Derek,
You don't say if the speedo was jerky immediately after the
conversion or if it's a recent problem. The overdrive's speedo pinion
is located further to the rear of the car than the pinion on the non-OD
gearbox and thus needs a longer cable Moss: 331-560. If the jerkiness
started right after the conversion, it could be that the original cable
is still in the car and is pulled too tight, resulting in a to-tight
bend somewhere. You should also check to see if the ange drives are
present on the speedo and gearbox. If the jerkiness is recent, it may
be that the cable is wearing out and sticking or needs lubrication. If
the cable is sticking in the housing it needs to be attended to right
away. I had a cable sieze in the housing right behind the speedo and
the drive pinion shaft sheared off the head. It's a rather expensive
part and it's a PITA to fish the broken gear out of the OD housing.
The drive pinion is located on the OD output and is driven by an
internal worm. AFAIK, the pinions aren't interchangable between ODs.
To correct the low reading, you need to find a speedo with the correct
turns-per-mile (TPM) ratio as the OD drive pinion. For the LH type
22/61972 OD w/21 tooth pinion OD you want a 1280 TPM speedo. These #s
are located in a plate on the bottom of the OD housing in the SN box. I
have the 61972 OD with the 21 tooth pinion & 1280 TPM speedo in my '69
B. I think the 20 tooth pinion wants a 1080 TPM speedo.
I'd take it back to the mechanic, in any case.
Cheers,
CR
Derek Vandivere wrote:
> So my mechanic, about a year ago, converted my 78 B to overdrive.
> Hooray! I now have two problems with it: the speedo is now very jerky,
> tending to settle down a bit more at speed (around 20 mph, it jerks
> between 0 and 40, but only oscillates by 10 mph or so at 70 mph).
> Second, it consistently reads about 10 mph (maybe 10%, actually) lower
> than my actual speed (as measured with a GPS).
>
SNIP
>
> (or should I just give up and go back to my mechanic and tell him it
> just don't work and he should fix it? it's been a bit of a saga with
> this speedo...)
>
> Thanks,
> Derek (78 B in Amsterdam, and now a 2002 Saab 93 diesel for the commute)
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