Battery Light

Assuming you're talking about a 1990 348 TS....

Did you start the car? There is nothing resettable in this system. It's either working or it isn't. Did you recharge the battery in the car? If not, did you put everything back together again properly? i.e. the battery terminals are clean and tight.

The light extinguishing usually means the alternator is charging the battery. If it's not doing this, either your alternator has broken or your battery is not accepting a charge from the alternator. How did you know your battery had recharged? Also, what's the voltage on the battery terminals when the car is running?

 

If the car started well and the charger showed it charged, then the battery should be good. You can check the battery volts without any tools (other than a multimeter) by checking the voltage on specific fuses in the forward luggage compartment. Fuses usually have cutouts in the plastic mouldings to allow you to put a multimeter probe (red one) on the metal section of the fuse without removing the fuse. e.g. fuses 3 and 4 have battery power on them at all times. Also fuse 15. Just put the other multimeter probe (black one) on a suitable earth. A fully charged battery should be around 12 volts dc. Start the car and then measure the voltage at the same fuse. It should be 13~14 volts. This is the voltage coming from the alternator. If the voltage doesn't go up and even goes down with the engine running, then, most likely, the alternator is faulty.

Since you didn't have a problem before, I'm not sure why your alternator would suddenly start playing up.

 

It will reset itself automatically.

 

Could you explain the automatic reset circuits in a bit more detail please, Radlu?

 

On the battery is perfectly fine, I assumed that getting access to the battery or the battery buss bar/s would be more difficult.

If the voltage remained lower, then the alternator or its connections may be faulty/corroded. In particular, check the (yellow-green) wire voltage sense wire and earth lugs.

Here's the wiring diagram:

Starter/Alternator Wiring Circuit

Basically, the battery/alternator warning light is a comparator circuit. The warning light gets two voltages, one from the battery and one from the alternator. If there is a reasonable difference between the two, the potential difference will be enough to illuminate the light. The light gets battery voltage via the ignition key.

Note that your car may be slightly different from the wiring diagram. Sometimes relays and additional earths are added for better reliability.