MS3X ignition coil driver

By default there is no direct MS3X ignition coil driver on board the Megasquirt MS3.  The MS3X has 5V logic drivers to trigger transistors whether these be on board with the coils or simply standard alone trigger system like a Nissan PRW-2 ignitor module or a smart coil.

If you are planning to use wasted spark or sequential ignition then it is likely that the ignition coils will have on board ignitor circuits.  But what if you are retaining an older carburettor style points triggered single coil distributor setup? Perhaps you are like me and prefer not to visually pollute your classic engine bay with another ‘new tech’ item.  There is a solution.

Some points to consider

Before jumping into this modification, there are some things to consider to determine if this modification is suitable for your build.

  1. This modification locates a high current ignition coil ground within the ECU case.  This may produce electrical noise and affect operation of your MS3.
  2. Know your ignition coil properties.  The V3 and V3.57 boards do not have infinite current carrying capacity.  If you coil draws more current than the board has capacity, use an external ignitor.
  3. This modification does require soldering and electronics skills.  If you are not comfortable with your skill set to be able to make this modification, use an external ignitor.

How to build the MS3X ignition coil driver

In reality this modification is taking the logic ignition output from the MS3X and using it to drive a BIP373 coil driver internally to the MS3 case.  So the first step is to build the high current coil driver (BIP373 at time of writing) available from DIYAutotune.  

  • Remove or do not install R57
  • Solder in a BIP373 into Q16 ensuring a mica washer is between the transistor and the heatsink
  • Tin a 330ohm 0.25w resistor and solder to a piece of 22ga wire for jumping
  • cover the assembly with heatsink
  • Install a the assembled jumper wire from Pin 14 (Spark A) on the back MS3X board to IGBTIN
  • Install a 20ga  jumper wire from IGBTOUT to IGN

You can now connect Pin 36 of the DB37 to the negative terminal of your ignition coil.  Additional BIP373 drivers can be added but may require mounting somewhere in the ECU case and an additional DB15 connector added.

See the original DIYAutoTune article here: https://www.diyautotune.com/support/tech/other/bosch-bip373-ignition/

The ignition settings will be specific to your build however one rule of thumb with BIP373 modules is the Spark Output must be set to “Going High (Inverted)”.  Not doing so will damage the BIP373.  Ensure your settings are correct by referring the original official documentation.