speedo drive

[The Hatter]

No, not driving along in swimwear...

I've just got the ebay sourced gearbox into the 350i and driven around a bit; it works fine and is much quieter (phew!) but the speedo is not working at all. I should have checked it before putting it in the car; ho hum.

So, from reading the web, LT77s had several speedo drive arrangements and it looks likely I've got an incompatable set of bits. It looks from the SD1 manual that the speedo drive pinion can be pulled out of the gearbox without disturbing anything else, so plan A is to try and swap the drives from the old gearbox to the new (Classic Hayn*s 'remove the speedometer drive'). Has anyone tried this or know of any problems I might encounter?

Thanks all!

[Hamish400]

Swopping drives between gearboxes should be OK, but note different colour (plastic gear part) speedo drive pinions have different numbers of teeth. If you need a new part Rimmers have a selection of the different sizes.

Rgds
Hamish

[Jack Valiant]

The 350i should have the plastic type brown/red gear wheel for the standard set-up bit fiddly to do but straight forward

Chris

[The Hatter]

I can confirm you can remove the speedo drive gear with the gearbox in situ; I swapped my old one into the new gearbox and it now works and reads the correct speed - It is a black gear though.

[Wedg1e]

I struggle to understand how you can have interchangeable gears that must be of the same diameter (in order to be able to mesh with the driving gear), have differing numbers of teeth (to effect a ratio change) and yet still mesh with the same driving gear... but it clearly does work!

[GV]

Mine reads a lot higher since my gearbox rebuild could I have the wrong speedo drive or do they all do that?

[hansdaal]

Morning and best wishes to all.

Where is the best place to get a speedo drive for an LT 77 box for a late 350i.
I presume they are all cable driven speedo's and not electric
Any partnumbers would be welcome.

greetings
hans.

[The Hatter]

Rimmers sell the speedo gears; as for part numbers it will depend on your final drive ratio and tyre size. Mine's an '86 350 with standard tyres and a powrlok diff (never checked the ratio) and has the black gear.

[adam quantrill]

Wedg1e said:

I struggle to understand how you can have interchangeable gears that must be of the same diameter (in order to be able to mesh with the driving gear), have differing numbers of teeth (to effect a ratio change) and yet still mesh with the same driving gear... but it clearly does work!

Ian - it works because the teeth are set at different angles depending on the ratio/colour. I've collected a few different ones over the years - must line them all up side by side sometime and take a piccy.

I think the internal gear is a worm drive so it's comfortable meshing with the different angles.

The exact drive ratio needed will depend on the final drive ratio and the rear tyre diameters.

[RCK974X]

Just for info, the Ford 5 speeder type 9 box has (had ?) worm drives of 6,7,8 teeth, which can be mated with 22,23,24,25 teeth drives to give (gbox) ratios of 4.17:1 through to 2.75:1, which is a big range of equiv diff ratios.

I think this was the same as many Ford boxes...

I agree it seems wierd that they all work, I just assumed that the teeth themselves were all the same but had bigger 'gaps' or 'loose' meshing ??

[honestjohntoo]

Does anyone know how to separate the gear wheel from the transducer?