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Permanent 12V with PDMs

EFI Wiring Fundamentals

Discussion and questions related to the course EFI Wiring Fundamentals


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How are permanent 12V wires normally connected in cars using solid state PDMs? For example, a digital dash might require a permanent 12V as well as ignition-switched 12V. Are traditional fuses used in these cases, or are they simply treated as switched 12V lines and wired to the PDM outputs?

You could allocate one circuit of the PDM to be HAAT (Hot At All Times, aka Permanent 12V). I would do that so I could monitor current consumption of those devices -- they might be turned off if they draw too much.

Thanks David. I'm looking at using a pair of ECU Master PMU 16s and they have what I believe is an activation input that is meant to be connected to the ignition, meaning they couldn't provide the permanent 12V supply. Having since looked at Motec's PDM documentation, it seems this may be an oddity of the ECU Master device, so I'll speak to them.

David is on point here (as usual :-) ), if your PDU can supply a constant 12V, that's the way to go, otherwise you will have to go 'old-school', like you've mentioned and use a traditional fuse, and most likely splice into the supply wire feeding the PDU from the battery.