I have a 3-wire Bosch alternator on my 617 that's going into the S-10. It uses a push-on plug with all three wires contained in the plug. Two of the wires are pretty big gauge (like 12ga in US sizes) and terminate in a single loop connector in the OEM wiring harness (which I saved from the 300SD). Pretty sure that they just go to the battery, which I'll accomplish by hooking to the starter B+ stud.

My question is what to do with the little green wire. I think it gets switched 12V with either an idiot light or a 10 ohm resistor in that loop. Am I right on this one?

Dan

Correct, the skinny wire is the D+. It needs to go to switched battery via an incandescent bulb or resistor. Preferably a bulb since that is your "no charge" indicator.

If you don't hook this up the regulator cannot excite the alternator and you will get no electiricity.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Thanks, JB! I thought I had that right but I wanted to be sure before I start running wires. I haven't wired a car from scratch in a long time.

Dan

Just to confirm jaybob, thats exactly what i did on mine sans the light or resistor. Just hooked it directly to key on power, the other two to B+. A charge light would defintitely be useful for later, but i never got around to hooking one up

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The guys who did my rebuild (Coastal Rebuilders here in Wilmington - great shop) REALLY stressed the light or a 10 ohm resistor. I'll confess that I don't know the technical reason behind that but obviously yours is working so it must be a "it's better if you do this" kind of a deal. Anyhow, it's not a big deal to add a light to the flat piece of aluminum that is my dash panel.

Dan

All correct. Both B+ wires simply run in parallel to BATT+ (via screw terminal block in my 300D). I don't know if running D+ directly to 12 V will risk damaging the voltage regulator. All cars I know of that use this "Motorola alternator" (GM, M-B, VW) power D+ either thru a bulb(s), resistor, or both in parallel. You need some current to "boot up" the alternator field, but might need limiting the current. If JB3's Vreg dies, there will be a good suspect.

Inside your alternator, there is an extra set of diodes called the "trio". Once the alternator starts, it will generate it's own bootstrap current through the trio to the D+ contact. If you don't have a bulb or resistor of some sort, the trio will be in parallel with the main charging circuit, and will be supplying power to the general electrical system. In time, this will kill the trio, which means you'll need to replace the alternator.

How do you establish what ohm resistance this light bulb or resistor should be?

I did about 8k without this arrangement in my van swap, BUT, the alternator did ultimately have an unrelated problem, the bearings were failing and the shaft was loose. It did charge fine the entire time though.

Because it was reliable i wired it the same for the new alternator and engine, it hasnt been running though, so no additional mileage.

When you say in time, what are we talking about? How long would the damage take to hsppen?

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Frank - Thanks for the explanation. I wondered what the deal was!

The rebuilders recommended a 10 to 15 ohm resistor (Radio Shack has them) or, of course, a light. It'll only come on when there is no charging going on. I decided to add a light to the dash so I have an indication of charging.

No idea of how long it might take to fail the diode trio - but I don't want to find out!

Dan

Ideally, resistance should be higher than the bulb. A 5W bulb would be about 30 ohms. The resistor doesn't isolate the D+ circuit, but it does limit current. A 30 ohm resistor will limit current draw through the D+ circuit to less than half an amp under the worst conditions.

Damage may not occur for a very long time. But if there's an unusual condition in the system...a weak primary diode, a short, or a completely flat battery...it can be quick.

Definitely glad you posted this info, id hate to do long term damage, especially since i have the big 120 amp alternator too. Need to upgrade the signal wire. Thanks for the data!

Dan can you post a pic of the light you will be using?
Would there be an issue using an led versus incadecent?

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You can use an LED but put the resistor in parallel. Need to use a power resistor, probably 3W minimum. Be careful it will get warm.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

This is a poor application for an LED, because it may not put enough current through the D+ circuit to bootstrap the alternator. If you use an LED, use a resistor as well. you can go higher on the ohm rating to reduce the wattage...say 330 ohms. This should put enough of a trickle through the field to bootstrap, but will reduce the load to half a watt.