Lotus Elan

no drive to speedo

PostPost by: nigelrbfurness » Mon Jul 21, 2014 4:45 pm

Hi all
I'd welcome a few suggestions as to what might be the problem here. First off, the car is a new rebuild S4SE, just completed. On my first test drive, everything worked except the speedo. I suspected the angle drive on the gearbox, and sure enough it was pretty rough so I replaced it and the cable. I reassembled it all, it seemed to go together without too much difficulty. Still no speedo. I disconnected the cable from the speedo, and the angle drive from the box and got someone to watch whilst I turned the quill by hand. Sure enough, the cable turned at the speedo end, so the angle drive seems ok. So I reassembled the angle drive to the box, and drove the car up the road - no movement of the cable inner at the speedo end. I'm wondering if it is possible that the quill is not engaging with the driven gear at the gearbox? Is there anything else that could go wrong at this end? I'm a bit stumped, short of jacking up the car and putting it in top gear whilst the engine idles to see if the driven gear is turning! I did notice that the aluminium extension piece the angle drive screws to is a bit loose even though the clamp fork is done up tight. Anyone got any bright ideas, please?!

Nigel F.
1970 S4SE/1760cc big valve/SA-AX block, L2s, 45DCOEs, 1978 Jensen GT, 1962 AH Sprite, Alfa-Romeo 159, 1966 Bristol Bus, 1947 AEC Regal bus.
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PostPost by: ericbushby » Mon Jul 21, 2014 6:52 pm

lotus elan may 2011.jpg and
Nigel,
Mine was just like that.
Turned out to be a mangled speedo drive gear in the gearbox. you can withdraw it after removing the angle drive.
First drain the gearbox oil, then release the clamp screw and fork. with a little prising it should come out. There is only the `O` ring making it stiff.
They are colour coded for the No. of teeth and I got one from Sue Miller.
Eric in Burnley
S3SE DHC
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PostPost by: nigelrbfurness » Mon Jul 21, 2014 7:49 pm

Eric
Thanks for that and the pictures, seems like a good bet that that is what it is. I'll have it out first thing in the morning. I've got some spares so with luck that will be it. I think green is 3.77:1, which is what mine should be.

Cheers again
Nigel F
1970 S4SE/1760cc big valve/SA-AX block, L2s, 45DCOEs, 1978 Jensen GT, 1962 AH Sprite, Alfa-Romeo 159, 1966 Bristol Bus, 1947 AEC Regal bus.
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PostPost by: Peter Richen » Tue Jul 22, 2014 1:28 am

I had a similar problem with my 65 S2.
It's pretty easy to tell if the speedo drive gear in the transmission is messed up, because it comes out pretty easily. Note that this gear floats, and it is hard to be sure if it has engaged the drive.
The replacement right angle drive was NLA (according to Ken Gray at Dave Bean Engr), but they sold me an MG right angle drive with an adaptor. It turns out that this adaptor was slightly too long, so it wouldn't engage the drive gear. I solved the problem by cutting about 1/4" off the threads with a hacksaw. Works fine now.
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PostPost by: nigelrbfurness » Wed Jul 23, 2014 1:19 pm

Thank you all who posted replies. I thought it would be useful to others to report the actual cause of the problem. It turned out that the quill section of the new angle drive was fractionally too short thus not engaging fully with the driven gear in the gearbox. I could only see this problem with the driven gear and housing removed from the gearbox and the whole lot assembled on the bench. I did consider returning the angle drive but a little thought produced two conclusions: (1) the part was obviously a modern repro of an original Smiths component so a replacement was unlikely to be any more dimensionally accurate and (2) the solution might lie in careful matching of components. I worked my way though half-a-dozen driven gears and they all failed to engage fully with the quill when all the parts were assembled. The solution then became a matter of how much longer did the quill need to be, and could it be lengthened (NO!) In fact it turned out that if it were 1/8" longer it would engage fully with the driven gear shaft. I then looked at the aluminium extension housing and measured the three I have - all were the same length to within 5 thou as accurately as I could measure them. In this process I deduced that there was enough material in the extension housing to machine off 1/16" from the flange where it is clamped to the gear box and 1/16" from the threaded section for the angle drive without loss of function. I also turned off 1/16" from the raised section that centres the housing in the gearbox plug to keep all the relative dimensions accurate. Reassembly on the bench showed I now had full engagement with a little end float allowable on the driven gear so I put it all back in the car and I now have a working speedo. That was the last niggling issue, so anyone want to buy the car? :)

Thanks once again to all who took the trouble to comment.

Nigel F.
1970 S4SE/1760cc big valve/SA-AX block, L2s, 45DCOEs, 1978 Jensen GT, 1962 AH Sprite, Alfa-Romeo 159, 1966 Bristol Bus, 1947 AEC Regal bus.
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