On "Microsquirting" the Porsche 914 - Part 7, "Ignition, Wiring, and Terminals, Oh My...Part Dos!"

"Ignition, Wiring, and Terminals, Oh My...Part Dos!"

Back to Part 6

Ok, so we know where we're mounting the Microsquirt ECU. And we know how much MS harness we have to work with. And we know the general wiring concepts and what pin housings and terminals we're going to use. But there's still some outstanding questions.

One issue is grounds. The MS needs two different grounds: a chassis one for the MS unit itself (it has two black chassis ground wires) and one back to the MS for the sensors. The chassis ground is pretty easy: there's a multi-pin ring terminal at a case half bolt in the back-middle of the engine. The engine grounds to the transaxle and there's a big ground strap on the back end of the transaxle to a stud on the chassis, and the battery negative wire goes to that same chassis stud. Soup. I can run MS, wideband controller, and ignition coil to that.

The sensor grounds require a bit more thought. Each sensor -- CPS, TPS, IAT, MPS, CLT -- needs to have a reference ground back to the Microsquirt itself through that sole sensor-ground wire. Five-wires-into-one presents a tad bit of a problem...do you splice them together? Do you create a ground block somewhere? What about my "no butt splices"?

On top of that many of those same sensors need access to a single +5V output wire from the MS box...again, splice them all together...? Sounds kludgy.

3/28/22 Update: Please read Part 11 Addendums. I no longer recommend using the Bussed Feedback Receptacles described below for the 5V sensor reference and ground. I instead discovered the SensorX Sensor Breakout Board which does it all for me, and can be neatly tucked into the harness. I changed the car from the below device to the SensorX. I'm leaving the info here because I still think they're pretty cool and could be useful for something else.

I happened to learn about something really cool. Deutsch makes something called "Bussed Feedback Receptacles". These things are 2-, 4-, 8, and 12-way female pin housings that connect together wires within the male plug in various ways. So, for example, I could use a "12-Way Bussed Feedback Receptacles" that connects the left 6 wires together in one circuit, and the right 6 wires in another circuit! This was how I was going to do it! I could wire all five ground wires to one side of the male 12-way plug with one wire going to the MS ground wire, then all five +5V wires to the other side of the male 12-way plug with one wire going to the MS +5V wire! Each of those circuits would be connected together!

A perfectly elegant solution. You can even get mounting tabs for them (as I did) to mount somewhere convenient.

In the same manner, I needed to deliver 12V power to each of the four injectors, the IGN4 coil, and the wideband controller. Let's see...four injectors plus one plus one is six, one wire going to the relay is seven...sounds like I'll also buy an 8-way "Bussed Feedback Receptacle"!

Sensor 5V reference power and signal ground issue -- resolved.

So what size wire to use for the relayed-12V power items? I figured the injectors didn't need many amps, but the IGN4 might need as much as 15. I spent a lot of time looking through various sizing charts, and decided in the end to go with 14g for all of it. These are short runs so I'm not too worried with that decision. A single 25' roll of red wire for all of it. Though I do admit those little injector terminals did not like getting crammed with 14g wire...but I managed.

For reference, the whole factory D-Jet system, including Bosch coil, low impendance injectors, and the ECU and its sensors, all ran on a single 25A fuse. I'm actually impressed...

At this point I was ready to start cutting and terminating wires. I installed all the parts, all the sensors, lightly draped the MS wiring harness across it all, and grabbed a beer to stare at it...and had no idea how much work I had ahead of me. So I grabbed another beer and decided that first wire snip would be put off for another day of drthinking...

1 - TPS, fabbed bracket

2 - GM MAP, mounted on the tab where aux air valve was

3 - GM IAT, fabbed bracket, where cold start valve was (see adddendums, it was moved later)

4 - CPS mounted behind cooling fan housing

5 - VW IGN4 coil, fabbed plate where distributor was (see adddendums, it was moved later)

6 - FiveO high-impedance 280cc injectors, stock mounting location

7 - Wideband controller, mounted on vertical shroud ((see adddendums, it was replaced later)

8 - Roughly where Microsquirt ECU will mount on chassis

9 - Going all-Deutsch, with stamped pins

I mounted all that up, looked at the coiled MS wiring harness, and thought to myself..."what's gonna be the first wire you cut? 'Cause once you cut, you can't un-cut..."

I'm'a gonna pull a Bob Villa on you and just show the final result. You can scroll down for some details.:

But this was a shat-ton of work; I probably have about 20 hours in it between design and implementation (could probably replicate it in 5). Some tips.

• Ensured all sensors and bussed receptacles were in their correct places and orientation;
• Separated the different MS wire functions and used Velcro straps to retain wires in desired routing;
• ID'd the wires I wasn't going to use (roughly half of them) and disassembled the MS connector and removed those wires entirely (put aside for future needs);
• Snipped off the obvious excess to minimize clutter;
• Worked from farthest point out, inward;
• Signals first, ground second, power last;
• Generous on strain to minimize wire fatigue;
• Terminated everything. Lots of crimped wires.

After all the wires were routed and terminated, I pulled the terminals from their pin housings and inserted them through PET Expandable wiring sleeve. The ends of each sleeve were cleaned up with wiring harness cloth tape, both to protect the wires and attempt to minimize water intrusion.

Highly useful tools:

• Very good wire cutter/stripper
• "W" crip connector. I used a ratcheting Weatherpack crimper from Eastwood
• Cheaper "w" connector tool to final-crimp wires and insulation strain-relief
• Deutsch pin and insert removal tool
• Deutsch pick removal tool
• Pin extractor tool - buy these, 'cause you know you're gonna need to remove a pin or two

Thank you to Mario of The  Dub Shop for generous publishing of required wiring diagrams. I just taped 'em to the cabinet and used them as my primary reference when building. Check, double check, triple check, write notes on the sheets....then cut the wire.

So now it looks like it's just about time to install the engine into the car...well, after installing the clutch, stabbing the transaxle, installing the starter and wiring, putting on the engine frame and mounts, transaxle mounts, install an oil cooler...you know, just a few small things.

On to Part 8: "Will it Run?"

TDC - Top Dead Center, the firing position for #1 cylinder
CPS - Crank Position Sensor, also known as CAS, Crank Angle Sensor
TPS - Throttle Position Sensor
MPS - Manifold Pressure Sensor
IAT - Intake air Temperature sensor
CLT - Coolant temp sensor
AFR - Air-Fuel Ratio
FLAPS - Friendly Local Auto Parts Store
CSV - Cold Start Valve
MS - Microsquirt