Forum » EFI Wiring Fundamentals » Powering an ECU from a PDM. Thought it would be simple???

Powering an ECU from a PDM. Thought it would be simple???

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Hi there,

I have started on a project to make a loom for an ECUMaster EMU, PMU16 PDM, ADU dash display, GPS module and Cartek XR isolator. I've installed ECU's before but this project has taken things out of my comfort zone. I'm trying to take my time and use best practices across the board and have bumped into my first road block.

I have some basic questions which don't seem to be covered in the user guide for the various components. Hoping someone has experience with this sort of thing, even with other brands. Apologies if it's been covered before.

The ECU, display and GPS module require both a 12v battery and 12v switched input. I'm comfortable in that the 12v switched inputs should come from the PDM but what about the 12V battery inputs? Should these also come from the PDM or should they actually be wired to the battery?

Wiring them to the battery would mean fuses are needed which the PDM is supposed to do away with.

If they're OK to wire to the PDM, should they both be wired to the same output? If so, then I'm thinking when the PDM switches the output off, the ECU and dash might not have the opportunity to run through their shutdown procedures as both battery and switched inputs would be isolated at the same time.

I'm stuck as both of the above options seem to have their issues. What are the best practices with this sort of thing?

You wire the 12V battery inputs from the PDM, and make them turned on with the master switch. This allows you to measure the current usage, and protect it against a short/fault. You would investigate whether the 12V switched input is just an "ignition on" signal, or whether it draws significant power. I bet it just a low-current signal that could be powered directly from a "Run/Stop" switch. That same switch might goes to the PDM, where it would switch the Ignition / Injector power (again, allowing you to measure it current, and detect faults). You will need to decide, should the display be on when the ECU is, or just when the engine is Running? I would normally make it on with the master switch, so that the engine could be cranked to monitor oil pressure without the engine running.

Got it. Thanks David! That makes sense. Yes I believe the ignition on signal is just a low current signal.