batt interference on rev counter

[chrisdurham]

Hi guys this is the first chance ive had in a while to re visit the rev counter issues ive got.where it started fluctuating badly, so i took it out the car, took a feed from the coil switching side, and a positive and negative off my spare batt on the bench, and guess what, all was fine and back to normal,
to try and track this down further, i ran a lead straight to the cars batt negative term. And the pos using the original connector and the fluctuating was back. Done the opposite ie lead straight to batt and used original neg, fluctuating back.
So where from here, i think this rules out the coil and any interference its picking up, i also tried running with the alternator disconnected, still the same.
any ideas guys

[taylormj4]

Good investigation method.
Sounds like coil and gauge are fine as you say.
Maybe poor joints or degraded cabling affecting both the +ve and -ve cables (possible if run close together in a loom arrangement).
If you have an ohmmeter, you could check the resistance of the two existing cable cores between the gauge and the battery (with them disconnected of course). Alternatively, check the volt drop across the cables with a voltmeter with it all connected up.
Cheers, Matt