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Old 04-10-2011, 05:12 PM   #1
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Default Need 12v switched during "start" and "on"

I've been running down power supply issues with my install for a few days now, and trying to stay in a Zen place and not push the car off a cliff. I need a switched 12v source that is hot both during "start" and "on", and am a total noob at reading wiring schematics, but it looks like I can find this at the Meter fuse under the dash. It looks like that fuse is fed off a blue wire from the ignition switch, and then a black/yellow wire runs up to the instrument cluster from there.

1) Did I read this correctly and is this in fact hot in start and on?
2) Is it better to grab the blue wire before the fuse box (with a new fused connection to my relay), or the blk/yel after the fusebox (probably with a new fused lead here too).
3) Is there another power source that functions the same way that would be better to use?

Thanks! I'd really like this thing to start today! This will get me a little closer.
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Old 04-10-2011, 05:19 PM   #2
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What exactly are you powering?

The white / red wire coming off the main relay is the one that supplies the ECU and most of the engine-control electronics. It is hot in start and run.
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Old 04-10-2011, 06:08 PM   #3
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Joe, thanks for the quick response.

I'm powering what's called the MPI relay on my Galant VR4 harness. It supplies power to the ECU and fuel pump, and requires both a constant 12v and a switched 12v. I had not thought the switched 12v through and just grabbed it at the cig lighter, and woke up this morning wondering if it needed to be hot at start and run which of course the lighter is not.

Here's the DSM ignition switch diagram I found earlier today, and it looks to me like the MPI gets power at both those times. So, given I'm reading this correctly, would I do better to grab my MPI power at the main relay white/red than at the Meter fuse? Is that what feeds pin 1B on my Miata ECU? If so can I grab it there, and if no where's the main relay located in my '92? (edit: looks like that's in the fuse box under the hood)


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Old 04-10-2011, 09:47 PM   #4
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I grabbed the new 12v switched at the blue ignition wire and the car started! I'd still like to know which source is preferable (if either is), this ignition switch source or the one Joe mentions above from the main relay.

Thanks again!
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Old 04-13-2011, 03:29 PM   #5
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For posterity:

I ended up using the source at the circuit opening relay, as the MPI relay with my new harness handles the fuel pump and I don't need to retain the factory relay setup.
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Old 04-14-2011, 12:49 AM   #6
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Where did you tap exactly? I'm assuming you didnt tare the column apart.
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Old 04-14-2011, 12:58 AM   #7
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Under the dash on the driver side. The plug in the photo below is what connects to the circuit opening relay. I pulled the white/red wire out of the connector and used it for this source.

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Old 04-14-2011, 01:09 AM   #8
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Thanks, I hope i can get my LC1 to work during cranking.
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