Thomas Buses-HDX Electrical System Repair

The electrical system on Thomas Buses vary depending on the model, pictured is an HDX model built in 2004. We are going to change a module that controls the main power to the body and chassis electrical systems. A hoist is handy but not totally required, I had to enlist a grinder with a cut-off wheel to remove the existing bolts.





This unit is called a dual power switch that is mounted on the frame on the passenger side across from the transmission. Since it is exposed to the elements the terminals are exposed so corrosion can develop. Consequently their is a possibility of an intermittent power problem to the lights or engine/trans controls when turning on the ignition.





As you can see the middle terminal is connected directly to the main batteries. The chassis and body battery supply terminals are located on each side while the ignition feed connector is plugged in on the top left corner.
When the ignition is energized a signal is sent to this module and power is directed to both systems. The chassis is the engine, trans. and running electrical while the body electrical is the lighting system.


Here is the module installed on the frame with four fasteners and an insulator between the module and the frame surface. It depends totally on what kind of environment you drive in for a problem to occur with this component. It is very well sealed internally with a hard epoxy type substance.
I hope this article has enlightened you with what to look for when experiencing electrical problems with Thomas Buses.

Thomas Bus Heater Blower Motor Replacement

There are 4 Front heater blower motors on the Thomas School Bus Saf-T-Liner Model. With Winter on it's way this is an important maintenance area, defrosting of the windshield has to be accomplished quickly.
The heater blower motors also serve as defrost motors with a quick change up on the controls. Each motor has 4 wires for high/low speed and 2 grounds.
The direction can be reversed using the ground wires. One is grounded directly while the other is connected internally back to the motor, reversing these 2 will change direction.

This Thomas Pusher Bus is a 1992 model, removing the middle and left side panels will allow access to all 4 heater blower motors.
The tandem motor in the middle looks after main defrost and heat while each motor on the ends provide floor heat to the driver and defrost/heat to both ends of the cab.
The main failures on these motors are dry bushings causing noise and seizure and blower cage noise.

Basic Electricity How It Works

Basic Electricity applies to any type of machinery running on a 12 Volt Direct Current system. Once you see how it works it's much easier to understand, instead of making it complicated I will explain the basics in layman's terms. This is the chasis electrical panel on a 1995 Thomas Saf-T-Liner[pusher]. note: Click on the Photos to Enlarge
It looks a lot more complicated than it really is. Any 12 Volt electrical system has a source of power from the batteries, in this case we run on 3 top post 900 CCA [cold cranking amp rated] batteries.

That's a lot of juice available to supply voltage to the body and chasis circuits, but there are environmental and mechanical factors that will cause a severe voltage drop if not detected.

Voltage is like a river, the heavier the flow the stronger the current. If there's a dam down river this will cause resistance and restrict the flow. If the source [the river] is cut off so is the flow.

Any resistance between the battery and the load will cause a voltage drop, this can be caused by a short circuit, open circuit or just plain resistance from corrosion. A good rule of thumb when troubleshooting a 12 volt circuit is to:

Step 1...Check the battery -Sufficient Voltage?
Step 2...Check the load i.e. A Light or Relay, is it working?
Step 3...Check the connections
Step 4...Check the wiring

Very basic steps but most electrical problems are found using these steps, check the simple things first.

Say for instance you have 13.5 volts at the battery and 0.0 volts at the load. This means you have an obvious Open Circuit problem [a rubbed through wire possibly]

Using a digital ohmmeter you can ground one end of the circuit and using the two ohmmeter leads at the other end connect one lead to the faulty wire and the other end to ground. What you're doing here is checking for Continuity [the ohmmeter has it's own battery voltage which runs through the faulty wire].

By the way, the multi-meter you use for newer equipment must be Digital to safeguard the electronic systems that are much more common in new trucks and buses. If the circuit breaker is popping on and off then there is obviously a Short Circuit which can be hunted down using a Short Finder [a very slick tool to have which saves a lot of time and grief]

You hook up the auto-reset breaker then slide the meter along the wiring until it finds the exact spot where the short is [the indicator needle stops moving] along the frame or body.
Clean the connections and check the load i.e. marker light or tail light for poor grounds & connections.
That's a Quick Overview on Troubleshooting a Basic 12 Volt Electrical Circuit.