I have a 1985 nissan 720 pickup. the other day the battery light started coming on. it stays very dim and brightens up as the rpm’s get higher. and the second i hit the brake pedal the light cuts off. and comes back on as i start driving agian. i went and talked to a mechanic and they tested everything. the altinator is getting power and everything looks fine but it is not charging the battery at all. the battery tested fine and i just replaced the altinator. still no luck.
Clean both ends of both cables,also look under coating of hot battery wire.
Look for positive resistance in the battery/ starter circuit. Weak or powdery connections, old starter cables, etc. Just look for poor connections everywhere.
Check to make sure the wire from the alternator to the battery is still good. Many car makers use a fusible link wire that can fail due to power spikes or corrosion resistance causing a build-up of heat. Also check the wires going to the voltage regulator on the backside of the alternator. These wires provide a signal voltage that ‘turns on’ the alternator.
Have the mechanic check the voltage between the alternator body and the negative post of the battery while the engine is running and there is a good load on the battery to see if there is a grounding problem to chassis. If there is a problem there will be a voltage drop across the bad ground. Be sure he makes the ground reference for the test on the battery post and nowhere else. If that is ok and the connections on the rear of the alternator check ok for proper voltage then the alternator may be bad. I assume that the mechanic has already made sure that the main connection between the alternator output and the positive battery post is ok.