GM 10/12Si alternator with Diode???

ftoy516 · Sep 15, 2015

i have a gm 10/12 si alternator on my buggy that has been working fine, but long story short i had to rewire some things because of an issue with my kill switch....

so i fixed the issue with the kill switch and this lead to the alternator feeding power back into the ignition (thus the buggy would not turn off) so i tried to find a solution to the problem and decided to wire a Diode into the circuit. i wired the Diode inline with the #1 (exciter wire) terminal and this allowed the kill switch to turn off the buggy and the Diode stopped the power from getting to the Ignition....HOWEVER i then noticed that the Alternator gauge showed that it was NOT charging the buggy...

i then stared to test it with my meter and it showed that it was indeed not charging (which it did before) so i took some jumper wires and put on terminals #1 and #2 to see if it would charge without the Diode....and it Charged with the jumpers and without the Diode in the picture, but that way does not allow the buggy to kill out which is why i went with the Diode in the first place.

the #1 (exciter) wire with the Diode gets 12v from a terminal block that is down stream of the kill switch. i also tried pulling 12v power straight out of the fuse box to terminal #1 instead of the way i have it and nothing changed when i went that route. the meter showed it was not charging that way as well...
the #2 wire (voltage sensing) wire gets 12v at a bus bar which is where the alternator batter wire is getting power as well.

12v is not something i know a whole lot about...and i just thought i would ask some of you guys to see if i may be missing something or if anyone has any experience with this type of set up.

any ideas or help would be greatly appreciated

clemsonjeep · Sep 16, 2015

As far as I know on my rig the excite wire is tied into the wire coming off of the 2nd terminal on the kill switch. In other words it's not the hot side coming from the battery...this is essentially a "key on" source. The 12v sensing wire is just lopped to the back of the alternator. With this setup I have to rev the motor up to around 1,200 rpm when I first start it in order to excite the alternator.

You didn't mention it so I have to ask...have you tried revving the motor after start up to see if that excites the alternator?

ftoy516 · Sep 16, 2015

yea, i have reved it up to around 2000 and i had my meter hooked up at the same time, and there was still no change in the voltage...it is just running off of the battery.

i have ask a few older guys that have hot rods at work today and no one seams to know what is causing it to do this...i have heard that people with 8n tractors have to do this kind of thing when they swap to a GM alternator, but i don't know anyone that has done this personally.

i have wondered if i should have the alternator tested just to see and go from there...at this point i have NO CLUE.

clemsonjeep · Sep 16, 2015

Yeah its definitely worth a trip down to advance for a quick test on that alternator.

ftoy516 · Sep 17, 2015

i just got through pulling the Alternator off and taking it to have it tested and it FAILED!! So i am going to try to get one at a local shop on Saturday

hopefully, that will solve the issues that i was having "where I thought the Diode was not working".

it may have been a BAD alternator all along.

jayphizzle · Sep 18, 2015

If that doesn't fix your problem, use a relay to open the circuit to the exciter wire on the alternator. Use the ignition power to excite the coil of said relay.

ftoy516 · Sep 20, 2015

well i have worked on this thing off and on all weekend.....to no avail......i installed a new GM 12Si 94 amp 3 wire alternator, i tried almost every combination of wiring that i can think of to stop the power from backfeeding, but i cant seam to get it figured out...

i wired the ign power straight from the battery... taking the fuse pannel out of the picture and that didn't work.
i tried different wiring directions with the diode which didn't work either. (I know the correct way it goes beacause i tested it with the meter to see which way the power flows / is blocked)
i have it blocking power from the alternator which is suppose to solve the issue......
some how power is backfeeding and will not let the buggy turn off with the master kill switch.

i can kill my IGN power, which kills my COIL power with the toggle switch that i have in the buggy currently but i really want to get it wired where the master battery kill switch cuts the rig off (mainly because if something happens where someone that is not use to my rig has to kill the power they will not be looking for a toggle switch...they will be looking for a master kill switch)

the kill switch i had was the Flamming River Battery and Alternator disconnect, and it worked fine, however i mounted it to close over the top of the drivetrain and i have hit it two different times because of the drivetrain flexing up enough to contact the bottom of the switch....and i do not really want to have to redo my switch panel and buy a third flamming river kill switch.

the battery disconnect part of the switch still works right, but when it ground out, that fried the alternator kill part to it....so that is the reason i went the route with the diode...which some hotrod guys say fixes the issue of alternator backfeed.

i know this is more than likely a simple fix in some fashion, but i can not seam to get it.

4truggy · Sep 21, 2015

Re:

Where does the wire, on the rear post of the alternator go?

ftoy516 · Sep 21, 2015

The wire that comes of the rear of the alternator "Battery terminal" as well a the #2 " voltage sensing" wire goes to a junction terminal that is on the frame rail, which is down stream of the kill switch.....

After sleeping on it last night i have been thinking that i need to put both of those wires to the other side of the Master battery disconnect
(closest to the Battery, before the disconnect)....because i think the power may be feeding through my ground on the fuse pannel back to the battery....that is the only thing i have not tried yet.

clemsonjeep · Sep 21, 2015

Why not eliminate the IGN toggle and run it to the battery kill switch?

4truggy · Sep 21, 2015

Re:

Yes that's exactly why I asked. If that wire is run to the battery side of the disconnet. It won't feed back to the electrical system.
I just noticed in another thread your in North Ga. I'm in dahlonega.

ftoy516 · Sep 21, 2015

yea, i am in Cleveland on the Dahlonega side....small world.

4truggy · Sep 21, 2015

Re:

Yeah it is. Your not by any chance the one that had the ftoy, at the rock garden in August are you?

ftoy516 · Sep 21, 2015

yea, that is me.....been workin on it trying to get some kinks work out of it......did you have a rig there?

ftoy516 · Sep 21, 2015

Sooooo after moving the Alternator Battery wire to the "battery side of the Master disconnect" everything is back to normal....it turns off with no issues.

also i put the "IDIOT" light into the system, so that if the alternator starts to act up the light will come on...i felt like that might be some CHEAP insurance.

thanks for the input fellas....12 volt can really try your sanity!