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Series 3/FC101 Speedo Issue


garrycol
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The speedo in a FC 101 is the same as that fitted to a series 3 - just has different gear ratios inside.

Last week I had to replace the trip meter cable which I did after a lot of mucking around (it is hard to get at the back of the speedo).

Initially all worked fine but yesterday the speedo needle started only showing half speed and was a bit bouncy but the oddometer and the tripmeter continued to read correctly.

As the oddometer and tripmeter are working fine I assume there is an internal issue rather than the cable which is relatively new.

I would appreciate any ideas.

Thanks

Garry

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Is the method of drive the same in a 101 as it is in a Series 3....pure friction between the speedo gear and the output flange - no key or spline? Could it be that the drive flange has worked slightly loose and just so happens to have occurred at the same time?

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Inside the head you'll find that there are 2 nearly separate mechanisms - the speed indicator - which is basically a rotating magnet trying to pull an aluminum disc against a spring and the odometer mechanism which has a reduction gear which drives the drums which give the reading. The drums are retained by a complicated cut-out flat spring.

Needle behaving oddly - usually a drive problem. The top end may not be engaged in the head properly (or may have dropped out especially if its that clip-on thing which often breaks. Bottom (gearbox) end should be square - there were many cables floating around with an oval end which was supposed to last about 1 mile.

The drive gear at the back of the transfer box on a Series box is pinched (or clamped if you prefer the term) when the output flange is tightened up. Often the flange works loose - when I had this on my LWB the thing would read OK when cold and at low revs - then would drop back at higher speed. Tightening the nut cured it.

However your 101 probably has the early Range Rover transmission - so its different.

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Thanks for that information - good stuff. If the issue was at the tfr case end surely the oddometer and tripmeter would also read slow along with the speedo needle. The plastic housing on the speedo cable us pushed up hard on the back of the speedo.

Thinking about your comments, can the speedo drive cable in the speedo have full contact with the oddometer drive but not the speedo drive.

Thanks

Garry

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This is a good manual with detailed explanation of the speedo and odometer operation.

http://www.tr3a.info/PDFs/speedo.pdf

I have been through several 101 speedos over the years and I tend to find it is either the magnetism driving the cup that fails or the cup/shaft/needle itself seems to develop excess friction. My current operates well except that it tends to stick at 40km/h. Above that it sweeps and below that it sweeps but it has that one sticky point. No idea why.

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