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HI so my 1981 320i bmw is having an issue with not wanting to crank over but if i jump it then it starts right

up.once i disconnect the jumper cables i tried to turn on the headlights and it starts to die. i have almost 12v

at batt when engine off but i dont know if my voltage is getting to the starter or what the problem is please

help i also disconnected my headlight relay to see if it helps the problem and if i do that and turn headlights

on nothing happens so im thinking there related.please help me get my baby back on the road

Can you drive it to an auto shop? They'll have a bettery/alternator tester you can normally use for nowt.

Have you tried a different battery?

i am a uti graduate with quite a bit of mechanical experience i think i will head out to the car and see if i jus put the other battery in and see what i get be right back

i have a multi meter and what not to do some of the testing just not to sure where to start with this issue

just tried a different batter not jumping it but putting it in and started right up

i jus used a different battery and it started right up not run batt voltage was around 12.3 and when running was only like 12 somthing as well

Please try to be to the point with your topic title and stay away from caps only

sorry for that and update i think it is in relation to the alternator removed to test

if the alternator is charging you should get about 13.5volts at the battery with the engine running and 12 volts with the engine switched off .....

could be earthing problem .
Check wiring upto fuse box old wiring

Most likely to be the alternator not giving enough output or having wiring problems. At idle the alternator should crank up the voltage to 13.5-13.8 Volts, if not here's a few checks, pls disconnect the battery first:

Remove the voltage regulator assembly from the back of the alternator and check if the brushes (correct name?) aren't too short. Check wiring on the back of the alternator and to the battery, usual suspect is the ground cable running to the front beam chassis connection. Usually that's where you have rust or other corrosion issues. You can also try to measure at the back of the alternator if you have a better voltage there than on the battery.

the brushes are fairly low could that be the problem

That is likely indeed

Just managed to get one from the local junkyard for $25 has a rebuild sticker on it works great