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How To: DIY MSD Ignition Driver box

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How To: DIY MSD Ignition Driver box

Postby Bob » Sat Mar 04, 2006 9:29 pm

This is an easy circuit to make.

The ECU signal is 0V and it sends +5V pulses when it wants to trigger the spark plugs

The aftermarket systems want to see +12V with pulses down to 0V when a spark trigger occurs.

So... it's just a simple inverter.

You take a resistor such as 4700 ohms and attach it between the ECU signal wire and the "base" of a regular NPN transistor ( Such as a TIP 122 )

Then you take the +12V voltage that normally comes to the coil pack and attach it to a 470 ohm 2 Watt resistor.

The other end of this resistor gets attached to the collector of the TIP122 transistor

You also connect a wire to the collector for a signal out to the aftermarket system.

You attach the emitter of the TIP 122 to ground

And that is it.... very simple.... only costs a couple of dollars to make.

I'd imagine the box is most of the cost in this "driver box"

You need one of these for spark plug 1&4 and one of these for 2&3 ..... you need two transistors to make it work.

So here is how it works...... when the base voltage is zero (no trigger signal from ECU) the transistor is off.

The +12V goes through the resistor and into the collector of the transistor.... which is attached to the MSD system or what ever aftermarket ignition you want.

Then when a +5V pulse comes from the ECU it turns on the transistor and connects the collector to the emitter.

Because the +12V is through a resistor the voltage at the collector will drop down to 0V

So.... 0V to 5V pulse from ECU makes a +12V to 0V pulse for the ignition system.

It's just that simple.

If I get time.... I will post a diagram or picture of the circuit.

Have fun.

Posts: 3
Joined: Sat Mar 04, 2006 9:17 pm

Postby Bob » Sun Mar 05, 2006 12:30 am

here is the circuit....


Now.... you would have one trigger for the 1 & 4 coil.... and one for the trigger on 2 & 3

Keep in mind that the collector is attached to the metal heat sink of the transistor so be carefull not to let the two transistors touch.

It would be best if you used shrink tube around the transistor.... or mounted them on a circuit board with a T220 "insulator" kit.

So with the two transistors separated via insulators you would tie the +12V and GND together..... giving you a total of 6 wires.

1) One for +12V

2) One for GND

3) One going to ECU trigger for 1 & 4

4) One going to Aftermarket Ignition for coil 1

5) One going to ECU trigger for 2 & 3

6) One going to Aftermarket Ignition for coil 2
Posts: 3
Joined: Sat Mar 04, 2006 9:17 pm

Postby Bob » Sun Mar 05, 2006 12:43 am

Here is how you hook it up


Have fun and be carefull

Bob the electronics dude
Posts: 3
Joined: Sat Mar 04, 2006 9:17 pm

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