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School Bus Body Wiring Modules

The video shows you where to find the wiring control modules for this bus model. The modules are a printed circuit and can’t be accessed but through the years we have replaced very few of them. The location of the fuses and relays for all the body components is located on the the inside of the door panel.

This is very handy to see what goes where and since everything is numbered makes troubleshooting that much easier. The LED lights on the modules are indicators of a closed circuit between either the switch and relay or the relay and the component. The green LED light indicating that the (for example) blower motor or clearance light circuit is live and receiving power.

The amber LED indicates that the control circuit is sending battery power to the coil of the relay meaning that the relay has been activated. This makes it so much easier to find the problem with lighting or htr/fan motors on the bus. If the amber LED is off then the green LED will not work as well. NOTE: Don’t count out the relay itself which could possible have a bad coil.

The first thing I would check (no amber LED) would be for power at the switch controlling the accessory. If there is power there the next step is to check the wiring harness running to the module. One remote possibility could be the module itself but like I mentioned before these modules are quite dependable unless there is a short circuit where high temperatures are involved.

If the amber LED is on and the green LED is off then the wiring out to the accessory (heater motor, wiper motor, marker light) needs to be traced to locate an open circuit or a failed component. This helps us a lot to diagnose a wiring problem. Once the schematic is located it’s just a matter of time before the fault is found. Anytime electrical troubleshooting is simplified I’m glad to mention it here and share with other school bus mechanics.

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