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How does the relay grounding work in this PDM

EFI Wiring Fundamentals

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Discussion and questions related to the course Motorsport Wiring Fundamentals

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I have been looking into doing a harness using a Eaton Bussman PDM and when I started looking at the diagram I got confused about the grounding of the Relays. Looking at the drawings of products similar to this in the Bussman lineup they all have a singular pinout that grounds the signal wires but this one does not show that.

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So the one shown in Capture2 allows you to use either all ground triggers, or all high triggers, since the 85 terminal is not always tied to the power in. You decide now it works by what you attached to the connector 3C. if 3C is tied to the Battery Voltage then all the relay triggers are grounded to activate. That's great if an ECU or other electronics is controlling them. If you connect 3C to Ground, then you need to provide +12V on the triggers, which is often easier if you have individual switches on your dash you wish to use.

So say if I wanted to run something like a starter or a fan using a switch to toggle it I would wire it like this?

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yes, that's correct.

Ok how about this situation, mocking up my hazards/turn signals. If the hazard switch is in the closed position would it create a short and blow a fuse?

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It would not blow a fuse, however it would just make both blinkers work if one was enabled.

I think your Hazard switch needs to connect both blinkers triggers to ground. I tried to update your drawing:

Actually what I've drawn won't work either, you need to use a two-pole switch so when the hazard to ground is active, so is the connection between the two turn signal triggers (like your drawing), otherwise those triggers need to remain independent.

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You need to sink to ground on both legs. Currently your image just shows you connecting them together with no ground to the low side of the relays coil. I also don't know how/if you're controlling the logic to the flasher to interrupt the signals to your lights as its drawn.

Edit: The other David's too quick for me. Still making coffee :)

So I am trying to keep the stock switches in my Datsun because I love the look, so if I can avoid using different switches that would be ideal.

So the way I'm understanding your explanations is that the way I have it in "test" makes it so the hazards will not work because the only route to a ground is though either one of the open Turn signal switches. Would that mean if I closed a turn signal switch along with the hazard switch I would then have working hazards? That would be a cheesy work around to the problem.

the T/S switch is pretty complex, and I might be able to use the pinouts in a funny way to make it work. I attached an Image of the pin out and I think if I connect pin 12 on the T/S Switch to 85 on the Relay 1/left side relay, then ground pin 14, when I have the left side closed it will flash. For the right I would start at 85 on the left relay to pin 11 then ground pin 13. For the hazards I would have a line from the 85 pins on the left and right relay, to the hazard switch, then it goes to pin 9 and ground through 10. In that configuration I think that would make it so the left and right are independent to ground, and the hazard can be switched on as long as the turn signal switch is in the neutral position, oops I guess it would work if the right side was closed as well. but that isn't a big issue if it means I can use stock switches. Does that make sense? I can draw it out if it makes it clearer.

Really appreciate all the help guys!

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Alright, back again,

I'm 99% sure this won't work, but I want to just make sure I'm thinking about this right. Will relay 4 be able to trigger from this set up? Using the line that has fuse 12 as a ground to trigger relay powering the starter solenoid. Still messing with the rest of it. Pretty much starting from scratch again haha so don't give me too much grief about the rest of it.

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