Thomas School Bus Electrical Problem Circuit Board

I’ve seen a lot of electrical problems over the years and this one is the first with ghost voltage connecting to a relay for the running lights. As described in the video the printed circuit board has all of the fuses and relays mounted on it and connects everything with internal wiring circuits. The LED system indicates which relays are on and off with the use of amber and green LEDs. In this case there was battery voltage going to the energizing coil in the relay feeding the running lights with all of the power turned off.

When I disconnected the relay from the circuit board the lights went out so either the switch on the dash was bypassing to this circuit or the printed circuit board is faulty shorting to the relay. From experience I highly doubted that the light switch was at fault. In the past we have had a few circuit boards fail on this school bus model. Going with experience we proceeded to troubleshoot the circuit board by checking the input connector from the dash controls.

With the key and light switches off we did not get any voltage at the connector pin that feeds the energized relay. I want to mention that having a wiring schematic is a tremendous advantage not unlike blueprints to a house that shows the wiring system from start to finish. Thomas Bus Online is a website our shop can use to find the proper schematic using the bus VIN for each of the four circuit boards on this bus.

So once we determined the power was not entering the circuit board from the outside it was ascertained that the there was a short circuit inside the circuit board. These boards are not repairable at least I’ve never tried. Perhaps if we had the time it might be worth it but instead we ordered a new board and the problem was eliminated. Another win using the power of deduction.

Electrical can be difficult at times especially if there is a just a corrosion build up in a wire that has a hole or cut in the insulation. Oxygen and moisture just love to get together and create a corrosion build up causing resistance and a voltage drop. I’ve scratched my head several times in these situations and sometimes the only thing you can do is start at the part of the faulty electrical circuit that is closest to the environment where it’s exposed to the elements.