School Bus Electrical Fault

This is the first time I’ve ever seen a problem like this on an HDX pusher school bus model. The Thomas HDX pusher bus has printed circuit boards that relay all of the supply power to run the lights, heater motors and operating circuits. There are 3 circuit boards in total and troubleshooting requires a schematic to see where the inputs and outputs are located.

The rear of the PCBs have a series of wire connectors that can be viewed on the schematic. They are numbered which makes locating a circuit quite easy. As I pointed out in the video the LED lights show inputs and outputs at the front of each board. The amber indicates an input voltage while the green LED indicates an output to the load.

The 2 green LEDs in this instance were for the tail lights and cluster lights. They were on even with all of the switches turned off. The yellow LED to #8 relay which runs the tail lights and clearance lights is on constantly. This input source will be the first thing we will check out. Some how the power is shorted to the relay energizing it with everything off and even though the relay is unplugged the cluster lights are still illuminated until the battery is disconnected.

There is a battery and ground connection to each PCB to supply direct voltage and grounds to the relays. It’s possible there is a short internally in the board or the head light switch is faulty but if the green LED for the cluster is off how come the lights on the dash are still on? This just makes the job at hand much more interesting and once I locate the schematic we will dig into it.

If you’re working on a lot of the pusher Thomas buses you need to get a user name and password for the thomasbusonline website. There you can input the bus VIN and find all sorts of great information like the electrical schematic for a PCB. Go to BOM or bill of materials and on your keyboard hit ‘Ctrl F’ and a search box will pop up to type in keywords and find what you’re looking for. In this case put in the keyword ‘schematic’. This will bring up lots of options with all of the parts that have wiring diagrams available.

This website is very good to have when it comes to locating Thomas pusher bus parts and schematics that you can print out and use to start troubleshooting. I will update what we found with this problem soon. Until then take it easy, never give up and happy troubleshooting!