2007 Western Star 4900EX - Intermittent Speedo and Shifting Issues

Question

I've got an old 4900EX with the Series 60 and an Eaton UltraShift. Occasionally while driving, the speedometer will just drop to zero or start bouncing between 0 and 60. When it does this, the transmission gets confused and sometimes drops into neutral until I come to a stop.

I've already replaced the output shaft speed sensor on the back of the transmission but the problem hasn't gone away. It seems to happen more often when it's raining or when I hit a big bump.

OTR_Carlos · Accepted Answer

✅ Confirmed fix: The problem was the wiring harness passing over the top of the bellhousing. The vibration over the years had rubbed the insulation off the VSS (Vehicle Speed Sensor) wires, and they were intermittently shorting to the frame.

Since it's a Western Star, the wiring is usually pretty robust, but that specific run is a rub point. Spliced in new shielded wire and secured it with loom and P-clamps. No more ghost needles or neutral drops.

Ms_Torque · Answer

Check the tone ring inside the transmission tail shaft. If a tooth is chipped or if there's excessive end-play in the output shaft, the sensor won't get a clean reading. Also, check the connector at the firewall for any pin push-outs.

2007 Western Star 4900EX - Intermittent Speedo and Shifting Issues

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