Paul M. wrote:
> I need help troubleshooting some gauge problems in my
> 1971 MGB tourer.
> The speedometer has always bounced around a little,
> but it's been getting increasingly bad recently, and
> over the past month, it's also been reading more and
> more "optimistically" (higher than true speed). At
> first it was just 5 mph or so, but then 7, then maybe
> 10... But when I went for a drive this afternoon,
> it's suddenly bouncing around wildly and reading about
> 40-50 mph high. But it literally bounces between, say
> 70 and 120 mph. Wild bouncing.
> But the odometer still reads perfectly, so am I
> correct in assuming that it's probably not a sender
> problem? (Or pickup, or sensor, or whatever it's
> called...) Is it likely to be a problem in the gauge
> itself? Or is there a drive unit or something that
> might be bad?
If your system is like the ones I have worked on, the Speedo and Odo
operate off separate systems after the cable input.
The speedo works off a magnetic coupling, but the odo operates off a
"picker" that moves the odo section one tooth at a time.
If your inner cable has a broken strand, it can catch on the outer
cable, causing the inner cable to wind up and release, making the Speedo
jump all over the place. This probably wouldn't cause the odo to read
wrong. Often the cable problems start when the outer wrap is breached,
allowing water to corrode the inner and outer cables and degrade the
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