12v Ignition source

golf4283 · Mar 26, 2009

So I've been reading through a few threads about 12v switched sources and I have a quick question. I am installing an electric fan as well as a 3g alternator. Now I have an MSD box and I hooked up the red wire to the old coil wire as indicated by the installation instructions.

Now I know the coil wire has a resistor built in which is fine cause the box draws power from the battery once the 12v ignition source is activated.

My question now is about the alternator and fan. The fan is using a relay so I think I can hook it to the same 12v coil wire because it just needs to be keyed to activate it and then draw power from the battery side of the starter solenoid.

I am also thinking the same holds true for the alternator 12v source that is needed because they recommend using a resistor inline anyways. I read that the original coil wire has a resistor built in so would this be ok to hook up the alternator, fan and MSD box to this wire considering none of them actually are drawing power from it.

Maybe I'm way off track here but it seems all just need to be "switched" on to do their function.

Any light shed on this would be great. Thanks.

BarnStang · Mar 26, 2009

THe MSD box is using the coil + to turn on the MSD box. THe MSD box is not "sending" 12v t the coil. THe level of voltage is not that critical as long as the MSD box nows if its on or off.

Hmmm, you could use the coil + to trigger the fan relay, however, remember that anything you tie to that coil + will draw current away from the coil. THe relay ma not be much, but it's still an electromagnet. The relay coil will stay energized all the time with the key on.

Alternanter 12v source? Oh, the field winding. What you will use to energize the alt as opposed to using a one wire alternater. Same thing applies, the alternater will be drawing currentfrom the ign all the time. Not sure what the current is to energize the alt, but I would think it would be more than the fan relay.

You can try it, see how much voltage is at the coil with the car running-if it starts.
If it were me, I wouldn't do it. You can make a simple switched 12v source with a relay. I've posted this a few times. If you have the original factory wiring, se that green with red strip wire going into the voltage regulater. It's switched 12v. Use that to trigger a relay that will provide a solid 12v from the battery through the relay. I have mounted a post beside the relay on other projects, so that you can use that switche 12v source for any electronic box that needs switched 12v.

golf4283 · Mar 28, 2009

BarnStang said:
You can make a simple switched 12v source with a relay. I've posted this a few times. If you have the original factory wiring, se that green with red strip wire going into the voltage regulater. It's switched 12v. Use that to trigger a relay that will provide a solid 12v from the battery through the relay. I have mounted a post beside the relay on other projects, so that you can use that switche 12v source for any electronic box that needs switched 12v.

Thanks so much. That's exactly what I was looking for. Since switching to the 3G I disconnected the relay and cut the wires and hid them. I'll just take that wire and connect it to the fan relay I already have.

12v Ignition source

golf4283

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BarnStang

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12v Ignition source

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