Repairing Coolant Leak Under Dash Thomas HDX

We recently had a phone call to our shop from one of our sports run bus operators. He was running a 2007 Thomas HDX Pusher. The operator stated that there was coolant on the floor and the inside windows were fogging up. It should be noted that this leak is very minor and the shutoff valves at the engine valves can be closed to isolate the leak and the bus can still be operated.

Since he was 4 hours away my hands were tied and I told him to take it into the nearest Thomas dealer to let them check it out. Shortly afterwards he said the leaking stopped and he limped the bus home without any further incident. Once we got a chance to inspect under the dash the coolant leak was no where to be found. The coolant temperature control valve is electrically controlled from a dash switch.

There is coolant running through the control valve where it is throttled on and off depending on the operators requirements. We decided to pull the front inspection door down and have a look, after a close look we found a leaking control valve where and o-ring seals the piping assembly. The operator must have adjusted the heat control to a position that stopped the valve from leaking temporarily. Thomas has a new valve available and replacement will remedy the problem. It's hard to detect where the leak is coming from until you physically pull out the front heater core and check the control valve itself.

Thomas heater cores are very durable and I have never seen one leak yet. The great thing about the HDX model buses is the accessibility to the front of the cab where you can drop down the front access door panel and work on the plumbing from the outside of the bus.

Thomas HDX Pusher Radiator Modification

In 2 of our new Thomas HDX Pushers we had rocks go through the radiator core. This was caused by an opening at the side facing the right rear dual wheels. Here the repair has been made by the rad repair shop using epoxy. The radiator has an aluminum core with plastic tanks.
[click picture to enlarge]




All that has to be done is to bend a piece of sheet metal to ninety degrees and use a 2 foot square mudflap strategically placed in front of the radiator.







Mount the sheet metal to the bus body with sheet metal screws and position the mudflap as low as possible for better protection.




















So far we've had no more incidents and hopefully this modification will fix the problem. It's a good days work to re and re the radiator so anything to improve the situation and avoid another core failure is well worth a try.

Thomas Buses-HDX Rear Emergency Window Modification

Thomas Bus HDX Pushers have a rear emergency exit window that has 2 ground switches. One for the alarm, warning the driver that the window is open and the other is an interlock that does not allow the bus to start when the window is latched.

The problem we were having was water seeping through the seal causing a circuit to ground tripping the alarm even when the window was fully closed. The quick fix was to grab an air line and dry off around the switch and grounds. Pictured here is the alarm ground switch[click on pic to enlarge]

This is the window locking latch that must be unlatched for the bus to start. The rivet on the outside is going to ground while the handle contacts a strip riveted on the inside of the frame.






We installed 2 intermittent switches in a currently available Thomas Part that encases the 2 switches and does the same job as the factory install.






The window must be removed to rewire the new installation. The 2 original ground switch wires are attached to the new switches in the casing and grounded at the case.






Once installed the [starting interlock] bolt style latch locks the window and the other switch is depressed [just above] to shut off the window alarm.
The latch catches on a "C" shaped flat bar that was fabricated to lock the window.




The latch bolt is in the open [run] position and the bus will start, this is a safety factor that newer Thomas Buses come with from the factory.

I will be updating this post soon with part numbers.

Thomas Buses HDX Electrical Panel

Thomas Buses have changed through the years and one model with a different electrical layout is the HDX. This 2004 has everything in a totally different layout compared to the Saf-T-Liner.





The Body Electrical [lights and accessories] Schematic is nicely displayed on the side door panel below the driver's side window.

[Click On Each Photo To Enlarge]





The circuit board shown here has been unfastened and tipped downward so you can see the many connectors that hook up with relays and battery supply junctions. There are 3 circuit boards in total with specific spots for each connector.








The circuit board is upright once again [located in the side panel] Here you can see the main battery power supply on the left with the many fuses and relays required to run lights, heater motors, solenoids etc.
The LED lights above indicate which relays are active, YELLOW
indicates the relay coil has been activated while the GREEN LED indicates the circuit is connected to the load.

Thomas Buses have evolved from the basic "battery to load with a switch in between" to complex relays & mult-functional switches that handle the demand for more circuits that can meet the load & safety component demands of today's School Buses.

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.