doing the tacho boogie

Joelipol · Post by **Joelipol** » Wed Aug 31, 2016 9:18 am

I have a 71 911T sporto with rebuilt gauges from Hollywood and a temperamental tacho.

The tacho swings wildly from nothing to redline+ when it is started, regardless as to if the car is hot or cold.
However if the car is 'hot' it takes very little time to settle down and read accurately - if it is cold it takes a bit longer.

It seems the car needs to be 'hot' to be reading accurately.

No idea what could be going on here but would be grateful for some advice.

Joelipol · Post by **Joelipol** » Wed Aug 31, 2016 3:46 pm

Oh I have:
- new voltage regulator (problem was there with old one as well)
- new batteries
- rebuilt alternator
- I've checked that the alternator is earthed properly
- I haven't checked the starter
- I haven't checked the connections to the tacho

jb · Post by jb » Wed Aug 31, 2016 4:02 pm

Maybe check the points gap as they seem to affect the rev counter sometimes.

Joelipol · Post by **Joelipol** » Wed Aug 31, 2016 4:09 pm

Points gap checked and a-ok - new points as well... the question is why would it work once the engine has been running for a bit... seems odd.

Mike Smith · Post by **Mike Smith** » Wed Aug 31, 2016 4:18 pm

Why not speak to Hollywood Speedo ?

Joelipol · Post by **Joelipol** » Mon Sep 19, 2016 6:35 pm

It seems there were multiple problems just to round off the thread:
1) There is a tacho 'deadening' capacitor on the cdi electrical board that had gone. Apparently as it heated up the resistance in the unit changed and it would start to work again... when bypassing it, bam - tacho worked.
2) alternator output is down and has likely been damaged by overcharging
3) multiple earths needed redoing...

anyway - now some things to replace and mystery solved

MT · Post by MT » Mon Sep 19, 2016 6:47 pm

Nothing to add technically, but thanks to Joel for rounding out the thread. Not so much here, but on Pelican and 991S I have been frustrated in the past when after being thrilled at finding a thread that describes my very problem, to then be frustrated when the OP never posts the end of the story about what fixed it!

Barry · Post by **Barry** » Tue Sep 20, 2016 10:27 am

MT wrote:Nothing to add technically, but thanks to Joel for rounding out the thread. Not so much here, but on Pelican and 991S I have been frustrated in the past when after being thrilled at finding a thread that describes my very problem, to then be frustrated when the OP never posts the end of the story about what fixed it!

Too right! How annoying is that, not to mention discourteous.

Good thread, excellent reference for the future

.

DDK - Die Deutschen Klassiker

doing the tacho boogie

doing the tacho boogie

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