Erratic Speedo

[Rice crispy]

I was out in the car this morning and for about the first 5 minutes the speedo was very erratic, one second showing true speed then dropping to a lower speed then back up again. After about 5 minutes it was back to normal? Any ideas, cable, plastic drive? Could it have been koved when the hoses were replaced?

[dapinky]

The actual drive for the speedo (and odometer) is a purely mechanical system whereby the cog in the gearbox drives the cable (rotational) which drives the mechanism in the instrument cluster. There is an electronic pick-up in the instrument cluster to provide an electronic signal to the ECU, but this has no bearing on your issue (except the ECU will be getting a false reading, so it may affect the running of the engine).

I would suspect that if you have been changing the hoses, you (or someone else) may have disturbed the routing/seating of the cable at the gearbox end. The cable always appears to be a bit "short" to me, and an extra 1" would be better, but that is me looking from my perspective of fitting it, not the engineers perspective of what works best!

The first place I'd look is where the cable enters (technically, I suppose it's exits) the gearbox and ensure that the clamp which holds it in place is correctly fitted. Then check the routing of the whole cable, and then remove the binnacle cover from the top of the dash to check where it enters the speedo. Somewhere there may be a noticeable fault that you can rectify, if not, then it may be a defective cable, speedo or drive gear - but hopefully not.

[Rice crispy]

Thanks Dave I will start at the easy end but I suspect its at the gearbox end

[Simon_P]

which drives the mechanism in the instrument cluster.

This is a magnetic disc and a steel disc with the needle attached and a spring. The faster the magnet rotates the more force is applied in the steel disk there isn't much to go wrong.

The cable can pull out of the speedo, the cable can break or the gear can sit in top of the drive gear in the gearbox - that's about it.

[Rice crispy]

Thanks Simon , I'm betting on it being unseated but it's puzzling that when the car warmed up it was ok. I suspect something is expanding just enough to make it work when warm

[Rice crispy]

So had a quick look and the Speedo cable runs between the hoses that go to the heater and as Dave says it looks like it could do with being longer. I wonder if it should travel down the inside of both which looks to be a slightly shorter run. I couldn't tell if it was connected properly but will take it back to my mechanic to check