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.

DT 466 Diesel Engine - engine oil pressure problem

We had a DT 466 Diesel Engine that could not build up engine oil pressure, especially in the cold weather. It is a 1998 International Thomas School Bus.

The electronic model requires high pressure engine oil supplied by a separate oil pump that feeds the electric over hydraulic operated injectors.

The engine ECU controls injection by sending an electrical signal to each injector solenoid at the specific time required.

This happens VERY QUICKLY. The problem we were having? no oil pressure was being supplied by the engine oil pump that in turn supplied the high pressure hydraulic unit injector oil pump.

So we stripped down the front end and replaced the oil pump and the pressure regulator (some scoring evident). We knew that since it came on the hook without any oil pressure if the engine fired up after the fix we obviously solved the problem.

CLICK ON PHOTOS TO ENLARGE

Oil Pump Pressure Regulator fits into the block oil gallery behind the front cover.
Pressure Regulator Disassembled (the body had some obvious scoring) If it was stuck open this would cause a no oil pressure problem. The nice thing about the electronic DT466 is the engine will not start without oil pressure...
Front cover disassembled. Oil pump is driven off the crankshaft, a very reliable drive system!


Ta Da! Once reassembled the DT466 Diesel Engine we know and love came back to life!I hope this post on diesel engine repairing and troubleshooting the DT466 has helped you....
PLEASE COMMENT! I LOVE THE FEEDBACK...

Cummins Diesel Repair - ISC Model

This particular Cummins diesel repair was necessary because of an indirect code that was finally traced to the camshaft endplay. There are 2 sensors installed to the backside of the timing cover just below the top drive gear (the 2 black round shaped sensors are visible to the right of the cam shaft bearing bore). One is the camshaft position sensor the other is the engine speed sensor.

The camshaft has been removed as well as the cam gear (above). You can see the tone wheel that the sensors read and send a voltage signal to the engine ECU. When there is a problem with these readings an engine code appears and a driveability problem arises. There is a specified air gap between the cam gear and sensors, if that gap is not maintained an inaccurate signal is produced. This is typical for most electronic diesel engines and care has to be taken when handling these components.
To remove the camshaft the rocker arms and shafts must be removed first. The wooden dowels act as holding devices that jam into the solid lifters that ride on top of the cam lobes. Once the dowels are installed elastic bands are used to hold them secure while removing the camshaft. This saves a lot of time not having to pull the cylinder head.

The camshaft endplay was excessive which caused the sensor problem/codes to become a regular occurrence. The first step we took was to change the sensors, this cured the problem for several weeks but the fault always came back to haunt us. The camshaft showed signs of wear on the shoulder where the gear sits and the cam lobes as well. There is a thrust washer that goes between the gear and the engine block to help with endplay movement.

If you have to do this job in a Thomas ER Pusher Bus you will have to remove the rear engine mounts and lower the engine enough to get the rear pushrods out of the engine. The solid floorboards above are not designed to allow the pushrods enough headroom for removal.

The cam gear has to be heated to 400 degrees Fahrenheit to install back onto the camshaft. We elected to replace the gear as well as the camshaft. If you are a Diesel Mechanic this is a Cummins Engine repair you may have to go through one day. I hope this mechanic info has helped you understand the procedures involved in this operation.

Diesel Engine Repairing & Troubleshooting.

One of my favorite tools for diesel engine troubleshooting and repairing is a digital heat sensor.

You can compare the block temperature to the coolant gauge reading, check for an engine miss, confirm any type of heat sensor that's sending a signal.

Case in point: I had a Cummins ISC (electronic) diesel engine that was not heating up properly. So the first thing you want to do is get the story first hand from the operator.

Questions like: How long until it DOES heat up or when did it START to develop?

In this case the operator stated the problem started when the cold weather showed up. So the first thing to do is a scenario with the bus (2000 Thomas Saf-t-liner school bus). So I went for a road test and the first thing I noticed was the engine was heating up very slowly.












I used the digital heat sensor and confirmed the coolant gauge was correct and the engine was running at 170 degrees after a 1/2 hour road trip.

From experience I know that the engine fan locks on with these engine models so I tested both the coolant sensor and charge air sensor.

They have a 3 to 3.5 ohms reading. No ohms means a failed sensor (open circuit).


These sensors determine when the engine fan should come on full which is usually around 200 degrees, in this case the fan came on full right from a cold start up.

Since both sensors were good I replaced the control module which cured the problem(located below the engine fan on the frame). Parts aren't readily available for this module and rebuilding it isn't worth the downtime.












When you have a fleet of school buses there is an advantage to numerous buses having the same specs.

The troubles usually happen more than once and history will help with diesel engine troubleshooting and repairing.

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.

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 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.

Found a Used school bus For Sale ? What to Look For...

Finding a used school bus for sale online is not as easy as driving down to the local auction house and kicking the tires of a potential school bus that has caught your eye. There is however a huge market available online with thousands of school districts all over North America retiring twelve to fifteen year old buses with a lot of life left in them.

The one thing most online shoppers may be thinking is how honest is the seller and how much of a hassle will they have to go through to get a great deal. With the diversity of the Internet buyers and sellers can interact with pictures, descriptions and email to make a great deal with confidence.

The following are areas on a used school bus for sale that you should check out carefully.
Most school districts have a strict maintenance program and every action performed on their school buses will be documented either in a log book or on a computer. This information is the bible on the school bus you're interested in. You can see what has been replaced, how frequent the servicing occurred and if it was a lemon with frequent breakdowns.

The maintenance log will also give you the mileage and the in-service date, if the mileage is extremely high [over 250,000] then caution should be observed. Diesel engines are very durable but they do have limits with high hours on bearings, pistons and rings. The advantage of a preventative maintenance program is regular oil changes and in some cases the analyzing of oil samples. Any repairs required are performed immediately because of the obligation of each school district to provide safe transportation for school children.

A close inspection of the interior and exterior of a school bus will reveal any rust that has formed. School buses must be washed and disinfected on a regular basis and one drawback is the floor with a wood base can deteriorate over time. The underside should be closely inspected for rust especially at the body crossmembers, the severity depends on how much salt and chemicals are applied on the local highways during the winter season.

Running the engine and observing for excessive smoke and unusual noise is important to determine the mechanical condition of the school bus in question. A cold diesel engine will run rough at first until warm with extra white smoke out the tail pipe. Check transmission shifting, brake performance and determine what is not working properly with the accessories and operating systems. Everything for sale is negotiable, don't walk away from a great deal because of a few minor repairs that may be required.

I hope this over-view on shopping for a used school bus for sale
has helped you.

Internatonal School Bus DT 466E Cam Sensor Replacement

Today we're working on a Thomas/ International School Bus that requires a Cam Sensor. It's located in the front of the engine in the gear cover and reads engine rotation sending a signal to the Engine Electronic Control Unit. The Engine will not run with a failed cam sensor and the Pro-link diagnostic tool will verify the sensor failure.






The depth is checked with a dial indicator that has been calibrated according to the new sensor part number. Shims may be removed or added to get the proper clearance which is between .025 and .030 of an inch. The belt has to be removed and the engine rotated twice to check the runout and get an accurate reading.





Here's a look at the calibrating kit that must be used to read the depth of the bore where the sensor is mounted. There can't be any interference fit, this will damage the sensor. The new sensor is seen here on the left while the rest of the calibration tools are nestled in the carrying case.
That's all there is to it when changing the cam sensor on a DT 466E Diesel Engine on a International School Bus.

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 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.