Tuesday, December 23, 2014

TPS Throttle Position Sensor Code

TPS Code Thomas School Bus With an Mercedes MBE 900 Diesel Engine 

The driver called us up on the radio and said he had no throttle. The bus he was driving was 10 minutes out of town. The bus was a Thomas HDX Pusher with a Mercedes MBE 900 diesel engine. The TPS which needs to send a signal to the engine ECU or in Mercedes terms it's called an MCM (Motor Control Module). This was not happening so some diagnostics were in order.

Without a throttle position signal to the MCM shifting and engine power goes crazy. Engine RPM will decrease and the transmission (Allison 3000 series automatic) will not shift properly. The 5 volt signal to the CPC (common powertrain controller) from the throttle position sensor will not happen. The CPC is the interface between the chassis controls and the engine / transmission. In other words it communicates the signals to the MCM.

After finding a proper wiring schematic and checking the circuits we traced the problem down to the TPS itself which as you saw in the video is mounted on the right side of the throttle pedal. It was a funny occurrence because we swapped throttle position sensor units with another bus with out any change. But...the problem was not with the TPS but the wiring to the throttle position sensor connector.

It was hanging by a thread and caused an intermittent throttle signal with a big voltage drop. This causes a bigger challenge for the mechanic doing the troubleshooting. Wiring is tricky at times but the only difference is the time involved to find the fault. So guess what...next time we have a pusher bus with no throttle that connector will be one of the first places to check.

The TPS wiring and switch is on the Thomas Bus (OEM) side so we can't blame the Mercedes diesel powering this piece of equipment. In conclusion it might have been the operator's boot that hit the connector wiring while going into full throttle. It's a hunch but it could very well happen. If you have time leave a comment or a question.

I don't get to questions right away but always check back periodically. Thanks for reading this post and I hope it helped you out. Remember that a diesel engine control module needs to be told what to do. It must have a signal from a sensor before it allows the diesel engine or transmission to operate. It's great technology if you don't get mad at it and lose patience. Work with it and get the experience..... you will not regret the knowledge that you accumulate over time.

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