Tacho not registering at idle but moves when revved

[AndrewH5797]

Hi hope someone can help, I took out the instrument cluster to replace some dead bulbs when I put it back the tacho sits at zero at idle but does register when revved. I checked with the haynes manual wiring diag and the wires to the tacho are different colours to those indicated have I got the wires switched ?
TIA
Andrew

[Jeroen]

That's funny, afaik you can only fit the connector one way, but there are two connectors right next to each other, maybe you didn't notice you pulled the other one out? See pic below, btw the 3 pins facing down is a more common version of the rev counter, but you can encounter both situations.

[jbrunken]

All of the contacts that I can see in the photo look oxidized. If the tach works at higher revs, it's likely because the system voltage is higher than it is at idle, and becomes sufficient to overcome the oxidation.

I noticed that the printed circuits on either side appear to be separating. They're old and the adhesive in them is probably expired by now. I had to re-laminate mine because many of the copper contacts came loose.

I re-laminated mine with some clear epoxy and cleaned the copper with some metal polish wadding. I supported them from behind with some wooden sticks, the ones that look like tongue depressors, while I worked on them. This solved the problems I was having with the gauge needles intermittently jumping around.

[Jeroen]

[jbrunken]

You should check the connection on the coil too, if you haven't already.