MGB & GT Forum

Cheers all:
The speedometer on my 68 MGB works fine (accurate, no wobbling) for the first 2-4 miles then suddenly stops working - speed indicator just drops like a rock to 0 mph. If the car is parked for 20-30 minutes - it will work fine for a bit but again suddenly just stop after a few miles. Installed a new cable but problem persisted. Read previous post regarding replacing angle drive and such. Was just wondering if anyone has experienced a similar issue or have a idea what might be causing this behavior. Suspect it might be a speedometer issue rather than cable/ tranny or somehow related to car warming up. Thanks

When this happens, does the odometer also stop working?

possible nearly broken cable. pop out speedo and take a drive to see if cable turns

With my '71 the odometer knob needs to be moved up and down to get it working.

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Be safe out there.
Kenny

I assume that the general design of the speedo for a '68 is the same as that in my '79 and, since I have speedo out and disassembled, I fave it a look. The speedo is very simple; the input shaft has a magnet on the end that sits inside a copper cup attached to the indicator needle shaft. The spinning of the magnet induces eddy currents in the copper cup and the magnetic fields associated with these eddy currents rotates the needle against a counterforce spring. The input shaft also has worm gearing to turn the odometer.

Where you live and this time of year, I imagine that the variations in cabin temperature are small and, even if the temperature did vary significantly, it doesn't seem that would affect the operation of the speedo gage.

Since you already replaced the speedo cable with no change in this symptom, that eliminates it from the list of suspects.

Does your car have a four synchro, non-overdrive transmission? if so, maybe the speedo drive gear on the transmission shaft is slipping once the transmission warms up. Here's a John Twist video where he discusses that problem:

Thanks for all the responses. Took it out yesterday evening - speedo dropped out after 4 miles. Odometer and trip meter were working but stopped at the same time. Local club member suggested it might be female housing on the speedometer where connects is connected is worn or rimmed out a bit and maybe when things warm up, it expands enough for the cable to start slippingp. Suggested making a thin shim out of a aluminum can to tighten things up. At least would have a reason to drink a beer after removing speedo. Thanks again for the replies.