TD rev counter repair

[garyd]

Hi,
my diesel rev counter has developed a habit of swinging quite wildly when the engine revs change significantly (during gear changes, etc). I am wondering whether perhaps there is an electrical component that acts to damp the needle action and which has perhaps failed.

Is there someone with more knowledge of electronics that could advise, please?

[silverbullet]

Hi Gary. Thinking out loud here:
TD tacho is driven from the alternator, so if the drive belt system is slipping somehow then maybe the alternator could overrun the engine speed at gearchanges because of its own inertia?

Sent from my Atari 7800

[syncroandy]

Hi gary, did you get my reply to your PM ? Electrolytic caps have a finite lifespan and they're often used in the sort of filter circuit which may provide a damping function for the tach needle. Simply replacing the caps with fresh equivalents often provides a fix.

E D I T: just checked an old tach I have here and it has 4 electrolytics.

[garyd]

Thanks Ian, I will check the belt but it has a spring tensioner so 'should' be OK.

Andy, yes I got your pm and sorry I didn't respond. I interpreted it as a surmised expectation of what one would find, rather than your more specific reply here. Any chance of a photo identifying these capacitors and or a specification for replacements, please?

[silverbullet]

It was just a thought (always check the obvious!) but having read Andy's reply, his prognosis seems much more likely.

Sent from my Atari 7800

[syncroandy]

Gary, the tach I have here is a petrol one, it's possible the component value may be different on your diesel one so you'd best check 'em yourself. The value (in microfarads typically) are printed on them.

[garyd]

Thanks Andy.

I will have to pull the dash out again after the weekend and see what I find.

[max and caddy]

I'm 99% sure I got a petrol PCB film of Vwnw Mick and used it on my van with the diesel tachometer...it worked fine..at least until I changed the alt for the biggest one I could get anyway,

[syncroandy]

Aye the flexi may well be the same but it's the little round PCB inside the tach that I'm on about, as the signal frequencies are quite different petrol vs diesel I think the component values will differ.