Kinda OT: Ford tach in my 914, What the hell am I up to Options

[DNHunt]

Follow me on this you guys.

My ignition is a Ford EDIS system off of an Escort. (Don't laugh it works) Wasted spark, no dizzy and a 2 coil 4 tower coil pack. The problem is that the 914 tach is triggered off of the negative side of the stock coil and it won't work with the EDIS system.

So, I snagged an Escort tach at a pick and pull and I'm going to swap the guts into an extra 914 tach. I just want to verify the connections. It has 3 connectors. Ground and signal in are self explanatory. The 3rd one is labelled B. This has to be switched 12V, right?

Thanks

Dave

[fiid]

One would think.

I think it may be possible to use some kind of setup with zener diodes to trigger the stock tach off an EDIS setup, but I haven't thought about it very much - so ...

The two signal wires into the coilpack (one on each side I think) represent the negative sides of the two coils, so it may be possible to get a large signal off of it. Alternatively - I wonder if a tach adapter from an MSD would work - they are designed to kick voltage out to old-school tachs.

l8r,

Fiid.

[DNHunt]

I've tried the diodes and zener diode circuits with various zeners from 16 to 51. No go. I've tried the MSD tach adapter. No go. The mistake I made was not getting a value and shape of the signal off of the stock coil.

I think I'll just jumper the Ford tach in with B as 12 volt+ and see what happens. Me thinks it will work. The Ford circuit board has a built in pot so I think i'll even be able to calibrate it. If it cooks I'm out my time and a couple of bucks.

Dave

[fiid]

Crap. I'm going to have the same problem - although I do have an aftermarket VDO tach which I plan to use - but I need to sort out moving all the lights out of the other dials.

[lapuwali]

I seem to remember from discussions here and the MS board that some tachs (and I think the early 911 and early 914 tachs are like this) depend on the spiky, noisy tach signal. The signal is something like 30-50V peak to peak. A later 914 tach ('75?) will swallow a clean 12V square wave. Instead of Zeners, you might try running the EDIS tach signal into an op-amp or a transistor and amplify it.

[Porsche Rescue]

Huh? Why did I read this thread? My head hurts.

[fiid]

QUOTE (lapuwali @ Mar 24 2005, 08:40 AM)

I seem to remember from discussions here and the MS board that some tachs (and I think the early 911 and early 914 tachs are like this) depend on the spiky, noisy tach signal. The signal is something like 30-50V peak to peak. A later 914 tach ('75?) will swallow a clean 12V square wave. Instead of Zeners, you might try running the EDIS tach signal into an op-amp or a transistor and amplify it.

The EDIS has two coilpacks and runs wasted spark. The problem is you need to join the noise from 2 coil grounds together into an output signal, without triggering the other coilpack from the one you just fired. You could then feed that signal into an OPamp, or maybe the MSD jobby. I don't know enough about the properties of zeners to know how to set it up right though.

I am suprised that you can't use the EDIS module tach output through an MSD tach adapter though.... makes me wonder what that signal actually looks like.

[fiid]

I just re-read what you said - you could also do the amp on the TACH output.

I wonder if you could also do something with a switching transister and a small coil, where each tach signal causes a small coil charge and letgo. the choil back-charge ought to be enough to trigger the tach.

Dave - do you have any pics of the ford tach in the VDO case?

[airsix]

Crap I'm going to have the same problem. Right now I'm triggering my tach off the coil. I tried using the tach signal from my aftermarket ECU (square wave 5v or 12v - can't remember which) and the tach doesn't budge. So I put it back on the coil. I was going to switch to waste-spark this summer, but I forgot that I'll have to deal with the tach again. (IMG:http://www.914world.com/bbs2/html/emoticons/mad.gif)

-Ben M.

[scotty914]

i dont know why this is being a problem my tach works fine off of the suby wasted spark, but it is connected to the brain, and that might do the conversion for you. it would be easy enought to make an amp circuit or even a seperate optical trigger to run the tach off the fan hub or flywheel, you could even do a hall sensor with a magnet.

i would think the tach signal from the edis should work the stock tach same kind of signal, its just a grounded pulse. just like a temp sensor just pulsed.

[lapuwali]

QUOTE (scott thacher @ Mar 24 2005, 10:04 AM)

i dont know why this is being a problem my tach works fine off of the suby wasted spark, but it is connected to the brain, and that might do the conversion for you. it would be easy enought to make an amp circuit or even a seperate optical trigger to run the tach off the fan hub or flywheel, you could even do a hall sensor with a magnet. i would think the tach signal from the edis should work the stock tach same kind of signal, its just a grounded pulse. just like a temp sensor just pulsed.

As I said, there appears to be a difference between the early and late 914 tachs. You may have a late tach.

This is all hearsay, and I can't find the relevant posts where I picked this up from, btw.

I don't think it's so much the noise as the high voltage that's required. Simply amplifying the square wave from the wasted spark system's tach out should be enough if I'm remembering at all correctly. If you tried to produce a second signal from another sensor, you'd end up with pretty much the same problem, only you're using even more hardware to get there. A simple op-amp or transistor drive shouldn't cost more than a couple of dollars to put together, so if I'm completely wrong, not much loss.

As for the MSD, I've heard tell some people can't get a stock tach to work with that in any scenario, so I suspect the MSD adapter is putting out a clean 12v square wave. It's there, I believe, to filter out the extra pulses from multi-spark setups.

I wish Dave luck, but calibrating the setup is going to be the real bitch on that project. I've got a handful of 914 tachs on the shelf I've considered producing stepper drives for, so you could have a modern drive with a square wave in, and easily switch to six or eight cylinders, all while keeping the stock look. Given how I operate, it will be years before I get around to that project. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)

[Mark Henry]

No issues here as well...wasted spark, plug and play.