Thomas Buses - Air Throttle Repair

During most of the 1990's before the electronic age the engine throttle was air controlled on Thomas Bus Saf-t-liner models. The high idle was a welcome addition since it was operated with the flick of a switch on the dash.
In the photo the air line can be loosened while the engine is running where there should be no air with the high idle switch turned on. The throttle linkage is moved into high idle with spring pressure[no air]. The regular foot throttle air line is attached to the underside of the cylinder. {Click Photos To Enlarge}

The air is controlled with an electrically actuated air valve that is normally open with full air pressure and no power from the switch. The air pressure moves the internal piston to the idle position compressing the internal springs. When the high idle switch is turned on, battery power closes the switch and blocks air pressure to the air throttle allowing spring pressure to kick the throttle linkage up to high idle[around 1000 rpms].



The disassembled air throttle consists of springs pistons and seals and is easily serviced. The one thing that does fail is the 1/4 inch adjustment bolt on the end cap wearing through the washer that seats on the internal spring. Thomas buses have changed since this decade and are electronically controlled with an onboard computer but if you still own a saf-t-liner pusher these systems are reliable and easy to maintain.

The Best Diesel Engine-DT 466 International School bus

The Best Diesel Engine? By far the DT 466 by International. For the Medium Duty fleet operator this engine is the most reliable of all. Pictured is the DT 466 E [electronic version]. Each injector is controlled electronically by the engine ECM and injection is actuated by high pressure oil. These injectors are HEUI [hydraulic electronic unit injector] that are reliable but typically have to be resealed if there's a fuel in the oil problem...for example.














Here I'm setting valves which is .025 for both the exhaust and intake. One of the differences with the electronic engine and the older non-electronic models is the newer engines have the intake manifold built into the valve cover so it weighs much more, making it difficult to remove. There is the Engine Control Module as well fastened to the valve cover on the driver side.

Below is an electronic unit injector that can be tested while running with a laptop for proper injection cycles.















On the International School Bus the dog house inside the cab also has to be removed to gain access to the rear cylinders. This aside the reliability of the DT 466 E and older DT 466 in my opinion is the most reliable model for a medium duty fleet operation. One characteristic with the electronic version is a very rough start up in cold weather because of the engine oil pressure feeding the high pressure system. It takes a bit of high idling to get smooth fuel injection happening.

If you can find a mechanical DT466 then you have reached diesel heaven! With no HEUI and a Bosch fuel injection system you will have very dependable power unit. I can remember in-framing these engines for $1200.00 they are very basic and easy to work on. But technology has taken over and fuel emissions are here to stay.

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 - Allison Transmission Wiring Modification

Thomas Buses are driven by an Allison MD 3000 Series Automatic transmission. This 1995 Saf-t-liner model had no power to the operators trans shift pad. The shift pad is also the Trans ECU [electronic control unit] so as I have said before always check the source , in this case the batteries.

There was a power feed open between the batteries and the rear control box at the engine compartment. I was able to limp the bus home by jumping battery power to the terminal board inside the control box which goes to directly to the operators cab supplying battery power to the shift pad/ECU.

This is obviously how I determined the open from the battery to the control box/terminal board [standard wiring on all Saf-t-liners]. The code for this fault is 35-00 [push both arrows on the shift pad at the same time with key on to retrieve code {under normal circumstances}

The control box at the engine compartment houses the engine intake heater components, lights etc. as well as the main battery power to the front electrical control box below the driver's side window.





My plan is to totally bypass the trans battery supply circuit from the battery to the ECU. Here you can see I've already cut the wires [larger #8 guage pos. and neg. wiring come from the battery while the smaller #10 guage pos. and neg. wires are running up front to the ECU]




This is the terminal board I mentioned earlier, factory wiring with spade connectors that connect the battery feed wiring to the trans. ECU wiring.

The first picture [top] shows the #8 pos. and neg. wires hooked up and running directly to the trans ECU. This will eliminate the extra #10 wires shown in the rear control box.


In the drivers cab the rear cover has been removed for easy access to the back of the trans. ECU. The upper plug can be removed and inspected for battery power. Both pos. and neg. #18 wires [2 each] run into this plug directly from the battery. The #8 wire is downsized under the dash to accommodate the smaller wiring into the ECU plug-in.



The 12V source from the battery is shown here with no connections in between. With a Digital Multimeter you can check for a battery voltage and a good ground.






So now you know when you have a shift pad with no reading [key on] the first thing to check is the battery voltage.

I hope this has helped you become more familiar with Thomas Buses equipped with Allison MD 3000 Series Automatic Transmissions.

Thomas Buses-How To Replace A Signal Switch

Thomas Buses have a signal switch that is very easy to replace. Most of the time the switch does not cancel properly which is a pain for drivers who have other things to look after and don't want to be constantly reaching for the the signal switch to manually turn it off after every corner.
The Thomas School Bus Saf-t-liners are similar with many similar components, the signal switch design did not change for many years.

Remove the upper and lower covers on the steering column. The upper cover is easier to remove when the steering column is tilted all the way down.






Use an allen wrench to remove the 2 signal switch screws holding it to the column.

Here is a top view of the upper screw being removed. As you can see the access to the signal switch is very easy.







Unplug the 2 wiring harnesses to the signal switch, they only fit one way.

Important Note:

One thing that you should watch for is to make sure the wiring running down the steering coluumn has enough slack. I've had problems where the signals do not work or short out on the column because of not having enough extra wiring to accommodate the tilt steering column movement .

The signal arm is held in the switch with a catch and can be removed by pulling on it aggressively until it releases from the switch.

Reverse this procedure and now you know how to replace the signal switch on Thomas Buses.

Cummins Engine Repair - Installing Injectors

Cummins Engine Repair
Replace o-ring on the injector body and copper sealing washer on injector tip. Position into head with orifice pointing towards fuel connector tube opening. [click photo to enlarge]





The injector hold down installs only one way to index the injector position aligning it with the fuel connector tube. The hold down retaining bolts are torqued to 89 in. lbs.






Install fuel connector tubes [with new o-ring] into the head bore. The tube has a tapered tip that aligns with injector body. Torquing the fuel line retaining nut forces contact and a proper seal between the injector body and connector tube.





The injector lines are what retains contact between the fuel injector and connector tube. Torque the fuel line nuts to 28 ft. lbs. using a crowfoot wrench.
That's it for this post with more to come on Cummins Engine Repair.





Recommended: Diesel Engine Repair Manuals

Haldex Brakes-Self Adjusting Slack Adjusters

Haldex Brakes self adjusting slack adjusters are installed at the factory on all Thomas Buses equipped with air brake systems. This allows much better control over brake adjustment especially for a large school bus fleet operation. At the same time there are operating and maintenance procedures that must be followed to get the best performance out of Haldex Self Adjusting Slack Adjusters.

Here Are Some Important Maintenance Steps To Follow:

• Visual and operational checks must be made at every scheduled PM check
• Check adjusters and anchor brackets for visual damage
• Automatic Adjusters should not be operated as manual adjusters
• Standard chasis grease may be used to lubricate Haldex Slack adjusters
• Lubricate the Standard 409-10 model every 50,000 miles or 3 mths
• Lubricate Reduced Maintenance 409-10 once a year
• No-Lube 429-10 adjusters are sealed and do not require lubrication
• DO NOT use Moly-disulfide grease as this will effect the operation of the slack adjuster
• Replace the slack adjuster if the TORQUE is less than 13 ft lbs when backing off the adjuster. You will hear the ratcheting sound while doing this.
• Haldex Slack Adjusters should be checked for operation between 90-100 p.s.i. in the air brake system reservoir. Anything over 100 p.s.i. starts to measure deflection in the foundation brakes and does not give a true reading
• To self adjust Haldex Brakes the operator has to make several full brake applications [spring brakes released] preferably with brakes hot and over 90 p.s.i in the air brake reservoir.
  
  Here is a very good video explaining proper Brake Adjustment on an air brake system. The self adjusting slack adjuster in this video is not a Haldex but the exact same procedure applies. In my experience with a properly adjusted Haldex slack adjuster the push rod travel averages at 1.5 inches of travel. Well below the Department Of Transport maximum allowed brake adjustment specs.
  
  Self adjusting slack adjusters are now standard equipment on all Thomas Buses and are a real asset for large fleets who must keep up with Department Of Transport Standards especially with air brakes and proper adjustment. I hope this post has informed you on the proper maintenance of Haldex Brakes.

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 Pusher School Bus - Removing Radiator

This Thomas School Bus had a leaking radiator that required removal and repair. All the body panels must be removed to gain access to the radiator. The air charge cooler must be removed as well, it cools the turbocharged intake air to create more density/volume.




Fortunately this radiator core was repairable being a 1999 model, This is the best way when running a fleet of 70 School buses. The Thomas Pushers have transversely mounted radiators since they have rear drive engines and transmissions.