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 Posted: 11-20-2016 05:58 pm
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NigelK
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Dear All

I am really struggling with the tachometer in my GT which, despite being calibrated 3 times, is still reading low. It is the standard Smiths RVI tachometer, but converted to RVC function with a Spiyda RVI-RVC conversion board when I upgraded to electronic ignition in 2015. I pulled the tacho out of the dashboard and calibrated it again yesterday, against my multimeter rev counter, and it was reading OK. But as soon as I put it back in the dashboard it started reading low again. I've checked behind the dashboard and all the wires are still connected. it's almost as if mounting the tacho in the dashboard is somehow impinging on the mechanism so it does not read properly. But this doesn't make sense as it's an electronic unit, not a mechanical unit like the speedo.

I am getting really bored of pulling the tacho and reinstalling it - which is very awkward and always results in at least 2 cuts and grazes on my hands. So any ideas would be gratefully received!

Best wishes,
Nigel

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 Posted: 11-21-2016 06:52 am
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subwoofer
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If it reads correctly when not mounted but incorrectly when in the car, there are really only two possible causes; either the voltage is too low for it to work correctly, or that the mounting in the cluster somehow distorts the housing causing extra friction on the mechanism.

--
Joachim

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 Posted: 11-21-2016 07:30 am
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NigelK
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Thanks Joachim. If the voltage was too low (possible given the age of the tacho electronics and the fact that I'm running with electronic ignition) then wouldn't the problem be constant irrespective of whether the gauge was mounted in the dash or not? Someone has suggested that a bad earth might be the problem, but again this should not be affected by whether the gauge is mounted or not. Do you know how the tacho should be earthed? Via the threaded mounting posts or via a separate wired connection? And, if the latter, which is the ground terminal on the back of the gauge? Best wishes, Nigel

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 Posted: 12-11-2016 03:44 pm
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NigelK
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An update. I had the tacho out again yesterday. Cleaned all the contacts with emery paper, replaced the female spade connector on the earth wire (as it had opened up too much to fit securely onto the male spade connector on the tacho casing) and checked that the earth was good. Then recalibrated the tacho again, this time ensuring the reading matched that on my multimeter when the tacho was in situ in the dashboard. All good, until I re-installed the tacho and it was reading low again. I started the car up again 24 hours later and the tacho was reading even lower. I am beginning to suspect that 41-year old electronics components are at fault here and that a full tacho refurb is required.

A fellow JOC member has suggested noise in the ignition circuit could be the problem. Worth noting that the tacho needle bounces to around 7,000 rpm when I start cranking the engine but settles down as soon as the engine fires. Is this normal? Or a sign of something amiss in the ignition circuit? Please note that I do not have the standard J-H ignition circuit in my GT - I have the Lucas Constant Energy system fitted i.e. 45DM4 optical trigger distributor and AB14 ignition amplifier.

Does anyone have any other ideas?

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 Posted: 12-12-2016 02:14 pm
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Brett Gibson JH5 20497
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What size coil, and do you have a ballast resistor in the circuit. ???

Brett

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 Posted: 12-12-2016 03:57 pm
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NigelK
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3 ohm coil, no ballast resistor (or resistor wire) in the circuit...

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 Posted: 12-12-2016 05:31 pm
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Brett Gibson JH5 20497
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From what I'm told that is the correct ohm's for optic ignition, have you tried running some temp. wires to the tach to see if it still fades, I'm thinking you might be losing voltage thru the harness ??

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 Posted: 12-12-2016 07:09 pm
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NigelK
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Thanks Brett. My next move, as suggested by Chris at Spiyda, is to install a relay wired so that the tacho is not energised during cranking. Chris thinks the wild needle bounce during cranking might be disturbing the balance weights in the tacho, with a knock-on effect on calibration. I'll try this out at the weekend and post the results...

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 Posted: 12-18-2016 09:12 pm
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NigelK
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I fitted a relay today, wired such that the tacho isn't energised during cranking (so no wild needle bounce), and it seems to have solved the problem. I'll keep an eye on it going forward though...

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 Posted: 12-21-2016 04:40 am
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Bfitz241
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Nigel,
that's good news..hope it's a permanent fix

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