Minimal wiring digram for SD22 Sd23 or SD25.

[elminero67]

I suspect that others that may be using SD2# series motors in cars/trucks other than the 720 series trucks may be in the same boat I am in. I needed a bare minimum wiring harness that provided basic charging/starting functions, but nothing not essential. Simply condensing the standard 720 wiring diagram is a little confusing...in particular the white and blue wire from the alternator to the charge light was puzzling as it is negative (ground) when the alternator/motor is not turning, and charged while it is turning. Without this charge light the alternator will not charge. This is something most people with mechanical ability already know, but not so obvious to knuckledraggers like myself... Note that I simplified it more by running the (3) thick white wires (8 or 10 ga) directly to the live starter lead rather than running them to the battery.

[elminero67]

Oh yeah, obviously only need the live wire to the IP on the rotary IP, the SD22 usually has the inline IP that needs no wires.

The IP wire needs to be live while the starter motor is engaged. If you use the 720 ignition none of the functions (accessories etc.) remain live while the ignition is turned all of the way on. You need to find a source that remains live while the starter is engaged or the motor will not start.

[waynosworld]

I spent most of a day with a volt meter figuring out a hot wire for both key positions, start position and run position, the start position was a bitch, I finally used a relay coming off the start exciter wire, and when I let the key go to the run position, I used the windshield washer motor wire to keep it running.
I am using a stock 720 diesel harness though.
I believe that a stock 1980 gas harness would work with with the extra wire that goes to the coil, but one would have to make a glow plug mini harness, as I like the automatic glow plug warm up system.

[asavage]

From memory, the idiot light is a field exciter and must be present to kick-start the alternator. Most later designs parallel a small resistor around the lamp, so if the lamp burns out the thing still charges. Let me go find the diagram again . . .

. . . yeah, here it is in the FSM.

And here's a colored-up version I did back in 2008. Click on this image, then in the original thread click on the image, and it'll expand large enough to read.