Ignition circuitry with MoTeC

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Number 7
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Post by Number 7 »

DaveEFI wrote:Notice you have one of those horrid pre-insulated crimps to the coil positive. Could the 12v feed have pulled out of that?

I'll repeat. It ain't going to work without 12v.
No "spare" wires floating around. I have pictures from last year for comparison. Agreed, where the hell does 12V come from??? The tacho was my only desperate idea, but cant see how - it was previously connected to the coil positive. Strangely, this wire from tacho to coil shows 3.8V with ignition on, but drops to 1V when attaching the wire to the coil negative.



Number 7
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Post by Number 7 »

Well, after much testing and puzzling over wiring diagrams from various sources, I finally fixed my ignition non-starting problem. As I thought all along, it was related to my tacho. Running a new cable from an ignition switched source to the coil positive terminal sorted it - and hey presto, the engine started. Been for a quick blat around the block and seems fine so far.

Many thanks to all who contributed guidance and suggestions. Amongst the many things I have learnt is that the single original connection between the tacho and coil positive wass supplying 12V to the coil, and also feeding a trigger signal back to the coil (this was one of my early questions), and also that my MoTeC installation does not follow the wiring diagram!
Thanks again

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Post by DaveEFI »

What is the wire colour between tach and coil that supplied the 12V?
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Number 7
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Post by Number 7 »

White and yellow. Why?

DaveEFI
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Post by DaveEFI »

Number 7 wrote:White and yellow. Why?
Because that tells you it was 12v+. A ground from the coil to rev counter would be white/black.
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Number 7
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Post by Number 7 »

Interesting - didn't know the colour was significant. Anyway, sorted now with appropriate notes for any future owner.

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Post by DaveEFI »

There is a BS colour list. Which allows you to identify what a wire does by its colour, on older UK cars. In the main.
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stevieturbo
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Post by stevieturbo »

Never assume though !! Always test.

A little testing at the start would have had this resolved a long time ago it seems.
9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

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Post by Number 7 »

stevieturbo wrote:Never assume though !! Always test.

A little testing at the start would have had this resolved a long time ago it seems.
One of my early questions which went unanswered was whether it was possible for a the single wire between coil positive and the old tacho was capable of supplying 12V to the coil and also sending the trigger signal to the tacho.

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Post by stevieturbo »

Number 7 wrote:
stevieturbo wrote:Never assume though !! Always test.

A little testing at the start would have had this resolved a long time ago it seems.
One of my early questions which went unanswered was whether it was possible for a the single wire between coil positive and the old tacho was capable of supplying 12V to the coil and also sending the trigger signal to the tacho.
No, I still say it is not capable of supplying the trigger.


But in a no spark scenario...one of the very first things you would do is check the coil had power !

I'm sure everyone here assumed you had done this, when you were pursuing what you believed to be an issue with the ecu or amplifier.
9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

DaveEFI
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Post by DaveEFI »

Number 7 wrote:
stevieturbo wrote:Never assume though !! Always test.

A little testing at the start would have had this resolved a long time ago it seems.
One of my early questions which went unanswered was whether it was possible for a the single wire between coil positive and the old tacho was capable of supplying 12V to the coil and also sending the trigger signal to the tacho.
The answer is yes. If it is supplying the current to the coil primary, that will change when the ground is removed at the sparking point. Such tachs are known as 'current driven', rather than 'voltage driven'.
Dave
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stevieturbo
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Post by stevieturbo »

True, I've heard of such tacho's, from some cars in the 60's or 70's or something ?

Never actually come across one myself though
9.85 @ 144.75mph
202mph standing mile
http://www.youtube.com/watch?v=XgWRCDtiTQ0

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Post by DaveEFI »

Not certain what cars used them. In Smith's parlance, there was the RV (voltage) and RC (current) series. Just why the choice, I dunno.

Out of interest, I used a similar principle to get the MPG side of the OBC on my SD1 working again after fitting MS. I have low impedance injectors driven PCM from MS, and the multiple pulses confused the OBC. So I derived an on/off pulse from the +12v feed to one injector.
Dave
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Rover SD1 VDP EFI
MegaSquirt2 V3
EDIS8
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