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Finally have my 68 running ,one small problem the motor keeps running even when key is shut off.Running a Powermaster alt. w/ either 1 or 3 wire hook-up. Am using 3 wire w/ adapter for int. reg. also MSD 6al,dist.& coil. Have red wire from box connected to white wire from ignition using mini starter so yellow wire not used.When I turn key off gen. lite stays lit nothing smells hot all wires seem fine. Any ideas?
 

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usually if there is nothing wired incorrectly a car that runs on will have one of/or two things wrong.

one, the idle is set too high and the throttle valve in the carb isn't shutting all the way when you turn it off.

two, timing is too advanced and you are getting some self detonation. the higher the compression of your motor the more this may occur.

check those for sure. then check to make sure the ignition wire to your distributor is hot only when the key is in run and start positions.

i'm not familiar with msd products but enough guys here are. i'm sure they can help you better than me. but do check your idle speed and your timing.

might also mention if it is "dieseling" or actually running smoothly and capable of high rpms as if the key was on and you were driving.

good luck, it'll stop when it runs out of gas i'm sure. :D
 

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The alternator is backfeeding the electrical system. Using an internal alt with alt in 3-wire configuration, the voltage sense wire should be on its own ign/acc terminal separate from the ignition contacts on the ignition switch.

As long as the sense wire has voltage supplied onto it, the alt is live. When the voltage is taken away, alt dies, engine quits.

Or somewhere along the wiring, you've wired the two circuits together. My bet is the alternator is still supplying volts.

Many 70's vehicles came with a two separate IGN terminals, one for ignition and the other for the alternator.
 

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I added the diode and it worked fine. Mine did the same thing yours did. If I disconnected the 2 wire plug on the alternator, then the motor would turn off with the ignition switch. Just make sure you install the diode in the right direction. It supplies voltage to the alternator and the diode blocks the voltage going back.
 
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