Allison Ep 40/50 Shifter issue

Michael Diagnostician Illinois Posted   Latest   Edited  

Hi everyone, sorry for the absence on here of late, been busy. I have something to share. I have a Allison Ep 40/50 Hybrid Transit Bus that has Active codes for the PBSS.  This stands for the Push Button Shift Selector. The Shifter is not lighting up and because of this the Bus will not start. As seen on the code list there is multiple codes for no PWM input from the Shifter and also it is showing codes for loss of CAN Messages to the Shifter. The codes also state it has multiple system speeds unknown and abnormal power down for TCM and VCM. As you can see their is quite a few codes on here so where do I start? When you are faced with this dilemma the best way to attack a problem like this is to go for the codes that are the highest count and look at the info for those codes and see how they interact with each other. So as you can see by the high count codes it is basically stating that the shifter has lost it's ability to provide range input and also that their is no Can Communication either. These buses use J1939 Can Bus at 250 K Baud. So at first I thought I'll just break out my Dvom and do some checks right? I did do some Voltage drop Testing with the Load Pro but decided that with the symptoms present I needed to use my  Picoscope. I decided I wanted to look at the Power, Ground, Ignition and the current being consumed by the shifter. The reason I'm using the Current Clamp is I want to see if the shifter is drawing not enough current or too much current and thus possibly interfering with the Can Bus Network. As you can see the shifter is dead.

So here is my first capture. Notice anything odd?

The ground side of this circuit for the shifter has excessive resistance. The Power feed and ignition check out ok. Here is also another Scope Trace showing how the Ground Circuit is being intermittent with High Resistance.

The next thing I did to confirm the issue is I ran a jumper wire to ground from my ground connection at the shifter.

Here I am showing my ground connections. So after I jumped the ground the Shifter powered up fine and the Bus started with no issues. Here is a video of me showing jumping the ground to prove the fault. Here are the good readings for the Shifter. And for the final nail in the coffin I show the fault. Now I bet your asking Mike why didn't the load test of the wires show the hard fault? It did not show the hard fault because of the intermittent nature of the problem. When I used the load pro the Voltage dropped a couple volts which I thought was borderline so to back up my suspicions I decided the scope was needed and as you can tell by my Picoscope captures the ground side voltage drop was varying. Having a scope with multiple channels to rule out out circuits is a must now in this day and age. And of course the final verification no codes after the ground wire fix. Hope everyone enjoyed that. I work on a ton of Hybrids on Buses so stayed tuned for more cases studies. Everyone have a good evening.

Nelson Manager
Nelson Default

Interesting. I've never worked on heavy duty trucks but i'm wondering..., where do you source your wiring diagrams on these buses? Seems to me that it could be a bit of a nightmare especially when you start getting into body electrical issues. Hopefully i'm wrong!

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Michael Diagnostician
Michael Default

It can sometimes be a little difficult but not too bad. We get wiring schematics for the actual bus and then we match the bus wires with the vendor wires. Of course sometimes their are mistakes on schematics.

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