MAINTENANCE MATTERS - Verifying a throttle position sensor

You may not give a lot of thought to what controls the throttle that gets — and keeps — your coach moving, but it's a handy bit of knowledge to have. Modern over-the-road diesel engines no longer use throttle cables. Instead, they use a type of three-wire variable potentiometer that we refer to as a throttle position sensor, or TPS. With a simplified Fly-By-Wire type of system, the engine RPM is controlled by comparing a known voltage from the engine's ECM with the voltage signal returned from the TPS between two separate resistance values. You can think of the TPS voltage as driver demand, or what the driver is telling the engine to do.

	5V Supply from ECM
	Signal
	Return

How it works, and what to do when it doesn't

So as a voltage is outputted from the ECM through the TPS resistor from pin C to A back to the ECM on the return a constant value is established. By varying the moveable signal (Pin B) on the throttle pedal we vary the voltage value as compared to C causing the ECM to interpret this value it then translates this as a count which is diplayable on an engine reader tool.

Use the appropriate engine manufacturer's troubleshooting manual to diagnose any codes emitted by the ECM.

The right tools for the job

Having a proper engine diagnostic tool in a mechanic's tool chest is a must today. This tool will help in diagnosing TPS issues by monitoring the count values being returned to the ECM in near real time, something that is especially importatnt when there is an intermittent issue to resolve.

If you have additional questions, or issues you can't resolve with your manual, call your fleet support manager or technical support at 800-241-2947.

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