Alternator / battery / charging issues
Just a brief listing of checks and possible fixes. Usually this starts with either the alternator light coming on (battery symbol on the dash) or a weak battery. Please note that the alternator light comes on at a very early stage. This means that in some cases the light can already start to glow very slightly when nothing is wrong really.
When driving at slow motorway speed say 80-100kms, 5th gear so low revs, high beams and some other big power users on, then in the dark you might already see a slight glow from the charge light. No worries as this is pretty ok.
If the light comes on or you have a low or dead battery, do a few checks first:
First of all, how old is that battery? If you can open up the cells for a check, open up (beware of pressure, gases ans the contnts are acid!). What will you see? Cells not straight or fluid low exposing cells to the air. Are the battery cables connected thoroughly and no corrosing or loose contacts?
Most E21's and E30's have an extra ground strap of abt 10-15 cms long running from the alternator housing to one of the bolts that go into the engine block. Secured?
Perhaps the most important one is the chassis ground strap that hooks up to the left front of the chassis, underneath the battery tray. Undo it and check it thoroughly for corrosion. Especially the chassis is very prone to rust there so be critical!
After these checks you can perform a check on voltages. With a multimeter you can check these:
1. with the engine running you should have abt 13.8V +/- 0.5V otherwise the brushes/voltage regulator combo may be up for replacement, see under Alternator. Hint: when welding is performed, the battery should always be disconnected, otherwise the voltage regulator can blow.
2. right after you turn off the engine the battery should give you at least 13.0V imo, otherwise it is likely that you may want to replace your battery
Warning: before performing any work on the alternator disconnect both wires from the battery to be safe. After this, you can check the connections of the wires, tight enough on the alternator and on connectors? Remember what wire goes where!
How good is the belt and its tension? Does the alternator sit straight, in line with the engine so the belt runs straight as well?
Most likely and an easy check is the bracket that holds both the voltage regulator and the voltage pick up brushes (see insert below). Only two small bolts undo this bracket. The brushes should move in and out their guides easily, there should be anough length left and no corroded contacts or disconnected wires.