backfeeding alternator

[zephyrc]

i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294

[Ken0069]

You need to add a diode to your dash light/exciter circuit to prevent current feedback to the MSD. The user manual from MSD has a section in it to deal with this issue.

[rick360]

i found today that if i switch the motor off it continues to run until it runs out of fuel,, is it possible that the alternator is backfeeding the ignition system.. if i disconnect the alt light wire at the alternator motor stops as soon as i turn it off, also if i remove the bulb it stops when turned off.. when i turn it off the warning light comes on and stays on until motor runs out off fuel..alternator is a powermaster 7294

When you turn off what switch? Main battery disconnect or the ignition switch?

Did you change something or when/how did this happen.

Rick

[zephyrc]

rick,, it is happening when i turn of the ignition switch,,, my ign live is fed from a fuse board to all ign live parts including the alt light and oil pressure light.. its been that way for 3 years or more, but only now has it started back feeding.. have i wired the car incorrectly or is the alternator faulty

[zephyrc]

alternator has 3 wire connectios,,. main power feed to battery,,, alternator warning light and another terminal which i am told is to sense battery voltage

[rick360]

It's strange that it did work and doesn't now, but I'd look into the MSD info like Ken suggested. You are definitely getting power back to your ignition from your alternator. A diode should prevent that. On 3-wire alts I always wired them as 2-wire using only the main large wire and a single wire from ignition with a diode between the terminals. I remember when the diode was backward the car wouldn't turn off.

Rick

[zephyrc]

have ordered the diodes, will be a couple of days i guess, will fit one and let you know.. is this a fault in the alternator that has arisen..

[Ken0069]

have ordered the diodes, will be a couple of days i guess, will fit one and let you know.. is this a fault in the alternator that has arisen..

Can't answer that but I run a 3 wire on both my cars. One has a 7AL and the other has a Digital 7 and both require that diode before they will shut off by the switch.