School Bus Wiring Problem - RH Signal/Marker Lights

Mechanics Hub - August 2, 2014

The video explains what is going on with the right hand side signal and marker lights on this Thomas C2 school bus. It just happened out of the blue as with all electrical faults so we needed to find out where the wiring originated and see if there was any output from the source.

I used the website to find the wiring schematic because it was a body issue. If it was a chassis problem we would check out the website. I know it's a lot of searching going on with websites but that's reality with today's technology.

After finding the wiring schematic as you see in the video we know the dark green 16 gauge wire comes out of one of the expansion modules. The expansion module is how the multiplex system adds on extra circuits such as the lighting in question but the advantage we have is the failed lights are the only load on this specific wire.

This is a good engineering tactic Thomas used when designing these wiring systems because everything is separated from each other so if one light shorts out it doesn't take down the entire lighting system. Smart thinking I would say... it's a way to protect the electrical system.

So we found there was power out of the expansion module but nothing in between that and one of the body connectors. We jumped power past the connector and the lights came on so all that had to be done was to connect a jumper wire to bypass the faulty area in the harness.

Believe me you don't want to get into trying to find an open or corroded wire in one of these harnesses because it will take you a lot of time to dig and prod through the menagerie of wiring sections that make up these school buses. We've been there and done that enough times to realize that it's much easier to run a new wire.

Without the wiring diagram we would have been up the creek without a paddle so if you own some of these bus models get a hold of your Freightliner dealer and get a user and password for both of these websites. You will have a much easier time troubleshooting this stuff in the long run. Thanks for reading and please leave a comment below.