Alternator D+ wire issue?

[DollyDagger]

Hey i have a 1990 325i Cabby and when i was driving home the other night my car died on me due to a low/dead battery. I tested my alternator at a few part stores, and it failed. I got a new alternator and battery (both currently test postive) but i still have the issue with the car dying out on me. The B+ wire reading matches the battery when i use the voltmeter(usually about 12.4, and when the car is on with radio and lights its about 12.0), so i think that takes that out of the equation, no? The only other wire i have is the D+ and i don't know where to go from here.

Thanks in advance,
Daniel

PS the belt is on securely, and not slipping

[Van Westervelt]

What does your battery test at while the car is running? If its at 12.4 or lower, you either have a bad connection, or a bad alt. Should be at least in the 13's to charge the battery while the car is running.

[DollyDagger]

When its running it stays at about 12 even, havent seen it drop below, but 12.0 is too low. Its a connection issue since the battery, voltage regulator, and alternator all pass tests. I guess my question is how do i follow the D+/small blue wire to figure out my issue because it seems to be the problem i think.

[DaveSmed]

Got a test light or a meter?

Attach the ground lead to the nut on the shock tower, and touch the positive lead to the metal alternator case. Does the test light illuminate/meter show voltage?

Also, what is your battery warning light up to?

[DollyDagger]

So i tested the alternator and it was at about 10.8 volts when the car was running. The bulb has burned out or doesn't function. So im assuming based on the wiring diagram i looked at but fully didnt understand that the D+ wire goes through the cluster, and is it related to that bulb/light? im looking at the bulb now but i cant tell if it is good or bad. this is a new cluster so maybe it burned out because it was sitting at pick n pull for awhile?