SDS Initial Wiring / Engine Test Box

SDS Initial Wiring

To work out where all the wires need to run from the SDS ECU and relays, i mounted the SDS box / relays onto the right hand avoinics shelf, then began to organise all the wires into logical bundles. I used wax string to tie the bundles into nice (hopefully) neat looking looms. 

The ECU will be accessible through the map box, but is also mounted on the RHS fold down shelf.

I realised that a bunch of the wires needed to go to the Forrest of Tabs, and a bunch needed to go firewall forward. Some of the wire lengths supplied from SDS were a little short to route these along the subpanel with the rest of the avionics wires, so i needed a way to run them through the lightning hole in the sub-panel rib. 

I decided to copy the idea from the fuselage baggage floor ribs, and make up some tabs which take 3/4" gromemts. 

The top grommet will get all the grounds from the SDS system, and the bottom will be the bundle which will snake out through the firewall passthrough. 

I added an ADEL clamp to one of the mounting bolts to hold the wire bundles securely. 

There are a few wires which need to be joined together - one is the Injector Relay power wires, one is the Injetor Revert ground (to activate the injector relays to revert from ECU1 control to ECU 2 control of the injectors), and the third was a new wire used to prevent kickbacks on start - this gets 12V when the starter is engaged. These come out of the SDS Relay / DB25 connectors as seperate wires, then need to be combined into one. I decided i didn't like the crimp connectors for these, so just mechanically wrapped them, then soldered them and covered them in heatshrink.

The 2 injector relay power supplies are tied together into one wire which gets power from the Engine bus fuse block.

The 3 blue wires from the DB25 connector get joined and sent to the start switch - this gives them 12V when the starter button is pressed, to allow the ECU to retard the spark and prevent kickbacks.

These are all the injector wires from the relays organised.

Every wire was labelled according to the wiring diagram.

The loop of wires are all the extra wires whish are not used in my application - such as the octane switch, and some spare wires included for future expansion.

The wires were then all bundled up and the ECU was removed for the next step.

Engine Stand Test Box 

In order to run the engine on the test truck (once it is finally built), i needed to make up a little test box which replicates the aircraft systems, and which will allow the ECU to run the engine. This included a basic electrical system (and starter), the fuel supply system (pumps and filters), the engine bus, a gounding system (since there is no airframe), all the required switches, the coil and fuel pump relays, and the ECU controller. 

I started by laying out the wiring harness on a big table, which had been marked up for the rough dimensions of the engine / firewall. This allowed me to lay the wires out and bundled them up logically.  I used Inkscape to make up a lasercut panel to hold all the switches, as well as the engine bus and ECU controller.

I teminated the Hall (crank) sensor wires so they could be plugged in and run with the rest of the FWF wire bundle

I then terminated the Air Temp (AT) sensor plug, and the throttle position sensor (TPS) plug to run to the front of the engine, to the throttle body. These sensors all use different types of automotive 'weatherpack' connectors - these have little rubber seals which are retained by some arms on the pin, to help keep water out of the connector. This wire run will run along the sump, so i covered it in spiral wrap.

The rubber seals need to be installed before crimping on the pin.

I used a $60 weatherpack crimper for the job

This is the crimper i used:

The wires are crimped as per a normal automotive / molex style pin.

Then the special hole in the crimper bends the tabs over the seals.

The manual has a "note connector orientation", however the drawing shows the pins in an incorrect orientation. It took me a while to work out which direction the pins had to go into the plugs - they go in horizontal, not vertical like the picture.

Next up i installed the AT (Air Temp) sensor into the throttle body and retained the wires with an ADEL clamp onto one of the mounting screws for the TPS sensor. I also added some chafe protection to these little 20AWG wires.

The spare plug was also sealed.

Loctite was used on the screws holding the TPS sensor to the throttle body.

The AT sensor plug gets a round style weatherpack pin, but it is crimped in the same manner. 

The coil wires were also terminated - this uses a different plug. You must unlock the orange tab or the pins won't slide in. 

The MAP sensors also got their green weatherpack plugs installed. 

I was working from the Nov 2024 verison of the manual which does not have the pinouts for the MAP sensors listed. The MAP plug bag had the correct pinout however (and the manual has now been updated). 

 Injector Wiring

Wiring of the injectors was interesting - the system is furnished with the injector plugs already terminated with white 18 AWG wire. Each plug has a long wire and a short wire - one needs to get power, and the other needs to return to the ECU relays so the ECU's can ground them to inject fuel. 

The Injector relays have long 20 AWG colored wires which are the grounds and need to be spliced to the engine wires already on the injector plugs. The manual calls for the use of 2 weahterpack style plugs for this purpose. 

The colored 20AWG wires are connected to weatherpack connectors, then to the larger 18AWG wires to the injector plugs (these came pre-wired). 

There are a couple of issues with this:

1. The longer white power wires from the injectors are not going to be long engough to reach from the engine, through the baffles, through the firewall passthrough to the engine bus. I would need to solder or crimp some extensions onto these (which i am not a fan of doing).  
2.  These wires would need to run through the baffles, as they are on the top of the engine. This would mean i would have to cut them if i ever wanted to remove the engine. The same applies to the Hall (crank) sensor wires.

The solution was to use some 8 position DTM connectors - this will allow me to bring the power wires back to the baffles, and on the firewall side run some 20AWG power wires back to the engine bus (which i can make as long as i like). The colored grounds from the relays would also pass through the same connector, and join the shorter 18 AWG white wires on the engine side. If i ever need to remove the engine, i can just unplug these connectors. 

I will do the same with the Hall (crank) sensor wires, using a second plug. 

These have all been left long and will be terminated once the engine is built and the baffles are installed. 

The DTM8 Deutsch connectors will be mounted into the baffles themselves, and held with a mounting clip and some RTV. I made up a test piece and will use this as a template to cut the holes in the baffles. 

Making up the Test Box

Now all the wiring was worked out, it was time to mount all this in a box. I repurposed a metal storage box, and used a piece of plywood to mount all the components which would be mounted on the firewall or in the tunnel. I made up some fuel lines from some scrap fuel tube, in order to plumb in the required pre and post pump filters. The fuel regualtor was mounted, along with the MAP sensors and the Battery and Starter contactors. 

Scrap fuel tube was used to make the lines.

I sealed all the extra plugs and fittings, and marked them with torque seal.

Fuel will come from the tank on the truck, to the smaller of the filters. Through the pumps and through the big filter, it will run to the distribution block on the baffles. It will return from this block to the black fitting on the fuel regulator, then through the blue bulkhead connector back to the tank. 

The Bussman RTA fuse block uses specal pins to hold the fuses, but these crimp using the normal molex style crimper.

The relays are mounted into Relay holders, which use these special sockets. Which, again, crimp using the normal molex style crimper. 

I printed out a cover for the test panel with a copy of the labelling i will use on the panel (with a dark carbon fibre hexagon pattern to represent some vinyl i am considering). 

I then added one fuse at a time and powered it all up - and NO SMOKE!

Next up i will go through the box and write down what is contained in the MAP.