MSD Ignition Power Supply
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MSD Ignition Power Supply
Can anyone recommend a switched 12v source to power an MSD 6A ignition box? I am having trouble finding a source that is "hot" in both the start and run key positions. 
1974 4-speed Corvette.
1974 4-speed Corvette.
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Joined: Aug 2006
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From: Columbia Missouri
Make your own.
Buy an off the shelf foglight relay. They are generic black 30 amp four pin relays.
Provide it with constant 12V from the BAT terminal on the alternator. Ground the relay to the engine or the frame. Run a trigger wire from the yellow wire from the wiper motor circuit and run the lead to the MSD box.
This will give you a solid 12V constant off of the BAT terminal on the alternator but only when the key is in the IGN position.
Buy an off the shelf foglight relay. They are generic black 30 amp four pin relays.
Provide it with constant 12V from the BAT terminal on the alternator. Ground the relay to the engine or the frame. Run a trigger wire from the yellow wire from the wiper motor circuit and run the lead to the MSD box.
This will give you a solid 12V constant off of the BAT terminal on the alternator but only when the key is in the IGN position.
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Thank you for the post Durango_boy
Will the yellow wiper wire be "hot" when the key is in the cranking position? Also, would the alternator produce enough voltage when the engine is cranking to close the relay?
Will the yellow wiper wire be "hot" when the key is in the cranking position? Also, would the alternator produce enough voltage when the engine is cranking to close the relay?
Last edited by thecorvette; Oct 6, 2007 at 08:40 AM.
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From: Columbia Missouri
Yes to both. The yellow wire is an IGN wire only, meaning it will have 12V ONLY when the key is in the IGN position.
Also, yes, the BAT terminal on the alt will have a solid 12V for cranking. If you're skeptical you can install a spike suppressor or capacitor.
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I like where this is going. Okay, what gauge wire should I run from the alt. to the relay/relay to ground? What is the spike suppressor and what is a capacitor and where/why would I wire them into the system?
Thanks again
Thanks again
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Joined: Aug 2006
Posts: 24,125
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From: Columbia Missouri
I would run at least a 12 gauge power wire from the BAT terminal on the alternator to the relay, and then 12 gauge to the MSD box.
Your ground should also be 12 gauge, or the same gauge as the power wire, and as short as possible.
The trigger wire does not support a heavy load so it can be a 16 or like stock harness yellow wire gauge.
A capacitor is just like a car battery only it stores a temporary charge. Once the power supply is cut, the charge will drain from a capacitor unlike a battery where the charge is held.
I doubt you'll need one, but it just installs inline with the power wire. some have a ground too.
I still think you're just fine running the power wire and won't need anything extra.
Your ground should also be 12 gauge, or the same gauge as the power wire, and as short as possible.
The trigger wire does not support a heavy load so it can be a 16 or like stock harness yellow wire gauge.
A capacitor is just like a car battery only it stores a temporary charge. Once the power supply is cut, the charge will drain from a capacitor unlike a battery where the charge is held.
I doubt you'll need one, but it just installs inline with the power wire. some have a ground too.
I still think you're just fine running the power wire and won't need anything extra.
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Thanks guys
I went ahead and dropped the steering column and wired into the ignition switch.
CorvetteDave, that is what I was hoping to do, but my '74 has resistor wire running to the distributor. Anyway, had a great time and thanks again guys.
CorvetteDave, that is what I was hoping to do, but my '74 has resistor wire running to the distributor. Anyway, had a great time and thanks again guys.
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