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.

Allison Automatic Transmission Wiring & Troubleshooting

The Allison Transmission Control Unit on the 1999 Saf-T-Liner is located under the floor directly below the driver. There are 3 plug-ins which is an easy way to tell this is a W-TEC 3 system [World Transmission Electronic Unit 3rd generation]. The Transmission vehicle speed signal enters here and exits at two different plugs, one for the speedometer and the other to the Cummins ISC ECU.



We will trace the exiting wire from the ECU that provides our Vehicle Speed Signal since that is the code and the speedometer is operating normally. Since we have speedo operation that gives us an obvious clue that the incoming signal to the Trans ECU is OK.[resistance should be checked regardless]
The wire we want to trace goes from the Trans ECU to the VIM [Vehicle Interface Module]
Removing the dash back cover makes this check MUCH easier.


Remove the cover from the VIM








The VIM is an interface between the Trans ECU and the chasis wiring. Various operations are accomplished through this module like the exhaust brake, back up lights, neutral start, high idle and of course our vehicle speed signal runs through here to the Engine ECU.
[Click the Pic to Enlarge]
Now we can check the continuity and see if we have a circuit. After checking this out there is no connection between the Trans. ECU and the VIM.
I'll run a new wire to repair the open circuit, an easy fix. It's nice to get a gimme once in awhile!

In the VIM you'll notice a series of relays and 2 fuses. If you experience a dead keypad and there is battery voltage to the VIM then check these fuses. One is main power & the other is ignition power.



The other vehicle speed signal wire [tan wire # 157] comes from the trans ECU directly to the speedo. The speed signal to the VIM is the same number [tan wire #157] since it comes from a different plug.




This is where it all begins from the output shaft housing of the transmission. The output sensor is a single coil unit that sends an AC voltage to the Trans ECU.
There is a toothed wheel rotating inside the housing that creates the signal to the sensor.
The Allison 3000 Automatic Transmission is a very reliable unit when maintained properly. The wiring problems come and go but are easy to trace and repair with the right tools and wiring diagrams.

Allison Automatic Transmission Troubleshooting and Repair

I'm working on an Allison MD 3000 Automatic Transmission with a wiring problem. The trouble started with a fault code on the engine side of a 1999 Thomas Saf-T-Liner equipped with a Cummins ISC Electronic Diesel Engine.[Click Pic To Enlarge]

The Cummins fault code was "241" meaning a loss of the Vehicle Speed signal to the Engine ECU, the amber warning light flashes on the dash alerting the driver. Without a vehicle speed signal the engine ECU [electronic control unit] does not know if the bus is moving or stationary so the exhaust brake and high idle will not work. There was also a drivability problem with loss of power which was related, since the fuel system also needs to know how fast the bus is travelling to regulate fuel pressure.

Normally the bus has to be moving over 5 mph and running over 1000 rpm for the exhaust brake to work and has to be parked for the high idle to come on.

Since the output speed signal is supplied from the transmission output speed sensor we have to start there and check the sensor for resistance and the wiring for continuity.
Here I'm checking the two wires between the sensor and the Transmission ECU for continuity with a Digital Multimeter. The output sensor at the trans should read around 300 ohms.The wire is a twisted pair coaxial wire which helps avoid interference, it checked OK so on to the next step.





















This is the connector at the Trans Ouput Speed Sensor which has been jumped across for the continuity check I performed above. you can see the twisted wire here, no other wiring can be used since we are working with a very sensitive ECU.
On my next post I will continue with this diagnosis and show you the conclusion.

Testing A Starter Solenoid

Check out this excellent video on Electromagnetism. This is the principle that solenoids and relays use to provide the most efficient electrical circuits. It's over 10 minutes long so grab your favorite beverage and have a look.

The principles here will give you a much better idea how components work like a starter solenoid. Following this video is my own on bench testing a starter solenoid which connects the Battery to the starter motor and engages the starter drive with the engine flywheel.

Below is a demonstration on bench testing a starter by simply connecting battery supply to the solenoid positive terminal and the ground terminal at the starter motor.

The switch terminal which is marked "S" is the signal from the ignition switch that energizes the coil in the solenoid and turns it into an electromagnet forcing the contact plate to connect the main battery power to the motor.



Here is an older post on the disassembly & repair of a Delco 37 MT Starter