MSD Wiring w/ Trunk-Mounted Battery

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mopowers

mopowers

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I'm curious how you folks with trunk-mounted batteries wire up your MSD ignition boxes. Instructions say wire the box directly to the battery. They also say not to use a fuse in that wire because the box apparently has automatically resetting short-circuit protection.

So if I run the large red wire to the battery in the trunk with no fuse, isn't that just asking for trouble if that wire shorts out?

I'm wiring all of the trunk components per Rob's (crackedback) diagram below. Is it worth tossing in an in-line fuse in the large 12v MSD wire. @Mattax , @crackedback - you guys are the experts. What do you think?

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Good question. I’ve had some strange issues with a couple of boxes I have out there and I’m pretty sure it’s ground issues.

I was told by someone I trust that if the battery is in the trunk you use a 1/0 cable from the battery to a ground on the engine.

Then ground the box to the same ground as the battery on the engine.

I haven’t done it yet.
 
MSD boxes tend to be sensitive to voltage issues.

I don't fuse the line as IIRC, there is a breaker inside the MSD box. You could put one inline if you like. MSD boxes usually draw about 1-1.5 amps per 1K/rpm.

Grounds definitely cause all sorts of problems with these boxes and other electronics like F.I. The large black wire goes to ground where I ground battery. You could also bring a clean ground wire forward and bus it up front.
 
I extended the MSD battery hot and ground wires with one size thicker wire and ran down the passenger side sill back to the trunk mount battery. The switched hot goes to the key switched lead from the column.
 
MSD boxes are very forgiving with supply voltage, not sensitive, will work with as low as 5v & deliver full voltage with 9-18v.
As far as the direct grd wire to the box, I would connect the thick black wire to the engine...or....grd strap if the body/frame is used for bat [-]. The MSD box only knows voltage, not where it comes from....
 
i run all wire to the battery in trunk. seperate fuse box hiden by the trunk hinge. also the switched 12V, The relais for the switched 12V is in the front. Battery mounting the wiring is to damping noises and power peaks.
 
I don't see an issue with using a 125A fuse near the battery as long as the MSD internal circuit breaker is rated at a good deal less than that. They're looking to protect their equipment and you're looking to protect yours.
 
A one wire alternator will back feed the ignition and the switch will not kill the engine. I tied in a normally open solenoid to kill the car. First picture no solenoid would not shut down , Second pic I added a solenoid and it worked great. The black capped post was my Positive battery connection from the trunk. This allowed me to retain my factory harness and battery cable to the starter with no modifications.

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Last edited:
ESP47 said:
I don't see an issue with using a 125A fuse near the battery as long as the MSD internal circuit breaker is rated at a good deal less than that. They're looking to protect their equipment and you're looking to protect yours.
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Thank you. That's likely what I'll end up doing. I figure a 20A fuse and 12g extension wires should work well. I like the idea of running a clean power and ground.
 
mopowers said:
Thank you. That's likely what I'll end up doing. I figure a 20A fuse and 12g extension wires should work well. I like the idea of running a clean power and ground.
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Copy that. I was mixing up the ignition wire with the battery cable, which is why I was thinking 125A.
 
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