Rev counter puzzler

[IslipMinor]

The car is being wired up after its respray and has the engine fitted, but not running yet.

Checking each circuit as completed, and when checking one of the additional 'ignition on' circuits, noticed that the rev counter was showing 3,000rpm - but no engine running?? Please see the picture below:
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First check was the rev counter connections, but they were OK as they are the same as before. Checked various other things, even disconnected the connections from the coil - still showing 3,000rpm with the ignition turned on! Obviously removing the supply to the rev counter resulted in 0rpm.

In the end I wondered if using the battery charger to test out all the circuits could be the problem, so carefully connected the battery and all OK!! So it looks like the rev counter is 'seeing' the 50Hz mains coming through the battery charger (50 x 60 = 3,000), even though it is supposed to be 12v DC. Maybe not full wave rectification? Anyway, an hour or so taken to find a 'non-problem'!

Good way to check the calibration of the rev counter I suppose?

[panky]

Very interesting, as you say a good calibration method.

[amgrave]

Most battery chargers are full wave rectified but not smoothed so the rev counter was picking up mains ripple. Some of the older chargers were only half wave rectified and slower charging.

[bmcecosse]

Interesting -and a great tip to check if a rev counter is working or not. I sold mine recently - not used for over 40 years -and had to sell it without guarantee. In the end, a Company that appears to specialise in refurbishing old car dials etc bought it, and for slightly more than I was expecting.