DIY Bench Harness

Here is all the info you need to build your own version of my bench harness set!

Tools needed:

  • Crimping tool suitable for Molex Mini-Fit Jr terminals; I use this one with good results:

  • Crimping tool that can crimp flag terminal connections: I use a modified noninsulated barrel terminal crimper (that I also use to add beads to aluminum intercooler piping). Can't really say what works best here, but mine is basically this, (but a cheaper version) trimmed back to where the crimping section is.

  • Wire strippers. These are amazing and worth the trouble if you ever need to strip wires. Life changing stuff:

  • Wire cutters, Screwdriver, and soldering iron for the power connector (whatever you want to get there is fine).

  • A terminal removing tool can help for depinning the ECU connectors, but most GM ones you can get by with a pick.

Parts List:

  • Case and Lid (3d printed) STL Files HERE or buy a pre-made case with screws here:

8-Pin 3D Printed Bench Harness Case with Lid and 4 Screws: $20 + Shipping

  • OBD2 port - Junkyard, slide in style. I usually pull them from mid-00s GM cars, as they have all of the wires I want already ready to go. Cut 4-5" of wire off with it.

  • 2x Switches - Amazon - Many different options, just needs to be this style. Here's one I've had success with:

  • 6x Flag terminals - You can use either the 22-16 or 14-16 Gauge depending on your wire size and method of wiring inside. You can also direct solder to the switch terminals if you're pretty decent at soldering and don't melt the switches. Example from Amazon. You'll be removing the insulation so just about anything works.

  • 11mm diameter mount hole power input (barrel plug or whatever you want. Alternatively you can just run wires out the hole and connect it however. Here's an example:

  • 1x 4x2 8 position Molex Mini-fit, Jr Male plug with mounting ears. Molex PN 3901-2081

  • 6x Molex Mini-fit, Jr male terminals. Molex PN 39-00-0083

  • Scrap wire, heat-shrink, solder (if necessary), etc. I recommend decent sized wire for the main power and ground stuff depending on how many modules you'll run at the same time.

  • 4x small screws to close the case lid.

  • Power Supply: This 12v 3amp one works well for most ECMs I've tried and is cheap.

  • Alternatively you can use something like this decent adjustable linear power supply in conjunction with banana clips to a 5.5x2.5mm plug.

This is a basic list of what I use to build the main unit, but you can customize, modify, and upgrade/downgrade as needed.

Each ECU connection you build will need:

  • 1x 4x2 8 position Molex Mini-Fit, Jr Female Plug. Molex PN 39-01-2080

  • Up to 6x Molex Mini-fit, Jr Female terminals. Molex PN 39-00-0080

  • Heat shrink, zip ties, wire covering of choice (I like the braided covering) if desired.

  • ECU connector. I just either buy them with 8" or so of wire harness attached at the junkyard or keep them with ECUs I buy.

If you want to make a splitter for multiple modules:

  • 1x 4x2 8 position Molex Mini-Fit, Jr Female Plug. Molex PN 39-01-2080

  • 2x 4x2 8 position Molex Mini-Fit, Jr Male Plugs. Molex PN 39-01-3083

  • 8x Molex Mini-fit, Jr Female terminals. Molex PN 39-00-0080

  • 16x Molex Mini-fit, Jr Male terminals. Molex PN 39-00-0083

  • Scrap wire, heat shrink, wire wrapping of your choice, etc.

You can make a CAN terminator easily with a 120 ohm resistor, one male Molex connector, and 2 male terminals.


I won't go into too much depth on the instructions because it's pretty self explanatory and may change depending on what ECUs you're targeting and how you power it, etc. I'll give some generic how-to and diagrams/pinouts and then if you have any specific questions feel free to contact me at the address above.

  • It's best to put together the OBD2 port and Mlex connector first before installing them into the case, with a little slack in the wires so they sit low and out of the way. This would be the data lines that'll pass straight through.

  • Once you have that, you should already have the other 3 wires coming out of the OBD2 port for grounds and battery power. Go ahead and crimp 3 more 4-5" wires for the Molex (Ground, 12v constant, and 12v ignition) and insert them with a few inches to spare.

  • Solder 4-5" of wire to the barrel power connector (center terminal is +, outer is -). Put the nut and washer on it as soon as you solder it so you don't crimp it to and aren't able to add it later! Also don't forget to feed it into the hole for it before you start crimping/soldering or you'll have to unhook and redo it.

  • At this point it should be easy to pop in the switches, the Molex, the OBD2 port, and the power connector into the case. You'll have a mess of stuff that looks like this:

  • At this point you'll just need a couple short jumper wires to go from one side to the other and then just need to wire up all the power/ground stuff according to the diagram. I use up to 3 wires crimped into the flag terminals as splices so no other splices are required but you could probably do it however you see fit to make it easy to understand. Refer to the wiring diagram and pinouts below to finish hooking everything up.

Diagrams and Pinouts:

Finished product:

Once you're done, you should have something like this. Click the menu above to get to the bench harness pinouts. That whole system is pretty self explanatory from that point on, just run the wires from the ECM to the female Molex connectors in the same positions you used in the main unit. This basic GM setup I put above will work for all modules 96 to current, and NEARLY everything from any make that uses pure CANbus (or just so happens to also use pin 1 or 2). You also have one pin open on the Molex to connect one other thing (like a FEPS signal for ford CAN stuff or whatever you can think of. If you're brave you can also edit the STL to put in a 10 or even 16 pin Molex connector, or just run cabling out and set it up outside the box even.