Bart your problem is unlikely to be the ECU as a car of that age will likely have a machine sensing voltage regulator within the alternator. If the voltages seen were on the cars own battery voltmeter they do indicate a problem but don't pay too much attention to the actual values as these instruments are notoriously innaccurate. Its the variations you saw that are relevant. With engine running above idle you should be seeing over 14 volts at the battery or alternator main output (thick cable from B terminal) unless the battery is well discharged but you will need to measure this accurately yourself with a multimeter.
Having checked for obvious stuff like loose drive belt, bad connections etc. and if problem remains, I would remove alternator and take it to an auto electrical engineer who should be able to test it fully in short order. If you want to have a go yourself, bear in mind a diode down would reduce output current by a third (each can be checked with multimeter) and might explain the low output volts, poor condition brushes could cause intermittent output and a regulator failure usually means no output.
Last edited by RichardV6; 22/02/21 08:24 AM.
