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One wire alternators converted to 3 wire operation require a external diode. The aftermarket has a fix. A simple plug and play diode. I personally just installed a diode inline and hid it inside the harness. But, you can just get a plug and play diode off ebay. This is extremely common and you really don't need to rebuild your alternator.
If I use the original alternator it needs to be rebuilt because it stopped charging. If I keep the new one yes I need a diode. The new one worked fine up until the internal diode failed. Because it was new I "assumed" it was not the issue so I replaced the ignition switch.
Is the battery light in the center cluster working? That bulb does help to clamp down on residual current.
I'll have to check that.
Edit: No the battery light does not come on when I turn the ignition switch on. IIRC that should come from the alternator 2 wire plug so I'll check it for voltage in a bit.
JT
Last edited by jthornton; May 29, 2026 at 08:19 AM.
Terminal 1 (Exciter / Light) Excites the alternator's internal regulator to start charging. It also provides the ground path that illuminates the dashboard battery light when the system is not charging.
Important Note: The light bulb (or an inline 1N5408 Diode) is absolutely required. Without it, power from the ignition will back feed into the alternator, meaning your engine will keep running even after you turn the key off.
And I just put new bulbs in everywhere and they are so much fun to change...
The old alternator checked out good and put out 15v so I picked up a new plug with pigtail. I think the plug was so abused and wires frayed and the multiple splices that was the original reason it would not charge.
So I put the old alternator back in and it barely fit, seems the body is a tad bigger. Plugged everything back in and key on no battery light so I assume it's burned out. And the engine will not stop until I unplug the alternator.
Pulled the glove box liner out and managed to get the battery light out and it's good... new flexible circuit board and now the negative battery cable won't screw back in and I'm pretty frustrated now.
I took the day off yesterday and went for a ride in my C8 to a favorite restaurant to clear my head. I put the old alternator back in after checking it at the auto parts store, I pulled the battery light and it tested good with my multi-meter so the issue must be between the two. The 2 wire plug is full of splices so there's no question about replacing that today and I hope I can get the battery light back in without taking the center gauge cluster out.
Is there supposed to be an inline fuse in the excitation wire or did Bubba put that there? I need to know before I replace the plug as there are more splices down from the fuse.
Started testing #25 at the alternator plug resistance to ground and it read open circuit. I unscrewed the fuse and tested there and read a few ohms so the light bulb showed up. Replacing the plug and all the splices now as soon as I remove the connectors and plug them in the correct way.
Yes, the inline fuse was added and the voltage sensing wire is something like 14 gauge. No reason to have a inline fuse being that circuit is already fused. There should be a number 1 and a number 2 on back of the alternator. Number 1 is the excite wire, number 2 is voltage sensing. Not positive about this but I would assume L stands for lamp, V stands for voltage.
Last edited by Fly skids up!; May 31, 2026 at 02:39 PM.
Got the new plug installed and with the key on no BAT light. With the engine off I have 12v at the left hand terminal with the key on and with the key off 0v. With the engine running I have 14v with the key on or off. I wish I had a 12v test light to draw it down with the key off. Sounds like a trip to town...
Edit: They are probably all LED so that won't work to test stopping the engine.
JT
Last edited by jthornton; May 31, 2026 at 02:50 PM.
Well I found a wire under the steering column and it's the blue wire that goes to the alternator... that should not be the issue but pulling it out of the way anyway. I did find the blue wire had a couple of feet of slack when I pulled on it in the engine compartment and the wire should be brown not blue...
Yes, the inline fuse was added and the voltage sensing wire is something like 14 gauge. No reason to have a inline fuse being that circuit is already fused. There should be a number 1 and a number 2 on back of the alternator. Number 1 is the excite wire, number 2 is voltage sensing. Not positive about this but I would assume L stands for lamp, V stands for voltage.
I didn't see any stamps on the alternator but L = Lamp and V = Voltage does make sense to me. I went back out and with a strong light I can see a very small 1 and 2 that's cast into the case with 1 on the left and 2 on the right viewed from the rear.
I think my next step to take before taking out the center gauge cluster is to take the drivers seat out, take my hat off and crawl in and check the fuses. There's one fuse that Bubba added that hangs down from the fuse block... not sure what that is or does.
I traced the wire from L on the alternator and it goes to the IGN connector just below the hazard flasher on the fuse block. According to the wiring diagram it should go to terminal 25 on the center gauge cluster plug which is the GEN light.
Looking at the circuit board the GEN light is the second one down on the left and the left side contact is ground instrument power and the right side goes to second pin from the bottom on the left so that has to be hot to ground to light up the GEN light.
JT
Last edited by jthornton; Jun 1, 2026 at 05:35 PM.
Reason: Correction
