Electric speedo sensor (gearbox mounting) - how does it work / how do I test it?

[AntonyH]

My speedo is still dicky... after many years of being rock solid, the needle recently started occasionally bouncing up to the 80mph mark, when actually travelling at, well, any speed really. Even, sometimes, when parked with the engine running.

The speedo sensor is in the bottom of the gearbox (K-series, 6-speed) not at the wheel.

On the advice of our local 7-friendly garage I replaced the quill as a first attempt and that made mostly no difference, other than occasionally now the speedo doesn't read at all. Hmm.

I'm guessing the problem is the sensor unit as a whole, or maybe the cables to it.

Question: How does the sensor work, in an electrical sense? Does the turning of the quill generate a voltage, or does it adjust an impedance?

Just trying to figure out how to trace the problem so I don't buy a replacement unnecessarily if the problem is elsewhere in the loom (or is the dial itself).

[AntonyH]

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[Jonathan Kay]

I've always assumed it was a rotary pulse generator, as on rear wheels and bikes. Replacement options. Check the feed and earth directly, and continuity on the signal return.

Jonathan

PS: Does anyone know if there's a clearance adjustment on these?

[AntonyH]

Thanks.

There is definitely no clearance adjustment on the sensor, it just screws on to the part that's captive in the gearbox. The sensor itself is sealed.

[OliverSedlacek]

I would guess that the sensor is a hall sensor. If so, it detects the presence of a magnetic field and generates an electrical pulse for every magnetic pole that passes under the sensor.

[AntonyH]

Seemingly now sorted, update in previous thread:

http://www.lotus7.club/comment/2178257#comment-2178257

[Jonathan Kay]

That sounds right.

Well done.

Jonathan

[AntonyH]

Just as well really, if that had made no difference I'd have been spending £70 on a speedo sensor that may or may not have made a difference!