Im sure bad ground but ill throw it out here

[justinc1081]

Engine running, when pressing the rear brake pedal the headlight dims..
The front brake light switch does NOT cause the headlight to dim, both switches light the tail light.
Anything other than a bad ground cause this?

[TopCatGr58]

Hey Justin,

It may NOT be bad grounds as much as overall bad connections throughout the harness. The charging wires....from the ALT to the harness behind the fuseblock backplate, the fuseblock itself, the harness connectors for the lighting system, but also the grounds for the reg/rect assembly under the tank, as well as the engine/frame grounds...both at the (-) of the battery as well as the other one at the starter motor engine to frame strap below the carbs. The OEM style brake lights are 27 watts each...and I think you have a special that uses 2 of them, so 54 watts when you hit the brakes.... and using 12 volts...some use the 13.5 value...divided into the watts will give you the AMP draw ~ 4 amps...the max output of the XS Alt is ~20 amps...the XJ ~25 amps. The headlight is also drawing ~4-5 amps, then you've got the ignition system as well as the other lights. And if you're slowing down...ie using the brakes, you may also be off the throttle, so low rpms....low charging output....loosing "smoke" from the headlight circuit when hitting the brakes....headlight dims!

Clean ALL of the connectors, especially inside the headlight, check to make sure your ALT is charging properly at the ~14.5 volts at ~2500 rpms.

T.C.

[DiverRay]

The rear brake switch is between the engine and swing arm. The wires are just above the swing arm, and MAY have rubbed against something. If so, the ground is MUCH better when you put on the rear brakes, and dim the lights.
Also, if you are at idle, the lights WILL dim a bit when you put on the brakes, as the alt really is NOT charging.
Other than that, T.C. covered it. check ALL connections, and use no-ox or anti-seize on ALL ground connections.

[crazy steve]

I'll go against the grain here and say that it's likely a bad rear brake switch. Unplug the switch, and check with a ohmmeter with the switch open and closed. Also check each wire to ground. It should read 'open' between the two wires when the switch is open, and zero ohms between when closed. Both wires should read open to ground either way. I doubt it's a simple ground problem if it only does this when using the rear brake switch.

Of course, cleaning all electrical connections on these bikes is always a good idea if it hasn't been done....

[justinc1081]

Thank you all, very good ideas.
Bike was at idle speed, i just thought it was weird that when rear brake switch lit the brake light the headlight dimmed a little, vs the front brake switch lit the brake light did not dim it.

Still cleaning up things on the bike, may get to put it out on the street this weekend as long as the new front caliper seals come in

[ViperRon]

I have had the same problem and the brake switch was bad internally. I would also check all wire grounds and connectors as it can save having wires and connectors melt when you are on a trip.