The first place I saw the diode trick to pull the voltage up the 0.6-0.8v was as a fix on a perfectly functional alternator, just one that wasn't pushing enough voltage for the application it was in.

The owner had installed an AGM (Optima type...) battery and it required above 14.1 to even charge.


After all my trouble with my car's failed diode, the new alternator only puts out 14.1 at peak, and the failed diode one was doing the same. It was setting numerous lights off on the dash when the diode failed, regardless.

I'm not sure what happens in a w123 that one might be running in the 13v range consistently, but I'd look into voltage drops in the main wiring runs before doing a mod on the alternator, but wouldn't stress over the alteration too much.

If someone does decide to modify their regulator, make sure to check the voltage drop on the diode you choose. The latest electronics project run at Radio Shack (due to being too impatient to wait for a Mauser or DigiKey order) was showing they're stocking .8v drop for the majority of their diode selection, and I wouldn't want to accidentally push the voltage too high and cause a battery to fail due to overcharging.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...