Sunday, March 05, 2017

School Bus Automatic Transmission Control Module

School Bus Allison Automatic Transmission Control Unit from John Whelan on Vimeo.

School buses these days come with Allison automatic transmissions. In my opinion there's no other choice for running gear. Allison has got it nailed with engineering design and software. We just purchased the newest Allison software and it does a nice job of laying everything out with troubleshooting and tutorial videos all in one. Recently one of our Navistar conventional buses was on a field trip and called in with the transmission not wanting to shift.

I went out and for some reason after cycling the key a couple of times the bus was limped back to the shop. The basic checks are transmission codes and wiring connections, bad wiring from the TCU to the transmission. The code P0614 was a torque data fault which is a communication problem between the TCU and ECU (engine module). There is no schematic to follow it's all about how much torque the transmission will allow from the engine.

This feature was put into place to protect the transmission from damage if for some reason i.e. the engine delivers too much torque due to over fueling. After checking all of the pin connections and the wiring the J1939 resistance was checked and we got a reading of 57 ohms which is normal. If you're not familiar with J1939 it is the network of communication between all of the vehicle control modules.The engine, transmission, ABS brakes, body control module etc.... so if there is an issue with this system the messages will not connect properly and strange things start to happen.

The green and yellow twisted pair of wires goes to each module so a high speed signal will occur when the operator wants to shift into a certain gear or signals the ABS module that braking is taking place and so on. The modules read it's sensors and adjusts to accommodate any changes such as driving up a hill or coasting on the highway. When there is a bad wire in this twisted pair the messages will be all mixed up and something in the operating system will fail followed by a fault code on the dash alerting the driver.

A great advantage we have running a fleet is there are buses with the exact same specs. which gives us the ability to swap modules to cancel it out during troubleshooting steps. In the case of the P0614 code when we changed the TCU with a good working unit from another bus the code disappeared. This saves the guessing game and the rules are when you order an electronic part and use it you own it. The price for these modules are well over a thousand dollars so be sure before you order one. They have to be specially programmed according to the part number of the original module and the specs related to the vehicle serial number so I'm sure returning it would be a hardship.

Thanks for reading this post I hope it helped out!!  


Bill Grimsley said...

Mr. Whelan,

Have a 5.9 Cummins mechanical diesel pusher (1994) with Allison MD 3060 transmission. When I turn the ignition on the ECU will not light up. By leaving key on and then unplugging connectors to ECU and then plugging them back in the N lights come on and I can start and drive the motor home. Every time the ignition is shut off I have to do the same thing. Any ideas sincerely appreciated. Also, new starter seems to kick in and out but will finally turn engine over and start.

John Whelan said...

There is a VIM vehicle interface module that is close to your shift pad. It has fuses and relays that will effect the operation. Also check at the batteries there will be a battery and ground larger wiring (with fuses) that go directly to your trans shifter make sure they are healthy.

amnion said...

Hello, John. We recently bought a 2008 Thomas C2 SafTLiner from a school district and your videos have been very helpful in diagnosing and solving problems. Our bus is in fantastic shape and running condition with exception of a couple gremlins here and there. One of these gremlins jumped aboard while we driving up a grade; as we were accelerating up the grade in the right lane we signaled to pass but a vehicle in the left lane came up fast behind us and I had to stop accelerating and brake suddenly. This sudden change of torque somehow stressed an electrical connection and 'check engine', 'check trans' amber lights illuminated on the instrument cluster. Within 1/2 mile the 'stop eng' light came on and luckily we were able to pull the bus into the Loves nearby and have the technician there read the codes. There were no mission critical mechanical faults present so the tech erased the codes and sent us on our way. Unfortunately these pesky codes will light up on the cluster every 100 miles or so but will seemingly self-correct and eventually disappear. I recently unplugged all the module connections, cleaned them out and added dielectric grease to them but the amber lights are still sporadically lighting on-off for a while-on again-off for a while...I suppose there might be a fusable link somewhere in the wiring harness that shorted out on that grade a while back.
Thanks so much for posting these vids and sharing your expertise here, John. We greatly appreciate it and if you have any specific advice for us regarding these pesky amber light gremlins we would love to send them elsewhere!

many thanks,