Keyed Hot 12 Volt
 

Hello All:

Well, I am still having issues with my build. It has never really run really good. It runs rough and seems to misfire a bit. I've checked the spark and it looks weak. Voltage across the + and - on the coil is only 4 - 5 volts. Shouldn't that be closer to 12 volts? I am using the keyed hot from the Miata ECU for the Keyed Hot 12 Volt supply for the coil. I don't think its getting the voltage it should. Is there a better Keyed Hot 12 Volt power supply that I can use?

Thanks,
Den

There is a lot of things ECUs put out only 5 volts for so you may very well be into one of those circuits. I have no clue what a Ford coil should have but don't ever recall one that ran on something other than 12v.

I can't speak to a 93 but in a 96 there is a live terminal in the under the hood fuse box for antilock brakes that I have pulled power for a couple of things for. It's wired on both sides so you just grab the output wire from the back of the fuse box and cut it for what you want to use it for. Then put a fuse in the box and boom keyed 12v. Wait maybe boom was a poor choice of words there...

Are you testing the voltage with an actual volt meter, or are you using a test light?

Voltmeter... a cheap one, but its still a voltmeter. I found new power supply, there is a blue plug near the drivers headlight with a solid 12v. I think its an AC power supply, and I got no AC.

Den

The blue plug near the drivers headlight is an auxiliary power. I came across it in wiring diagrams but I don't remember how many amps it's rated for.

And it's fused I hope!

I would not use that blue plug for anything other than a timing light personally. That's really it's only point. Most recently I used the white with red to switch a relay to a separate fuse panel which comes directly from the battery. This gives me about 8 keyed hot sources with their own fuses.

-Jason

Do I have to use the mustang fuse box from under the hood? and harness from under the dash?

no, just the under hood engine related wiring.

Make sure your 12v constant is pulled from the Miata fuse box, not the old ecu connections. If you are worried about the Miata 12v keyed hot (white with red) being strong enough pull another lead off the Miata fuse box or right off the battery and use the white/red to turn it off/on with a relay.

-Jason