Mechanic Tips-School Bus Fleet Preventive Maintenance

Every Fleet Mechanic will have there own ideas on how to set up the best PM program for their equipment which will vary for different locations. With different climates, road conditions and power trains the procedures will never be exactly the same. Our shop likes to condense the annual PM since the regular checks are carried out every 5000 Kilometers (3000 Miles). Oil changes are done every 15,000 Km with an oil sample to be analyzed. Fuel filters are replaced at every oil change.

The Allison 2000 and 3000 Transmissions  are all running on TES-295 Synthetic oil which has drastically cut down on repairs. Right now we change the oil every 3 years, the newer buses have prognostics which tells us with a dash light indicator when the filter and oil needs to be changed. Air filters are replaced every year regardless of the mileage. Steering Filters are serviced along with changing the oil every 2 years. Valve Sets are carried out according to the Manufacturer i.e Cummins every 5000 Hours or 240,000 Km (Electronic Models). With the introduction to emission controls in 2007 we now have to perform manual regens if required and replace the DPF (diesel particulate filter) every 150,000 Km (90,000 Miles)

Air Dryers every 2 years with a new unloader kit and dessicant filter. Dirt is a factor on school buses so we service interior heater filters and the entire lighting system (Non-LED lights). We check the starting and charging systems including the battery pack. If necessary we will run tests on the batteries and alternator. years ago alternators and starters were removed for rebuild but the time and parts did not add up as an economic strategy so we decided to give these components a 5 year life span before determining if they would need replacement. The cost has come down so much on electrical reman units, it's feasible to go this route with the added bonus of a warranty period.
 
Spring brake chambers are replaced every 5 years regardless of mileage and condition. The Mercedes MBE 900 requires a coolant and thermostat change every 2 years, the other diesel engines in our fleet do not have a time or mileage limit but we're working on that to prevent antifreeze deterioration and thermostat failures. Our tires tread depth is measured every Summer and pressures checked during each service.

Common sense plays a role with any preventive maintenance program and we found the best way to set one up is to sit all the Mechanics down in one room and go through the fleet  figuring out the best program to cover all the bases. Breakdowns will quickly eat up your labor time and parts budget like wildfire so keeping on top of maintenance with a feasible program will pay you back 10 times over.                                                                     

Allison 3060 Automatic Transmission Fault Codes

 Here are fault codes that a reader left on a comment wanting to know what they were. The Allison 3060 Automatic Transmission is all electronic with sensors, solenoids a wiring harness and the Transmission ECU (electronic control unit).

The codes can be retrieved by pushing the 2 arrows at the same time (key on) to retrieve the 4 digit fault codes. Another method is to use the Pro Link Hand Held Reader. To clear the codes after you've repaired the problem hold the mode button until you hear 2 beeps. This has to be done during code retrieving.

Code 2511 means the output speed sensor has a zero speed reading. Check the speed sensor and connection, the ohm reading should around 300 ohms. Also check the ECU connectors. Make sure they are tight,clean and undamaged. Check the fluid level and check wiring to the sensor has no opens, shorts between wires or shorts to ground.

Code 2522 is related to 2511.... 22 being a sub-code.

Code 2312 is the shift selector. Check ECU for connectors, shift selector connector, check selector for proper operation and check the wiring harness for opens, shorts between wires and shorts to ground.

We have had lots of wiring issues and repaired a lot of these problems by running a new coaxial wire from the sensor to the ECU. There seems to be some resistance in the wire that causes improper signals to be relayed to the ECU.

Another area there has been faults is the shift selector pad. You can get a kit for the selector which isn't hard to replace.

If it's hard to push the selector to get into gear that's the first sign of trouble and a new selector pad would fix the problem. Here are all the posts I have produced on the Allison 3060 transmission with troubleshooting and repairs included. Scroll down and you will see some more Mechanic Information on the output sensor circuit.
Got Questions ? Please a comment..

Allison Automatic Transmission Troubleshooting MD 3060 Code 2116

The Allison automatic transmission MD 3060 is all electronic running off direct battery power to a series of relays, fuses and modules. The ECU (electronic control unit) for the transmission reads and controls inputs and outputs. Inputs include engine speed, output speed, turbine speed (the trans internal drive shaft) and throttle position. It computes these inputs into smooth shifts according to these variables.

The TPS Cable Complete - It's Located In The Engine Compartment Area

If it receives a bad signal everything goes to hell in a handbag. Code 21-16 is a trouble code for the Throttle Position Sensor. One of my newsletter subscribers from North Carolina has a 1996 Holiday Rambler motor home with a 5.9 Cummins diesel engine. He's dealing with this code and asked me to help him out.

The TPS control has 3 wires to it, one sends a 0 to 5 Volt signal to the ECU which regulates shifting patterns using these voltage readings. Full throttle and idle can be determined this way, if the wrong reading is sent then you will get a hard shifting scenario.The other 2 wires are the battery voltage and ground.

21-12 Code indicates that the ECU is receiving 14 counts or less. The TPS works with counts and 14 is too low so the problem might be at the closed throttle end of the TPS travel. The first check to make is the cable for frayed wires hanging up somewhere or a bad wire in the 3 wire harness.

My Crude Wiring Diagram Showing The Simplicity Of The TPS Circuit.

I've seen a couple of bad cables and have also replaced all three wires from the TPS to the Trans. ECU. You can check the wires for grounds, shorts or open circuits. If you replace the TPS you can reset it by cycling the ignition 5 times to reset the 0% and 100% throttle position or use the 'ProLink' Reader to reset the system.

Mechanic Troubleshooting - Allison Transmission Wiring

Our fleet has absolutely zero manual transmissions, school bus drive trains are unless requested differently come standard with Allison MD 3000 electronic transmissions. Once they get a few years use there are electrical glitches that occur.

In this case the code pointed to the C3 pressure switch. This switch reads the clutch pack #3 oil pressure in the transmission module. It activates at around 37 psi when pressure is operating the clutch release and engagement.

If it fails, a code 32 will pop up. Most times the wiring is at fault so we will check the continuity from the ECU to the Trans. Module. (shown in the photo above)
At the front end the blue connector has to be disconnected from the ECU where the appropriate terminals have to be probed and checked for resistance. The Allison troubleshooting manual is a "must have" for this procedure. For newer units the WTEC 3 is required(world transmissions electronic controls). These manuals are available at an Allison dealer.

You can tell if the system is in the 3rd generation from the 3 connectors at the ECU. In this case the ECU is located under the driver side floorboard attached to the body(Thomas 1999 Saf-t-liner).

The resistance read 25 ohms which is excessive. As with other codes replacing wiring has become a common occurrence. We use shielded wiring which we run front to back to eliminate the defective wire. Many times if transmissions act up and shift awkwardly in wet weather there's a good chance you have a wiring issue.

How To Manually Check for Transmission Codes

Press the 2 arrows on the shift pad with key on and read the 4 digit code that comes up. Record the first 4 number code then press mode which will bring up a second code(if any) . I hope this mechanic information has helped you out.

Allison Transmission Problems-Hard Shifting

Allison Transmission Problems vary and one typical complaint is hard shifting. I recently looked at a 1999 Thomas Saf-t-liner [pusher] with an Allison MD 3000 series transmission. It had a hard shift into 2nd gear and a hard downshift as well. No codes came up so it was time to check the mechanical side and do a clutch pressure test. The test has to be turned on "enable clutch test" with the "ProLink" so each shifting range can be turned on with the unit running in the shop.

The pressures are compared to main convertor pressure and sits around 270 psi on this test. The individual clutch pressures must be within 5 psi. If there is a big difference that tells us the clutch itself is worn or there is a hydraulic problem. This test revealed no problem in this area and one conclusion could be a clutch that has to be physically inspected for imperfections. There is always 2 clutches applied to get a forward gear in this case it's C1 and C4 for 2nd gear.

The next step is a big one, whether to disassemble the trans and inspect for problems or live with the hard shift. I tried to "fast adapt" the shift parameters with the "ProLink" which did not change anything. This is one of the allison transmission problems you might encounter, if we get into this project any deeper I'll keep you posted.

Cat 3116 Engine Installation-Thomas Buses

This is a Cat 3116 Engine installation into a Thomas Pusher. This process is much easier with the transmission installed [MD 3000 series]

The transmission jack supports the trans. and the hydraulic crane hooks on nicely to the front engine lifting eye. With some assistance we rolled the whole assembly into the backend with ease. You'll notice the rear subframe and bumper have been removed. This is one great advantage of owning a Thomas pusher although we don't want to be doing this particular operation too often...

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.