Diesel Bombers - Retaining the stock Over Drive switch function

If you have had to resort to using a pressure switch on the transmissions governor pressure port and cringe at the thought of drilling another hole in your dash for a toggle switch, here is a solution.

You can use your exiting OD on/off switch with this simple to build circuit. The switch, and light will act as though it were still attached to the ECM but isn’t. The circuit defaults to the OD on position when the ignition switch is turned off just like factory. One push of the button and OD turns off and illuminates the OD light. Another push, the light goes out and OD engages. Just like factory.

Building the toggle switch board is fairly easy with basic soldering skills and an Exacto knife to cut the traces on the board. All of the parts I used I ordered through Digikey except for the relay I used.

Installation is also fairly simple, I love simple. Once you have installed your pressure switch (I would recommend an adjustable pressure switch) in the governor pressure port on the right side of the transmission, it’s time to do some wiring.
You can run separate wiring or you can creatively cut and splice the original to give it that stealthy look.
Starting at the OD connector on the transmission, cut the ground wire (orange/lg) and run it from the OD connector over to your pressure switch. Now you can either run a new wire or tie back into the wire that is there.
Now run your new wire (or find the other end of the old wire if you are reusing it) through the firewall into the cabin.
Now you need to find the wires that go to the OD switch and light. From these wire you can also get power and ground by splicing into them to run the toggle switch board. You should use a separate heavier ground for the common terminal of your rely. Cut the wires that run back to the ECM and hook the light to the NO (normally open) side of your relay and the OD switch wire to the appropriate terminal on the toggle switch board. Now all that is left is to hook up the wire from the transmission pressure switch to the NC (normally closed) side of your relay.

Tidy up your wiring and you’re ready to go. The transmission will shift into overdrive based on the governor pressure that you set the pressure switch for. And you can turn it off if needed.