i have mounted a 1995 subaru outback alternator to a 1987 bmw and im trying to figure out how to get the battery light to function on the bmw. what does the subaru alternator put out to make the battery light come on when charging stops? how exactly does that bulb circuit work on a 1995 outback? i currently have it installed and functioning as it charges the battery but the bulb on the dash never lights.

usually that bulb comes on when the alt is not working. Sometimes very dim or flickering if one of the diodes in the 'trio' go bad.

 

Unless BMW is weirdly different.

Edited May 10, 20232 yr by 1 Lucky Texan

4 hours ago, sirtokesalot said:

i have mounted a 1995 subaru outback alternator to a 1987 bmw and im trying to figure out how to get the battery light to function on the bmw. what does the subaru alternator put out to make the battery light come on when charging stops? how exactly does that bulb circuit work on a 1995 outback? i currently have it installed and functioning as it charges the battery but the bulb on the dash never lights.

Have you tested and noted what you are getting out of all the wires while running, then pull the drive belt to see what differs?

 

i have tried it in a few configurations and i beleive i cannot get the light on the bmw to work with the subaru alternator. oh well it charges anyways.

Edited May 11, 20232 yr by sirtokesalot

That BATT light should be tested at key-0n, before start, with the other warning lights.

From my hazy understanding the Subarus have their alternator (battery) light wired in to the positive side of the instrument cluster. I believe the alternator wire for the light is negative when the alternator isn’t spinning. 

Once the alternator is spinning and putting out positive voltage both sides of the alternator light are positive and thus the light goes out. 

That’s what I believe happens but I could be wrong. Someone explained it to me years ago… 

Cheers 

Bennie

have you tried checking/comparing wiring diagrams for the two vehicles in question?

ie: the Subaru and the BMW...  see what is different between them... might give you a clue on what needs to be done

This may be a dumb question, but is the bulb burned out on the BMW?

sorry i disappeared. the bmw developed a new issue and havent managed to get parts to fix it yet so its been parked again. the batt bulb on the bmw does work. it lights up with key on and goes out with key off but i know if u unhook the plug on the alternator it stops charging and the light does not come on. it is difficult to get to the alternator plugs on the bmw with the air box installed and it has the old school air flow meter in the airbox so it wont run without it installed. i would up just installing a volt gauge in the dash to monitor charging but id like to make the idiot light function at some point because the water pump is driven by the alternator belt as well and would like any warnings i could to notify me of overheating possibility's.