Tach with EDIS and MSII

[z-ya]

It seems like a number of people were looking for a way to drive a tach with MSII and EDIS. Some have had varying results using the EDIS tach output from the module. But it seems that it is not so reliable.

 

Since there is no way that I could find to use the MSII spare outputs to driver a tach, this is what I did. These MSII spare outputs are really software outputs. I'm using one of those for a shift light BTW, works great.

 

Here is a circuit that is working for me. It basically buffers and inverts the EDIS PIP signal so that it can drive a 280Z tach directly. So far it seems to be working well. You need to build the circuit in the proto area of the V3 mainboard. Then pick a free spare DB37 pin and run a new wire in your harness to the tach.

 

[X64v]

I've got a super-trick, super-simple way of running the 240 style current-sensing tach on MSI/MSII with EDIS, a magnetic crank trigger, LS1 coils, any setup that doesn't use a single coil. I'll post it up when I test it out.

[1 fast z]

For edis 6 and edis 8, pin number two on the ign module works every install I have done (a few).

[cygnusx1]

I get a tach signal from PIN#2 on my EDIS-6 module. Unless, it's cold out, or the idle drops below 900rpms, it works. It is temperature dependant...it's a Southwest/Northeast thing...

[z-ya]

I've got a super-trick, super-simple way of running the 240 style current-sensing tach on MSI/MSII with EDIS, a magnetic crank trigger, LS1 coils, any setup that doesn't use a single coil. I'll post it up when I test it out.

 

With the current sensing tach (70-72) you just need to power all of the LS1 coils from the same black/white wire that powers the original coil. It will count the current pulses from all 6 LS1 coils.

 

Since I was previously running Extra code and using the tacho output signal, it was easier to add this circuit as I already had the harness wired to the tach.

 

Dave, have you tried a different EDIS module?

[X64v]

With the current sensing tach (70-72) you just need to power all of the LS1 coils from the same black/white wire that powers the original coil. It will count the current pulses from all 6 LS1 coils.

 

Sure, that would work for the lower rpm range, but what about when you get past 5714rpm (assuming 3.5ms dwell) where their charge times begin to overlap? Then wouldn't the tach go dead, as there would always be current flowing through the wire?

[z-ya]

Sure, that would work for the lower rpm range, but what about when you get past 5714rpm (assuming 3.5ms dwell) where their charge times begin to overlap? Then wouldn't the tach go dead, as there would always be current flowing through the wire?

 

First of all, why would it be any different than the stock coil and distributor? Still, only one coil would fire at a time. What is the stock 240Z dwell, I'm sure it is in the 3mS ball park?

 

Secondly, I think you math is off:

 

5714RPM = 95.2 RPS

 

So the time used to charge the coils per second is 95 x 3.5mS = 333mS

 

This leaves over 600mS idle time per second.

 

At 8000RPM there would still be over 500mS of idle time.

 

Pete

[Matt Cramer]

Not quite. 5714 RPM = 95.23 rps = 285.7 sparks / sec. Next step is to flip that over: 285.7 sparks / sec -> 0.0035 sec / spark. So at that point a stock ignition system is going to start cutting dwell (actually, since the coil needs some time in between sparks, it would have been cutting dwell a while ago). But a coil on plug setup would still be charging the coils for 3.5 msec. As the RPM goes up, the dwell will overlap, with one coil starting its charging before the previous one has discharged. This will still result in uneven current flow, but the current won't stop like on a distributor.

[z-ya]

Not quite. 5714 RPM = 95.23 rps = 285.7 sparks / sec. Next step is to flip that over: 285.7 sparks / sec -> 0.0035 sec / spark. So at that point a stock ignition system is going to start cutting dwell (actually, since the coil needs some time in between sparks, it would have been cutting dwell a while ago). But a coil on plug setup would still be charging the coils for 3.5 msec. As the RPM goes up, the dwell will overlap, with one coil starting its charging before the previous one has discharged. This will still result in uneven current flow, but the current won't stop like on a distributor.

 

OK, forgot about that (duh), 3 sparks per revolution. So the stock dwell must be much less than 3.5mS because you can easily run a stock ignition to 7000RPM.

 

So yes, it appears that the stock current sensing tach may stop working after 5700 or so RPM with modern ignitions have long coil charge times. But, many modern systems vary coil charge time with RPM. Higher the RPM, the shorter the charge time. I think this depends on the system.

 

I'll be finding out at some point as I plan on using the stock tach on a RB25 build I am doing. I plan on powering the COPs through the tach like the stock coil was connected.

 

Pete

[X64v]

If the 240z tach will work based on simply uneven flow then it would work. I don't know if it needs that full stop of current or not.

 

As far as modern systems changing dwell, I don't know if any other system does it, but if Megasquirt is running the coil or coil packs directly (as I plan to do) then the dwell will be fixed as long as it can pause for the minimum discharge time. So a wasted spark set up would most definitely overlap charge times when run directly by Megasquirt.

 

As to the stock dwell, it's not fixed in milliseconds, it's fixed in degrees of crankshaft rotation (I believe 30-35 degrees is what the FSM called for for my 73) because of the way a points distributor works. Not sure how the 280Zs do it.

 

 

How's this for a test of the current-sensing tach: Ground a low-ohm resistor off the coil positive and run the car (I've got a 5ohm handy that will handle the power). This will pull 2.8 amps through the tach at 14v with the coil off, then 12.1 amps with the 1.5ohm coil charging. This won't perfectly simulate the conditions with edis coils (which are at most 1.5ohm, so one charging draws at least 9.3 amps), but if the tach doesn't work this way, it surely won't work with the edis coils.

 

Edit: I just performed the above test on my car (5/73, but it does have the current-sensing tach) and the tach indeed kept working. It's not a perfect test, but it's a good sign.

[z-ya]

If the 240z tach will work based on simply uneven flow then it would work. I don't know if it needs that full stop of current or not.

 

As far as modern systems changing dwell, I don't know if any other system does it, but if Megasquirt is running the coil or coil packs directly (as I plan to do) then the dwell will be fixed as long as it can pause for the minimum discharge time. So a wasted spark set up would most definitely overlap charge times when run directly by Megasquirt.

 

As to the stock dwell, it's not fixed in milliseconds, it's fixed in degrees of crankshaft rotation (I believe 30-35 degrees is what the FSM called for for my 73) because of the way a points distributor works. Not sure how the 280Zs do it.

 

 

How's this for a test of the current-sensing tach: Ground a low-ohm resistor off the coil positive and run the car (I've got a 5ohm handy that will handle the power). This will pull 2.8 amps through the tach at 14v with the coil off, then 12.1 amps with the 1.5ohm coil charging. This won't perfectly simulate the conditions with edis coils (which are at most 1.5ohm, so one charging draws at least 9.3 amps), but if the tach doesn't work this way, it surely won't work with the edis coils.

 

Edit: I just performed the above test on my car (5/73, but it does have the current-sensing tach) and the tach indeed kept working. It's not a perfect test, but it's a good sign.

 

The early tach measures current pulses. A fixed DC current will not show anything on the tach.

 

I'll take some measurements to see if the charge time is fixed. I know with the Wolf 3D systems, the charge time changes with RPM and load.

 

I guess I was wondering if anyone had put a scope on a stock Z ignition to see exactly how the charge time behaved.

 

I think you will find that the EDIS coil pack will work just fine with your tach as long as you power the coil through the tach. You should not need any additional components.

[X64v]

I guess I was wondering if anyone had put a scope on a stock Z ignition to see exactly how the charge time behaved.

 

I think you will find that the EDIS coil pack will work just fine with your tach as long as you power the coil through the tach. You should not need any additional components.

 

A 240z points-type ignition, or the electronic 280z?

 

And that would be wonderful if it would work directly through the stock tach.

[Zmanco]

MegaSquirt-II attempts to set up a charge/ discharge cycle that charges the coil for the specified charge time, discharges it for at least the specified spark duration time, and then remains in the discharge state until it is time to start charging for the next cylinder. At high rpm, there may not be time to fit in the specified charge + discharge times. In this case, MegaSquirt-II (or MicroSquirt) scales down both the charge (max. dwell duration) and discharge (max. spark duration) times proportionally so they just fit within the time between tach pulses.

This is from MSII/E documentation: http://www.msextra.com/ms2extra/MS2-Extra_Gen_Ignition.htm#ignsets. I believe this was taken straight from MSII.

 

FWIW, I'm running MSII/E driving an MSD Blaster coil on my 73 with the stock tachometer. Today I tweaked the tach adjustment on the back to get it to match the MS display on megatune, and then ran a datalog where I took the tach up to 7k rpm. The datalog shows about the same rpm (there's a bit of noise at that speed). So there doesn't seem to be any problem with the stock tach accurately reading the pulses as they get shorter at higher rpm.

[dum-bass]

Hey Z-ya, I know this is a really old thread, but I've been beatin my brains out tryin to get my stock '75 280Z tach to work with EDIS! I've tried the diode method, works on one coil but not two So, I implemented your circuit design into my MS III and voila! it doesn't work! I mean when you look at the design of the circuit, it's perfect! how could it not work? Now my only question is, what year is your 280 tach?

[z-ya]

Did you try using pin #2 on the EDIS module as cygnusx1 suggested?

 

Check you wiring again, that circuit has been working in the race car for almost two years now.

 

Pete

[TrumpetRhapsody]

I've had the same issue, '74 tach with edis, doesn't work with pin 2 of edis, or the diode method. Haven't tried Z-ya's circuit yet.

[dum-bass]

The circuit does work, I'm getting a 12v switched signal but apparently it's still not enough juice to trigger my tach even with the inline resistor removed. I'll try bypassing the resistor in the tach itself, I'm fairly confident that will work but what a pain in the a@#!

[TrumpetRhapsody]

Keep us updated, i'd be interested to see if the internal resistor bypass works.

[z-ya]

The circuit does work, I'm getting a 12v switched signal but apparently it's still not enough juice to trigger my tach even with the inline resistor removed. I'll try bypassing the resistor in the tach itself, I'm fairly confident that will work but what a pain in the a@#!

 

What inline resistor? The transistor's collector should connect directly to the tach. There should be no series resistor. On the back of the tach there should be a male bullet connector. This is where the collector should connect.

 

I'm using it with a 280Z tach, but is should be the same since all Z coils are 12V.

[cygnusx1]

The wire that went from the engine bay to the tach in the 280Z's passed through a resistor that is behind the glove box. I believe I did, and that you do need to bypass the resistor to make the tach work with EDIS6 pin2 and/or megasquirt.