Some quick background:

2003 FLHTCU (Dad's) and I noticed that his brake lights weren't working on the tail lamp or the Tour Pak.

When using either front or rear brake a click from the relay can be heard and felt.

Of course, start with the simplest items: checked the fuses and checked & tried new bulbs (one of them had a broken filament) but still no brake lights. Then next I whip out the factory manual wiring diagrams and do my homework. (I must admit the lack of color ink on the wire traces is disappointing)

I pulled the brake relay, jumper'ed the brake light to the ground and all lights worked fine this way. So either the relay or something in front of it.

(After we called it a night I saw another post that said to use the fuel or starter relay, didn't get a chance to try this, will attempt but I have a question below)

Next I pull out the voltmeter and check the two smaller leads into the relay (85 and 86) with the relay still removed.. These are the ones that are supposed to energize the relay and connect the ground to the brake lights when either the front or rear brake switches go. For some reason, these two leads were showing ~10.3 volts. When either the front or rear brake switches would be engaged (brake applied) the voltage would go down to ~3 volts.

Is this correct operation of the wiring to the brake relay? I don't think it is. This is with the brake relay disconnected. I supposed next I'm going to try with a relay connected and see if the coil draws the voltage closer to 0 volts as maybe the above voltages are just small milliamps not enough to energize the relay.

Thanks

 

Found the problems. Needed a new relay and replaced the Accessories fuse.

Turns out that I must've blew the Accessories fuse somehow when I was testing out the relay because I know those turns signals worked before I pulled the relay.

With a working fuse but with the relay still unplugged, the initial voltage is 10 volts and then goes up to 12 volts when either brake switch is activated. With the blown fuse the initial is 10 volts but goes down to 3 volts, so that explains that.

Testing the voltage with the new relay plugged in, the initial voltage is less than a volt so that coil relay absorbs the less then 10 milliamps that I measured.

Everything works well now!