Something to try/think about for some rev counter problems?

[MartinM]

For rev counters that are happy with a low voltage source (ie don't rely on the high voltage flyback) the standard feed is the TACH OUT from the MJLJ.

This is a +12v variable pulse train that is the inverse of the SAW output:
- as the rpm advances, the frequency (ie pulses per second) increases
- as the ignition advances, the mark:space ratio increases ie the pulse train spends most of its time at +12v but dips to zero for briefer and briefer periods
...so as the rpm rises, the frequency increases and the mark:space ratio increases (since advance increase with rpm, normally)

Some rev counters seem to work up to about 3000rpm and then read zero. Could it be that the advance at 3000 rpm is such that the "dips to zero" in TACH OUT aren't wide enough for the rev counter to notice??

I certainly know of one installation where the fix was to set the trigger wheel so that it was 10 degrees out to what it should be and then reduce the advance bins in the map by 10 degrees. That reduced the width of the "dips" in the TACH OUT signal and seemed to solve the problem. That's not a great idea for a couple of reasons, but it sorted the rev counter problem.

So are there any other signals floating around that might be useful? Yes - it's the PIP pulsetrain from the EDIS. This is also a +12v pulse train of the same frequency as SAW ie increasing as rpm increases (strictly speaking, SAW is the same frequency as PIP, of course) but it is a square wave ie always 1:1 mark space ratio. So it might suit rev counters better...

So, how about connecting the rev counter to the PIP input instead?

The other (more off the wall) thought is that maybe the rev counters are designed to respond to pulses rather than "dips" and if the TACH OUT signal was inverted (ie followed SAW) then maybe that would help?

Thoughts? ...apart from the fact that a diagram might help!

[Gilesy998]

That's bloody well thought through mate, and what you say about the 0V "dips" would explain a lot of strangely working tachos.
I have been thinking what the effect of connecting to the PIP or saw lines might be, but I'm not in a position to try it out yet. I do think though that connecting into the PIP line is certainly worth at least a go (as it is for the SAW line I suppose!), just to see what happens. I wish I though like you!

[brentp]

That's a fine idea! Testing with PIP is certainly worth a try- with little risk. The PIP line would simply need to supply enough current to drive both the MJLJ input and the Tachometer in question.

[MartinM]

...and all we need is a little add-on circuit that takes a copy of PIP and adds a high voltage spike on it to simulate the flyback and we can feed that to the other troublesome rev counters

[brentp]

MSD has something that appears to do this task already:
http://www.msdignition.com/pdf/8910_21247.pdf

But for those tachometers that need the HV pulse don't we already have the ingredients to create the necessary signal from the coil packs?

Understanding why some Smiths tachometers do work with the diode circuit and some don't will get us down the road further with less effort.

Once this is explained we will probably quickly know how to get all of them to work properly.

[Paul f]

I have been driving my tacho from the the pip line for about 6 months with no problems at all. It is a ford unit but would not work from the edis tacho output for some reason and as my megajolt is a v2 with 5v tacho output it did not work from that either, I did use the diode method but the glass zener degraded and caused me to have a difficult to trace missfire so looked for an alternative method.
Paul.