Re: Alternator on SM/TA

Posted by John T on June 21, 2006 at 20:13:56 from (66.244.90.5):

In Reply to: Alternator on SM/TA posted by SDE on June 21, 2006 at 19:46:43:

In cases where an alternator like the 3 wire GM 10 SI was fed its lil No 1 terminal excitation voltage from the coils high input (NOT to distributor) terminal or the ignition switches IGN output (same point electrically) even if the ignition is turned off its possible for the alternator to backfeed voltage to the coil and they can run on even after the ignition is turned off.

One way to prevent that is to wire a diode (one way current flow check valve) in series from the coil or ignition switch to the alternators lil No. 1 excitation terminal. Assuming its Negative ground for the alternator, the diode MUST BE WIRED with its + anode to the coil/switch and its - cathode to the alternator. Its bigger then required but I use a 5 amp 50 PIV diode cuz the bigger leads are easier to wire. The diode allows current flow INTO the alternator to excite it but not OUT OF the alternator to power the ignition coil.

I recommend a 10 ohm current limiting protection resistor in the excitation circuit whereby the diode may not even be required but if thats in place n she still runs on, then I add the diode.

John T

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