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I just picked up my first CRF450X (titled, with the brake light on a switch already). When I picked up the bike, the rear brake light did not work. So I ordered a new one and wired it up and it still won't turn on. I checked the only (how many fuses are there)? fuse that I am aware of. What should I be checking next?

I've measured the following voltages at the brake light with the bike off:
Red wire: 13V
Yellow wire: 13V
Black wire: ~8V

From the best I can tell, the red wire runs thru the brake light switch, connecting to a yellow wire on the other side. The yellow wire splits off to the brake light and to what LOOKS grey (to be honest, the grey and black wires look similar to me). The black wire looks like its connected to the black wire...

What should I be checking next?

I just picked up my first CRF450X (titled, with the brake light on a switch already). When I picked up the bike, the rear brake light did not work. So I ordered a new one and wired it up and it still won't turn on. I checked the only (how many fuses are there)? fuse that I am aware of. What should I be checking next?

I've measured the following voltages at the brake light with the bike off:
Red wire: 13V
Yellow wire: 13V
Black wire: ~8V

From the best I can tell, the red wire runs thru the brake light switch, connecting to a yellow wire on the other side. The yellow wire splits off to the brake light and to what LOOKS grey (to be honest, the grey and black wires look similar to me). The black wire looks like its connected to the black wire...

What should I be checking next?

Welcome to the site !!

I am surprised no one else has replied yet.....

Off the top of my head ....which does not mean i am 100% correct , but from wiring basics , this "should" be correct

There should be 1 wire going INTO the Brake Switch ,the wire going TO the brake light "SHOULD" be fused somewhere in between the Battery and the Brake Light Switch , and ONE going out from the Brake Light Switch TO the Red Wire on the Brake Light itself (generally the brake light wire is Red) , the other colored wire on the Brake Light itself should be the Tail Light (generally a Yellow wire) , and NOT connected to the Red wire , but should tie into the Tail Light wiring of the stock harness going to the fender (it "should" be a Gray wire), the Black wire should be Ground ONLY and should also tie into the stock wiring harness to the fender , it should not be connected to anything else except to a ground point somewhere , preferably at the stock harness to the fender (it "should' be a Black wire) ....hope that helped

It did help some...I'm 90% certain I'm dealing with a bad ICM (I just bought the bike... ) but regardless, I think I know a way to verify that the ICM is bad (to my knowledge, there is no good way to verify that the ICM is bad other than the process of elimination). Disconnect/cut the ground wire of the tail light (black in my case) from the grey wire (presumed ground wire - which routes through the ICM). Take a random piece of wire and connect it between the tail light wire to chassis ground. If it lights up..well, than your ICM is bad. I haven't ordered a new ICM to confirm my suspicions, but I will update this post when I receive a new ICM and verify that this is the issue.

Here is a final update (this is for future people with an ICM issue..).

A new ICM fixed the brake light. In conclusion, here is how to verify a bad ICM quite easily. Disconnect the ground wire connected to the ICM module (typically a bullet style, should be able to pull apart). Touch the ground wire to any part of the chassis, and if the light turns on you have a bad ICM.

This works because CRF450x (2005-2014, at least) uses a ground interruption to turn on/off the tail light. If your ICM is bad, you have no ground.