Tacho Woes!....

[Darren65]

When I had problems with the modified 10K tacho in my T I took the 8K tacho out of my E to test it in the other car and it worked perfectly.....10K gauge back to North Hollywood Speedometer for repair (who incidentally have been excellent and have upgraded the gauge free of charge even though it worked fine on their test bed ).

When I put the 8K gauge back in the E it had ceased working.....thorough testing of all electrics seemed fine so a quick call to NHS who suggested I try it back in the other car to be certain it was the gauge that was at fault.

I've kindly been lent a 10K tacho from Dougie which is in the T whilst my original is away for repair.......so this evening I took the 10K tacho out of the T and tried it in the E where it works fine, confirms good electrics....

....this is where it gets weird....I then tried the 8K gauge back in the T, where it works fine!.....tried it back in the E and it doesn't work!.....switch to the 10K gauge in the E, works fine......switch back to the 8K gauge and it doesn't work!!!!

I again called NHS who tell me there's nothing complicated about these tachos and it must be a wiring fault.......however I can switch from the 10K to 8K gauges in the E multiple times and each and every time the 10K gauge will work and the 8K gauge doesn't.......both gauges work fine in the T.......get your head around that!


The 8K tacho that currently doesn't work in the E is original to the car and has been fine for over 20,000klms since refurb......all I've done is try it in another car.

This makes no sense whatsoever, can anyone help???

Cheers,

[hot66]

what exactly is the ignition system in the ST

[impmad2000]

So the ST is Canems, and the E is CDI. Do you know where the tach wire in the engine bay of the E is wired to.?

[sladey]

Don't think he's got Canems has he? That's me that is

[Darren65]

Guys, cannot be wiring or ignition system???

This is the original tacho that's been in the car from day one and was working fine until taken out.....it works in the other car but not in the E.......a different gauge does work in the E!

[Jonny Hart]

Sometime around 72/73 the tachos changed between needing a high voltage input with the correct dwell vs any old square wave at a lower voltage.

My guess is your T is running an adapter of sorts as it has aftermarket ignition. This steps up the voltage to drive the earlier 72 tacho. If you plugged the later E tacho in, it may have damaged it.

[impmad2000]

Some tachs work with a voltage spike, some with square waves, some with 5v signals and some with 12v.
911s over the years have used all of these methods and so without knowing what had been connected to what I can't answer your question.
If a high voltage spike has damaged the input circuit of the E tachs no longer works in the E it might still work in the ST If the Canems is wired with a tacho driver unit that creates a high voltage spike ( possibly low energy, high voltage) to drive the Tacho. A direct connect to the coil will generate a high energy high voltage differentiating it from the the low energy and so may have damaged thing in the E tach that the low energy on wouldn't have damaged .
Make sense ?

[Jonny Hart]

^ that's basically what I said which is jolly lucky isnt it

[Darren65]

Thanks guys that does make sense....I think!

The T is currently taking the feed directly from the coil which suggests connecting my E gauge has in some way damaged it.........I just couldn't understand why it still works in the T......the guys at North Hollywood Speedometer didn't think this could be so but I guess there's no other explanation for it!

Bugger!

Thanks for the feedback.

Cheers,