EDIS TACH Driver Trick

[Jim]

A TACH Trick for EDIS users :
The output from the EDIS module ( PIP ) to MS will directly drive a
power MOSFET or IGBT ,
(Metal Oxide Semiconductor Field Effect Transistor) or
(Isolated Gate Bipolar Transistor)
either one of these types of transistors put virtually no load on
whatever is driving them and their outputs handle lots of power .
( unfortunately their "Gate" input is easily destroyed by
the slightest static charge on your body or tools, make sure the gate
has at least a 100K or less resistor to ground , do not let it float ,
keep a small strand of wire wrapped around the pins until
it's soldered into place )

This enables you to create a TACH Output without the very questionable
and dangerous Diodes connected to the Ignition Coils !!!

I use the output of the "Opto" circuit to drive not only MS , but also an
International Rectifier #IRF540 "HexFet" MosFet power transistor
with a 2 watt 680 ohm pull up resistor to provide a
perfect 12 volt square wave output for my Tach (with NO spikes) .

Works great .

[Anders Karlsson]

MSnS-EDIS, V2 board, extracode 021u3 on a 4 cyl Opel engine.
Sounds like the solution to my problems with my Autometer tacho.
The tacho needle jiggles very much at idle, and the tacho shows a lower value than the tacho in Megatune.
I noticed that the Autometer stabilized and showed more reliable values when i turned on the lights and some other electric stuff in the car. First i thougt the alternator was bad, because i have 15 V right after start, but thats because the battery needs charging of course.
The tacho doesn´t stabilize after some time of running however, so i guess the signal voltage is too high or too noisy.
I will change the voltage control unit in the alternator first, and see if it makes any difference.
Is there any schematics or pictures on where to connect the MOSFET/IGBT?
I didn´t quite understand where to connect it.

[EstateAgent]

Why would the diodes be 'questionable and dangerous'?

They dont touch the HT side of things, they just passively tie onto the switched ground side of the coils.

Many many tacho's tie direct to the switched ground directly! So if its dangerous, theres certainly a few million cars we should stay away from! (Even Fords using the EDIS!)

[Jim]

If you have ever used an ocilloscope to look at what goes on ,
even on the primary side of the coil , the trash and spikes would
astound you , it's not really "dangerous" to you , but to
semiconductors it can be instant death just waiting to happen ,
most Tachs have built-in resistor and capacitor filtering networks
as well as Zener clamp diodes to prevent frying the chip .

Why would you want to attach a wire carrying this type of trash
to your precious MegaSquirt ECU ? where if some how, some way,
this trash got into the wrong place on your board it could
instantly fry it ?

To Me , that's a risk I would like to avoid .
Therefore I call it "dangerous"

And that's not all , connecting a wire to the coil acts as an antenna
broadcasting noise into the air , maybe it will get into your stereo
and maybe not , (try tuning in a weak AM radio Station) ,
I'd rather not have one more thing that needs to be shielded or filtered .

By the way , just for kicks , try this :
put your left finger on a grounded screw or bolt on your car and
take your right finger and touch your "Tach" wire ,
IT'S ONLY 12 volts it can't hurt you !!!

[Jim]

To Anders :
I'm sorry I can't give you specific instructions on were to
"tap into" your MS board because I use the new Version 3 board .

However I can provide more info on how it can be done .
First , basic information :

On the V-3 Board there are 2 separate stand-alone curcuits for
the Tach Input , one of them is designed for use with a direct input
from a "Variable Reluctance" or VR pick-up coil inside some
electronic distributors , the other curcuit is called "Opto" which is
short for Opto Isolator (Optically isolated transistor) , it's purpose
is to isolate MS from the trashy noisy signal that you get when
wiring straight to the primary of an Ignition Coil , this is also the
curcuit recomended for use with the PIP Signal coming from
an EDIS Ignition control unit .
One of these 2 circuits must be selected by soldering jumpers on the
printed curcuit board , one jumper for the input to the curcuit from
the DB-37 connector and one jumper on the output which goes to the
MS Chip for it's "tach" input .

You may "tap into" the PIP signal from EDIS before it gets to MS or
after it goes through the "Opto" conditioning circuit .

I chose to tap into the signal after the Opto circuit because it was
easy to do while I was wiring my board and because the output of
the Opto Curcuit was a known quanity , I could look at the schematics
of the circuit and see that it would not be a problem to drive an
additional load from it .
However , I am reasonably confident that the PIP output of the
EDIS unit could also drive an additional load , provided that the
load was only the Gate Input of a MOSFET or IGBT transistor .
These types of transistors have an input that acts just like a very
small capacitor , they have an almost infinite input resistance , so
tapping on one of these transistors is almost the same as simply
adding a very small capacitor to ground .

Here's how to do it : (See the attached Picture)

Also , if anything starts working or stops working when you
turn something else on or off , you need to start looking at your grounds
ALL OF THEM , if you can avoid it , never use the car body as a ground .
Run a separate ground wire from the battery or engine block .
The very best setup for grounds is to have them ALL go to the same
point .
The Engine Block is the best place .
Also make sure that you have a BATTERY CABLE from your
Engine Block to the Body .

[tony2ltr]

Can't you use pin 2 on the edis for tach input?

[sevenDriver]

I have connect the tach to pin 11 of my EDIS4 module.

EDIS4 Pin 2 is the EDM = E-DIS Diagnostic Monitor signal.

[EstateAgent]

If you have ever used an ocilloscope to look at what goes on ,
even on the primary side of the coil , the trash and spikes would
astound you , it's not really "dangerous" to you , but to
semiconductors it can be instant death just waiting to happen ,
most Tachs have built-in resistor and capacitor filtering networks
as well as Zener clamp diodes to prevent frying the chip .

Why would you want to attach a wire carrying this type of trash
to your precious MegaSquirt ECU ? where if some how, some way,
this trash got into the wrong place on your board it could
instantly fry it ?

To Me , that's a risk I would like to avoid .
Therefore I call it "dangerous"

And that's not all , connecting a wire to the coil acts as an antenna
broadcasting noise into the air , maybe it will get into your stereo
and maybe not , (try tuning in a weak AM radio Station) ,
I'd rather not have one more thing that needs to be shielded or filtered .

By the way , just for kicks , try this :
put your left finger on a grounded screw or bolt on your car and
take your right finger and touch your "Tach" wire ,
IT'S ONLY 12 volts it can't hurt you !!!

But your not connecting those lines to the MS anyway. The only lines the MS use are PIP and SAW.
And like you saw, Tacho's have circuitry to suppress the spikes.

I can see your point about it being a good way to suppress noise, though using Fords suppressors in the places they put them never has interfered with the radios on my cars at least.

[tony2ltr]

Sevendriver, are you just going straight to the tach from pin 11? or are you using the diode and zener setup? I have some of the EDIS instructions printed out and it says to try pin 2 from the EDIS 4 right to the tach. I haven't fired the engine yet, but is this going to work?

[Anders Karlsson]

Great explanation...

Thank´s a lot! Now i understand.