Tacho peculiarities

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AndyLotus
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Tacho peculiarities

Post by AndyLotus »

Slightly strange one this.....

I currently run mapped ignition via a Nodiz ECU. This ECU has a 12v output to specifically run the tacho.

Normally this works fine, however sometimes (particularly if the engine is cold/or the weather) it will start up with the tacho not registering.
The weird thing is, you can get it to work instantly by switching on anything electrical i.e. sidelights, heater fan, electric windows etc.

Once it's been running a few minutes it all behaves and the tacho works correctly with other electrical items off.

Is there some strange electrical phenomenon going on here? :?

Cheers,
Andy.

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Re: Tacho peculiarities

Post by Sirclip »

I guess you have already tried a little 'tap' on the rev counter, eg is it a mechanical 'stiction' problem?

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Re: Tacho peculiarities

Post by AndyLotus »

Unfortunately that doesn't help.

It does help the fuel gauge though. :D

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Re: Tacho peculiarities

Post by Zag »

I too am running a nodiz but do not have these symptoms.
Does not narrow the fault down other than it can work correctly.
Did you have the symptoms on the original ignition?
The way I wired it there is only be 3 connections to check, the plug on the back of the instrument panel, the wire from the nodiz to the white / slate wire on the bulkhead where the coil used to be and the wire into pin 2 on the plug on the back of the nodiz.
Last edited by Zag on Mon Nov 25, 2019 18:34, edited 1 time in total.

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Re: Tacho peculiarities

Post by Lotus-e-Clan »

My guess is that the 12v pulse output from the 'Nodiz' might be flakey because of earth issues?

As ever, it's difficult to replicate intermittent faults. :|

I have an old automotive multimeter that has a tacho mode. If it were my issue to trouble-shoot, I'd be inclined to connect my multimeter (switched to tacho) directly to the Nodiz 12v pulsed output to see if it simultaneously replicated the tacho fault during these 'faulty cold starts'.

A lot of (most) ECU's rely on a PRISTINE earth connection with no 'loops'.. ie a reliable '0V reference'. It may be that the Nodiz is 'complaining' about its earth quality..but somehow when you switch on any other electrical device you create an alternative sink to earth (breaking a dominant bad earth loop) from which the Nodiz can properly reference 0v?

I'm guessing that the main earth to the nodiz is via the engine earth strap(s) and that your tacho is earthed via instrument panel earths. It might be that the tacho earth is the flakey one or the engine earth(s) are ...or a bit of both?

The 'cold issue' is also consistent with this theory given the effect of temperature on the resistance of bad earths.
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Hawaiis0
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Re: Tacho peculiarities

Post by Hawaiis0 »

Try making earth lead jumpers for the tacho first and then the ECU to a good known earth
Umm! What to do next?

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Re: Tacho peculiarities

Post by AndyLotus »

Thanks for the thoughts guys.

I must admit the earth thing does make sense, especially the cold and increased resistance.

The engine earth was remade as I know earth's are so important but the earth to the tacho could be an issue I guess.

The tacho worked ok with the standard ignition, but I haven't run on that for the last 4 years.

Looks like a bit of wiring testing coming up this weekend!

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Re: Tacho peculiarities

Post by Zag »

I don't know where you have your unit mounted, mine is next to the fuse box and I use the earth point under the steering column.which works fine.

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Re: Tacho peculiarities

Post by AndyLotus »

Hi Zag,
Snap! Exactly the same as mine.

Out of interest did you get your map setup on a rolling road?
I've recently had mine redone on the rollers after some changes I made to the breathing. It's going back again in couple of weeks for some final fine tuning. (I needed to buy some new main jets) Would you fancy exchanging maps for interest more than anything?

Cheers,
Andy.

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Re: Tacho peculiarities

Post by bash »

My question is where did you get the ignition map from ? I know megajolt have a map sharing platform, what about nodiz ?

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Re: Tacho peculiarities

Post by Zag »

Got my map from you around a year ago!!

I've been working abroad for quite a while so not had chance to do much playing with it, hopefully back in the Uk next year and will get it mapped on a rolling road then

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Re: Tacho peculiarities

Post by AndyLotus »

Ha, ha you did, I remember now. :D

Bash, there aren't many maps to share for Nodiz but they do have a conversation program that takes a megajolt map and converts it to Nodiz.

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Re: Tacho peculiarities

Post by bash »

Oooo, interesting, cos there are a few Rover V8's that have been done by them.

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Re: Tacho peculiarities

Post by AndyLotus »

Hmmm, tried running another earth to the tacho....no change.

Will keep looking for wiring issues.
Spoke to the Nodiz people, they seem stumped. They said the tacho output was a simple 12v square wave and have never had a problem with it.

I love electrics :roll:

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Re: Tacho peculiarities

Post by Sirclip »

If the output is a 12v square wave, you could put a multimeter on it set to AC (rather than DC). It should read 6v irrespective of the engine revs(assuming that the peak amplitude of the square wave doesn't change with engine revs). So you could try Peter's suggestion to see if there's an output when the rev counter is reading zero. This should confirm if the problem is with the rev counter, or the 'Nodiz' itself.

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