driving the factory tach with FORD EDIS

[pete]

I need to get my tach back working again and the EDIS8 module's pin 2 will not drive it for whatever reason. The simple way is following this drawing


But I have a question. Is the Zener really necessary and if so why is it an 18volt? I would think I would get no tach signal unless using a 12V zener which by the way is all I can find at radio shack.

[rs2000]

Not sure about that, but you could also use the tacho output direct from ms.. Heres a link

http://megasquirt.sourceforge.net/extra/tachopin.html

andrew

[ptegler]

I needed some extra 'oomph' to drive my stock tach. Since I'm running wasted spark with my VB's external to the MS box, I rewired the drive to the original coil drive VB on the heat sink. There is a V+ track on the back of the board right above the vias where the leads of that VB pass through the PCB board. It was very easy to add a pull up resistor to the drain lead. The VB now drives my tach with a nice clean signal.

Paul Tegler
ptegler@cablespeed.com (ptegler@cablespeed.com)
www.teglerizer.com

----- Original Message -----
From: rs2000 (andrewr@iinet.net.au)
To: megasquirt-mss@msefi.com (megasquirt-mss@msefi.com)
Sent: Sunday, August 27, 2006 9:30 AM
Subject: driving the factory tach with FORD EDIS


Not sure about that, but you could also use the tacho output direct from ms.. Heres a link

http://megasquirt.sourceforge.net/extra/tachopin.html

andrew




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[pete]

Yeah I know I can use MS but that requires a change to the board wiring and a place to put that little driver circuit. The diode set up is really simple and can be embedded in the harnes under the hood.

I built the ciruit and will be trying it today. I used a 12V zener instead of the 18 volt that the diagram called for. It seems to me a 18V zener would never conduct as the battery is 14.8 max.

[superchargingmachine]

I think the Zener limits the power coming back through the ciruit. I'm not and Electrical Eng but I seem to remember something like that.

I used a 15V because it was all I could get my hands on quickly and it works fine.

[pete]

Well it doesn't work. Tach worked fine prior to changing to EDIS. Can't go back to check to see if the tach failed for some reason.

I don't have an oscope so all I could do was read voltage. I get battery voltage at the tach output from my circuit. I also get battery voltage on the wire coming from the tach when I measure to Battery +. That seemed weird.

Maybe I blew the tach.

[pete]

Installed a Sun tach and that did not work either. I know little to nothing about electronics but I can't see why the system doesn't work. The diodes are installed correctly the only difference was the rating of the zener. It still seems as though the zener would not be necessary as stock configuration just uses the pulsed ground signal at the coil.

Anyway I hooked the sun tach up to the EDIS tach terminal and it works fine. I'll just be useing that but would really like to know why it doesn't work

[Ferret]

I just connected the EDIS's tach output directly to my tach and it works fine.

[eliotmansfield]

Matt (Midnightblue) emulated an ignition coil by using the IDM to drive the coil of a relay via a transistor, the collapsing field current was enough to drive his tach in his TVR. Quite a novel idea!



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