Brother needs help with alt not charging.

[GRINCH]

Russell, don't worry about stirring the pot. We all are here to learn. The "S" terminal may be connected externally to a voltage, such as battery voltage, to sense the voltage to be controlled. I read that the S voltage will provide a better regulated voltage. Read more here. http://www.madelectrical.com/electricaltech/remotevoltagesensing.shtml

Pins S is the large one on the end of the connector, then F, L and P.

I hope someone doesn't get confused by the diagram you posted and apply 12V to pin L.

 

[Russell]

That was a really good read! I am absolutely going to go and hook up the voltage sensing wire on my K5 now!

 

[Blue85]

Fordum is talking about load dump. The alternator is a big inductor and you can't change the current in an inductor rapidly. So when there is a lot of current (i.e. charging a low battery) and you break the circuit, the voltage increases as much as it needs to for the current to continue flowing and dissipate wherever it can (V=LdI/dt = inductance x the rate of change of current). The current goes through whatever it can - the ECM, the radio, the lights, whatever. This can last as long as half a second, which is a LOT of energy. So....disconnecting the alternator while spinning is only OK when the output current is low.

A lot of newer cars have zener (actually avalanche) diodes for rectifiers to limit the load dump voltage. Either that or they use another device to absorb the energy (CLD protection).