2008 Western Star 4900 - Speedometer Jumps and Engine Stumbles

Question

Cruising down the highway and my speedometer starts dancing all over the place. Whenever it spikes, the engine feels like it hits a wall for a split second, then recovers. It's like the ECM thinks I'm hitting the speed limiter and cutting fuel.

I've checked the VSS (Vehicle Speed Sensor) on the back of the transmission and the wires look okay for a 15-year-old truck. Is there anything else in that circuit that can cause the engine to cut out like that?

RustBeltRandy · Accepted Answer

✅ Confirmed fix: The problem was a bad ground at the battery box. The VSS shield wire was picking up electromagnetic interference from the alternator because the main system ground was weak. Cleaned the frame-to-battery ground strap and the 'ghost' speeds disappeared.

SuspSpecSal · Answer

Check the tone ring inside the transmission. If it's loose or has a chipped tooth, it will send a garbage signal to the VSS and cause exactly what you're seeing.

2008 Western Star 4900 - Speedometer Jumps and Engine Stumbles

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