No Alt. light after start up but battery not topping off .. Options

[Literati914]

My battery was bought last month.. Alternator light comes on with the key on, but goes out at start up. Battery keeps running low and eventually won't start the car. Does this point to anything usual - VR maybe?

[Superhawk996]

I’ve been reluctant to chime in . . . . Cause I’m just going to preach the same ol’ same ol’

There is a voltage drop (~ 0.8 - 1.1v) going on somewhere between the alternator D+ and the battery.

Ground cable between chassis and engine trans is always the first suspect on 914’s.

Next on the hit parade would be the battery cables themselves (both positive and negative).

Don’t forget the B+ wire between alternator and the starter.

So four wires / cables / ground strap portions of wiring need to be checked.

Until I had verified all those with a DMM by measuring voltage drops I wouldn’t move forward. Note: per Greg’s comments above - I don’t think a diode can be ruled out but I’d verify voltage drops in wiring first before chasing a bad diode theory.

Edit:
Note: measuring resistance of the battery cables and ground strap CANNOT be done with a DMM by measuring ohms. You have to measure voltage drop while cranking the engine to gauge whether or not the cable has excess resistance

The above pertains more to starting issues.

Since the 0.8 - 1.1v voltage drop is occurring across wiring with the engine running - voltage drops could just be measured with the engine running and ought to be apparent. .

Here’s an intro (too long . . I know . . . but necessary to understand):
https://www.fenderbender.com/running-a-shop...ircuits-part-11
Plenty of YouTubz videos on checking voltage drops too if wall of text isn’t your thing.

Will try to assist if OP wants to go down this rabbit hole.