Monday, September 19, 2011

Wiring the harness adapter

I finished wiring the harness adapter this weekend.  My goal was to make this plug compatible with my existing harness so I could swap the Megasquirt in and out as I felt like it.  Keeps the project from being an "all or nothing" conversion.

The covers are removed left and right, exposing the fuel injector drivers on the left and the 6 coil drivers on the right. 
Once the circuit card is removed, the injector drivers can be harvested easily by using a heat gun on the back of the board.  Heat the back of the board while gently tugging on the driver.  It will pull out when the solder softens.

Remove 88 pin connector with the heat gun, or carefully clip each lead.  The 88-pin attaches to the rail from the bottom with Torx screws.  Notice that the coil driver rail has an insulating cover.

The coil drivers clip back into place on the rail.  Here I've wired the output to the 88-pin and the grounds are tied together.  I use de-solder wick for the ground.  The power-on circuit is on the left.

The power on circuit (credit to Goat Thumper).  The transistor uses power from Ign to switch the main relay on.  I couldn't find a 2n2222a at Fry's so I used a generic NPN low power transistor.  I measured total current through the coil and transistor at 100mA.  If the transistor has .6v of voltage drop it dissipates .06 Watts, so don't worry about the heat.  The resistor limits current through the base, the diode is flyback from the relay coil.  The capacitor keeps small glitches in IGN power from resetting the ECU.
UPDATE (27 Nov 2011):  I finally solved my remaining starting problems by identifying a fault with my power-on circuit.  The symptom was that in colder weather the motor would never fire on the first attempt, or it wouldn't fire at all.  It was totally sporadic and difficult to identify, but what was happening is that the cold reduced my battery voltage sufficiently so that cranking caused enough of a voltage drop that the transistor no longer had enough current through the resistor to keep it saturated.  IT WAS TURNING OFF THE MEGASQUIRT!   I solved the problem by reducing the resistance on my 7.5kOhm resistor to match my transistor.  If you stick with the 2n2222a as specified you'll be fine.  I need a new battery, too!



Added the wiring for the main connector.  I used the 18" labeled wires from DIYautotune.  Only 1/2 of the labels matched because it was meant for bank injection, but it's still nice having different  wire colors.  I cut 10mm off the bar on the right to route the wires out the same direction that the 88-pin connector wires go.
Wiring for M3X plug added.  I used the stock DB37 solder cup connectors but if I did this again I would use crimp connection DB37 connectors.  Also added cable retention clamp where the bundle exits the case.
Done for now.  When I'm sure that I won't need to re-arrange or add wires I'll add a conduit for the wires.

 Chris didn't screw this up too bad, so I followed his wiring guide.  Since I didn't mount the power-on circuit on the Megaquirt proto area, I didn't use the two connections that I crossed out.  (credit to Chris Seigman.. he has some great info at

A note about grounds- they're not all the same.  Just because your 88-pin plug connection is labeled "ground" doesn't mean it's a ground.  Some are grounded to, well, ground and some are the ground connection for a sensor.  You can't connect a Megasquirt ground to a sensor ground, it needs to connect to the chassis.  Know the difference.  Pins 6, 28, 34, 53 and 87 connect to the chassis.  Of those, pins 6, 28 and 55 are on a larger pin with more capacity.

Bosch coil driver.  These are bullet-proof and you save $48.00 for 6 over purchasing BIP337s.

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