Adapting an oldschool tacho to work with Microsquirt?

[petew]

In the throws of rewiring my oval for safety, EFI and microsquirt. I'd really like to use this oldschool smiths tacho in it...



These are the connections on the back of the tacho...



Yellow = 12v +
Blue = Negative
Black = backlight
White = tacho signal trigger wire

As you can see, the white wire is not actually connected. Instead what the tacho appears to do is pickup up (via induction changes) the breaks in power as the coil fires. It's supposed to be wired in series in the ignition wire. Any thoughts on how I might use this OR change the tacho internally so it could be signalled by the Megasquirt?

[Piledriver]

First: If you are still firing the plugs through a distributor and just using the MS to control the timing, hook it up to the coil as original.


If you must use ECU outputs:
The method usually used on "old school" HV kickback tachs is the "relay coil" setup.

Short: It uses the coil of a relay to generate the required HV kickback signal.


Also:
If you have "dumb" coils you can use a little diode circuit to combine their kickback pulses and free up an output.

[petew]

I'm using a crank triggered ignition system... So yeah, I need to use the ECU output.

I'm still a little mystified by your explanation. (Sorry, electronics newb)

If this is any help, here's how the circuit of the Smith's 1960's tach works...

http://www.accutach.com/smiths-tach-cal ... background

[Piledriver]

The last circuit works for a crank fired ignition with dumb coils.
Including EDIS.

Its about $1 worth of diodes, and even a caveman could do it.

What exact coil setup do you have?

[petew]

Hyundai excel wasted spark type twin coils...



P.s. can you explain to this "dumbie" what a Dumb coil is?

[Piledriver]

Its easier to explain what a "smart" coil is:
It has a built in ignition driver and is controlled by low power logic signals.
Usually have current limiting and overdwell protection, sometimes ion current sensing as well.

I do not know what kind you have there, some research may be required.

Dumb coils are plain old coils that have to be power switched by an external driver, usually (but not always) in the ECU.
The stock coil was a dumb coil. Feed it 12v and ground it, then lift, it sparks via the plug
(thank you Mr. Kettering and Tesla)

On a 4 tower coil like you have there, (waste spark) they would have 3 input pins if a "dumb" coil.
(12V and 2 driver connections) but the vehicles schematics or diagnostic procedures may prove handy to be sure.

[jhoefer]

That is an inductive pickup tach. You typically loop the positive coil power wire going from the fusebox to the coil positive terminal through the inductive pickup (replacing that white wire or splicing the existing white wire into the power wire). It doesn't get signals from the negative side of the coil like a more common tach pickup does. I'm not sure the circuits presented above are going to help. Can't say I've ever seen an inductive tach used with a coil pack instead of a standard coil, but since it is still an actual coil, first place to start is to just try looping the coil pack's power wire through the pickup and see if it works.

You also need to make sure the coil power wire is going through the induction pickup in the right direction. It won't work if the current direction is backwards.

[petew]

Cheers, I'll try that.

[Dale M.]

As a thought......

RelayOutput.JPG

Dale

[tango]

I tried several coils and combos some time ago to try to drive my 914 tacho off the MS2 (driving seq LS2 coils) but didn't have any success. It's something I need to revisit. I am just finishing a rebuild/refresh and going to try a Air-Air IC with ducting so engine cooling air is pulled through the IC so it might be a little while till I get to the tach..

__________________
73' 914 2.0 undergoing a bit of a refresh. Yes, on axle stands.

[jhoefer]

As a thought......

RelayOutput.JPG

Dale

It's an option, but I'm not sure the relay coil will draw enough current to trigger the tach since its resistance is higher than an ignition coil. I know with higher current coils and current sense tachs, it can be necessary to modify the wire to only go through the pickup once instead of looping around to pass through twice. This reduces the signal level to the tach. You could try the opposite modification to try and increase the signal level, by using a smaller gauge wire and looping it several times through the pickup instead of just the one loop. Since the current through the relay coil is less, it's ok to use a smaller gauge wire to power it.

The reason I suggested trying the coil pack's +12V power wire was that it might already provide enough current to drive the tach as is and you can try it out for free before attempting alternatives.

Otherwise, everyone seems to either replace current sense tachs (Smiths RVI) like these with a common voltage sense tach (Smiths RVC or any modern tach stock or aftermarket) or have the tach's internal circuitry modified to a voltage sense pickup instead when used with modern ignition.