Lotus Elan

Tacho problem with electronic ignition - but NOT bouncing.

PostPost by: Johnfm » Thu Nov 14, 2013 10:42 am

Got car started last night after replacing points with Accuspark black box and ignition and repalcing dynamo with dynamator.

Not getting any tach bouncing, but I noticed that when I switched on the headlights, the tacho was definitely under reading.

The pulse feeding the tacho is the switched wire that goes form ignition, through the tacho and then on to the +ve coil terminal. It is providing the power for ignition.

If lighting load is affecting the current flow through this wire, will it in fact affect my ignition?
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PostPost by: rgh0 » Thu Nov 14, 2013 11:18 am

The tach has a 12V supply to it behind the dash and a voltage regulator in its circuits to feed the electronics a constant voltage. it sounds like this voltage is dropping when the lights are turned on and this is dropping sufficiently to overcome the tach voltage regulator and affect the tach accuracy.

I presume you have powered your electronic ignition not off the positive supply to the oil and the headlight wiring is standard.

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PostPost by: Johnfm » Thu Nov 14, 2013 11:26 am

Ignition is powered by +ve side of coil.
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PostPost by: rgh0 » Thu Nov 14, 2013 2:12 pm

this is guaranteed to give you problems --- move it to an independent supply. The electronic ignition takes its power in pulses just like the coil for the ignition and the tach has a tendency to sense both

This is why everyone wastes money converting their tachs as they do the wiring like you have when they don't need to

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PostPost by: Johnfm » Thu Nov 14, 2013 3:48 pm

Will do!

No reason I can't find a switched power source elsewhere in the engine bay or inside the car.

Cheers
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PostPost by: billwill » Thu Nov 14, 2013 6:05 pm

Johnfm wrote:Will do!

No reason I can't find a switched power source elsewhere in the engine bay or inside the car.

Cheers



Wires powered through the ignition switch are generaly WHITE.

You can make a T-joint to the wire from the ignition switch , BEFORE it goes into the tacho loop.
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PostPost by: Johnfm » Fri Nov 15, 2013 10:34 am

All is good.

The tach was only dropping if I switched the panel lights on (not the headlights), but that isn't happenibng now.

I will rewire teh ignition feed anyway, but all is good.

Need to replace water temp sender (as it seems to over read and is a bit inconsistent).

Other than that - these engines love revs!!!
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