This is ingenious! Once you know, it makes total sense but damn if it took years for someone to figure it out. When I get a chance I'll have to put a diode on mine and report back. I wonder how the S5/S6 deal with this issue in their harness...

Jon

Edit: Hm, actually no can do since in the intervening years I re-did my wiring harness and eliminated the now-unused FB wire from my harness entirely. I guess I could try it on a friend's car. So do you get less draw with the stock FB wire and diode than supplying constant 12v, or just compared to stock FB wiring without the diode because of the grounding when off?

 

This also might help. https://www.maximintegrated.com/en/a...dex.mvp/id/636

 

When I was testing the circuit with my Power Probe (essentially a test light that can also apply power or ground, although the new ones are much more advanced) I found that the circuit at the alternator is "dead", no power or ground. Supplying battery voltage to the circuit did not affect current draw on the battery.
So a loop line to the alternator will have the same effect as a diode in-circuit as far as parasitic draw is concerned. (No draw whatsoever) However the voltage sense will not be optimal and the alternator will undercharge, since it is getting its voltage sense at the alternator output instead of far downstream of the alternator. Probably not as much of a problem when the battery is fully charged and the alternator is providing nothing more than maintenance current, but it does make a difference if the battery is discharged to any degree and the alternator is trying to cram 60-70 amps through that tiny little alternator cable.

Ideally, the voltage sense would come straight off of the battery, not the alternator or deep from the chassis wiring. I may change my car to do that, someday, if I ever install a proper battery kill switch, which would also require running the alternator B+ cable to the battery side of the kill switch. If you do that, there is no way for the car to keep running if you hit the switch, it kills the car electrically 100%.

But then I'd want to run a separate power source to my stereo, like a low current 12v NiCad, and a diode in-series, so that I wouldn't lose memory power to the stereo even after hitting the switch, because resetting all of my settings and re-pairing the BlueTooth is a bitch

 

hi guys, i want to make sure i'm understanding the wiring so if you can make sure the following is correct, that would be great (i'll be attempting to wire an FD alternator to FB chassis).

B - goes directly to battery
S - goes directly to fuse box
L - dash (but I believe this is just a switched power source, am not understanding the volt gauge wiring in the wiring diagram of 85 13b)

and you then added a diode to S to make sure it doesn't get grounded when car is shut off?