset if we powered up with no light at the end of the wire. I don't see anything in the wiring diagram that would cause this behavior. Any ideas?

[genen8kua]

Most peculiar, Chaz. Did you ever give it just a bit more time between the LED and the incandescent - it sounds as if you tripped a self-resetting breaker (but there ain't such a thing on that bike, at least in the relevant wiring diagram in my Clymer manual).

I do agree that it sounds like a short in the LED (did you check it with an ohmmeter?) that would bring the potential across the relay close to zero, but should've tripped some protection (fuse or breaker) or else generated some real smoke! Fuse 2 should be protecting the tail light circuit, but it's a FUSE, right?

It makes me wonder if there isn't a corroded connection somewhere in the circuit and heat is making it behave strangely (a natural semiconductor if you will). Of course if the LED is shorted I'd stop plugging it in.

[dbrick]

I agree with Gene: there's a bad connection somewhere.

Did you try connecting the LED directly to the battery, bypassing the bike's wiring harness?

[MOTORRADMIKE]

That is a bit baffling.

If the LED is wired backwards it would be almost a short. Wouldn't light either and you haven't said you ever saw it on.

I'd check the original tap into the tail light wire for the Centech relay. It should be a properly soldered splice, not one of those crimp through insulation abominations.

Lastly, if you have a DVM, maybe post voltage readings with at least one decimal place rather than 'got power' or 'no power'.

My guess is:
LED is shorted
Original splice for Centech is high resistance.
Tail light is now wired downstream of relay.


Good luck!

[MOTOR31]

It sounds like there is a diode in there somewhere that is wired backwards to the LED. Since a diode will only pass electricity one way it would explain the symptoms you have.

[guitardad]

I'll take some measurements next time I get back to the bike. At this point, both taillight and brakelight LEDs have been replaced by the original incandescents. Everything is working fine right now. I'll let ya'll know what I find out.

[PETDOC]

Are you sure the LED taillight is compatible with that model BMW's electrical system?
Only reason I ask is the first LED taillight I got for my '04 didn't work, but the company subsequently made one with (I believe) greater resistance that looked identical and worked fine.

[rt1to3]

Does anyone know how to hook up rear moto lights on this bike? I certainly do not want to mess up the computer system.

I had a local shop add the moto lights to the front, but wondering if I can do the rear lights myself.