Rev counter behaving badly

[PierreW]

No, I don't have an AC generator, as the incoming current is anyway rectified in the meter by a selenium rectifier without being followed by a capacitor, it only causes an error at low voltages, which is probably shown on the curve in low revs.

[cactusman]

If it worked.ok beforehand then I'd suspect the wiring to the tacho and not the tacho itself in the first instance..if it's driven as per original from the generator on the cam then there are only two wires between the two...I reckon it has been modified in some way and the new loom, as you say, doesn't have the modifications that were present in your old one....just my thoughts...

[dlgis]

Quick question: Is there meant to be any earthing to the Rev Counter. I have 2 wires from the rev generator- both good, and a single wire to the clock. No earth?

[cactusman]

If your tacho operates directly from the generator on the cam as per the original design then for the tacho to work all it needs is the two wires to the generator. No connection to ground is required. If this is your tacho configuration then the ignition system probably isn't the issue . Either there is a wire loose, a fault in the tacho or a problem with the generator. If your HT leads are.copper and you don't have resistance plugs the ignition system will generate a LOT of high frequency interference that may possibly interfere with the radio but IMO is unlikely.
The clock is fed from the 12 volt supply and obviously does need a path to ground but given the minuscule current the connection is made.by the retaining clamps on the back of the radio securing it to the dash. No harm in adding a ground...

[dlgis]

Thanks Julian, i did find when removing the tacho that the clamps were very loose, which might explain why the clock has stopped working. I was hoping that it could also be the reason for the erratic behaviour of the rev counter (only goes from 500-1500 rpm and not necessarily in sync with actual engine revs). I’ll try an earth and see what happens.

[dlgis]

Quick update - I removed the tacho from the car as I’d ruled out rev generator/drive dog and wiring as the fault. Took the bezel and glass off and flicked the needle around a bit. Reassembled and hey presto it seems to be working again. So maybe just a bit of arthritis in the mechanism. Just have to hope it stays fixed

The clock was also not working and so I’ve run an earth directly to it and now that works too.