A truck came into the workshop with a speedo that wasn’t working properly. It is an easy one to blame on a dodgy gauge, but this one had a very different cause.
Finding the fault
First thing we did was take it for a drive. We gave the dash the odd whack, worked it through corners, and ran it through stops and starts to see when the fault showed up. Sure enough, the speedo cut in and out whenever you held it around 60k an hour.
That test drive is where our diagnostics always start. The way a fault behaves out on the road tells us where to look before we touch a tool.
Getting underneath
Back in the workshop, I wanted to know whether it was a cable-driven speed sensor or an electronic one, because that changes where the problem sits. So I got underneath, and the cause showed itself straight away.
The speed sensor reads its signal off the tailshaft. The tailshaft had excessive play in it, either a nut that had come completely loose or a failed centre bearing, the bearing that supports the tailshaft as it spins. With that much movement, the sensor could not pick up a clean reading, so the speedo dropped in and out.
Why it matters
The gauge was never the problem. It was just the messenger, telling us something further down the driveline was not right.
If we had swapped the dash and left it there, the real fault would still be sitting under the truck, getting worse. A tailshaft with that much play is not something you want to leave, because when it lets go it can do a lot of damage in a hurry.
This is the same reason we chase the cause on every job rather than the first thing that looks likely. A vibration, a warning light, or a gauge playing up is usually pointing at something else, and finding that something is the whole job.
If a gauge or a warning light in your truck is behaving strangely, get it looked at properly rather than tapping it and hoping.
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