School Bus Short Circuit

imported_John Whelan - November 13, 2015
In the video you heard the fuse that was popping off every few seconds. That is a short circuit or more commonly called a 'dead short' directly to ground. The fuse resets itself so it's handy to have that plugged in so we can check wiring exposed around the printed circuit board where all this is happening.

NOTE: Thomas had a recall years ago on replacing all of the resetting fuses to regular ATO fuses that require replacement instead of leaving the reset fuse in the panel. Why Thomas Buses had the recall was they were seeing some of the reset fuses fail from shorting out due to high heat. We are using the reset fuse in the video just for troubleshooting.

The first challenge is finding the electrical schematic for this year of bus. It's a 2002 HDX Thomas pusher so getting the correct wiring diagram is essential. I phoned Thomas built buses tech support and they are always very helpful especially with wiring diagrams. I tried to find the schematic in the but could not find anything on PCB #4.

Tech support sent me the correct schematic by email and once looking that over the fuse or F3 as they call it on the circuit board is the ignition supply to the ignition relay. the relay I pointed to in the video only has the amber light on. This means there is battery feed to the relay but the green LED beside it indicates no output from the relay.

The output from the relay feeds the ignition supply output voltage and there was none at all. The schematic also showed us that there was a diode between the ignition stud and ground on the PCB. After checking for ground and power it was clear the diode had failed allowing battery voltage from the ignition stud to go directly to ground.

The printed circuit board had obviously failed and luckily has a part number stamped right on it. After our Thomas parts supplier found the superseded number they put in an order and informed me that there was one in Memphis, Tennessee. This is not a surprise so I asked for air freight and usually it's not that much more to get it up to us in a matter of a few days.

So now we wait and park this bus until the circuit board arrives. There's a warehouse 8 hours away in Alberta but since this board is not a regular requested part it will be a bit longer to get it here. The cost is around 350.00 before shipping and tax (Canadian) which is not an extravagant price for what we're getting.

Just another experience under our belts. It all gets recorded in our memories for next time. Once you here the complaint and see what exactly is happening it's nice to realize we've gone through the same scenario before. I love it when that happens!
thomas school bus