revisiting alternator upgrades and rewiring - PeachParts Mercedes-Benz Forum

JHZR2 · #1 09-10-2010, 12:46 AM

Hi,

Thinking about my options for alternators for my 82 300cd, and the options of using the 80 or 115A saab alternators. The reality is that especially for the 115A the system needs to be re-wired, in order to handle the extra current, but I think Im missing something.

From what I see on a w123, the electrical setup about the battery is as follows:
Negative to body. Positive to the starter - the "heavy" cable is for discharge purposes in reality.

Now, there is a single cable that goes from the starter to the terminal block. Hot heavy, but not 18ga either. There is also the other smaller wire to the starter which engages it.

So, in reality, the entire charging current from the alternator to the battery goes through that one small wire that connects the terminal block to the starter. I believe it is 6mm^2 area per my manual, making it a 10ga wire.

OK, now here is where my confusion grows... There are two other 6mm^2 wires that are the connection from the alternator to the inside of the car (dc distribution I suppose at the fuse panel), which go in the upper grommet next to the battery. The connection back out comes from the lower grommet next to the battery and to the terminal block.

So is it correct to say that the current to charge the battery takes the trip of 2x10ga wire into the car, back out to the terminal block, then via 1x10ga wire to the starter and then via the 25mm^2 (6 ga) battery cable to the positive terminal of the battery?

OK, so where do we want heavier wires then? Bigger ground and maybe an extra one to the alternator body is good, right? Bigger one to the starter aides in faster cranking. But the limiting step is still going to be the old, 10ga wire that makes the distribution inside the car, right? Isnt that dangerous? Even if you connect a nice heavy conductor from the alt to the terminal block, so it charges the battery via heavy cables in the route from alt to terminal to battery directly, there still is the chance for a high-current alternator to put excessive current through the old OE cables that connect there.

Additionally, if one re-wires the alternator, what do we do with the connection that originally connected to the alternator? Just wrap it in electrical tape? One of the wires is still needed, and the harness as a whole goes straight into the car.

It seems to me that if the alternator is re-wired, whether it goes straight to the battery or straight to the terminal block (or both), we end up feeding the DC bus inside the car backwards from what was OE (since we are no longer sending current up the three-spade alternator connector). Is this an issue?

Should we be concerned about safety of a high amperage alternator given that the wiring to all of the loads (except battery charging) would still be the original small gauge wires? Shouldnt fuses be put inline with the existing distribution wiring to protect from excessive current from the upgraded alternator, NOT just bigger wires in some sections?

Sorry for the lengthy post.