Yet another alternator question Options

[ThePaintedMan]

Sorry for asking one that I'm sure has been answered before. Searched but couldn't find a thread that sounded quite like my issue. Getting ready for a track day next week, and of course having charging issues. Typically I see the alt light come on with ignition on, but it shuts off as soon as I start the car. Occasionally it does come on when I turn the ignition on, but the next time it doesn't. Now, it doesn't come on when the ignition is on or when it's running. I suspect it isn't charging regardless because I see 12.2 volts with the car off and about the same while it's running. The car has been sitting but it was charging prior to this and it was covered with one of Perry's excellent covers. I wonder if the brushes are bad, but I think it sounds more like a bad relay board.

[Spoke]

Below is the schematic of the alternator/battery/Voltage Regulator circuit in the 914.

Your GEN light doesn't light now. Trace the path to light the GEN light from the Battery when you turn on the key and engine off:

Battery through Fuse F9 to one side of the GEN light.
Other side of the GEN light to the D+ connector at the VR.
From D+, path is through the relay in the VR to DF at the alternator.
From DF in the alternator through the armature (rotating part) and its brushes to ground.

To test the GEN light and alternator, remove the VR and short D+ to DF at the VR pins on the relay board. This will cause the alternator to produce maximum voltage when the engine is running.

With the engine OFF, when you turn the key ON with D+ <> DF connected, the GEN light should light.

This will test all the wiring, the relay board, the armature and its brushes.

If the GEN light is off during this test, several items may be suspect:
GEN light bulb is bad; test separately out of the socket.
Visually inspect the GEN light socket and connections to make sure they are clean and secure.
Fuse F9 is bad or dirty contacts. Check F9 with an ohm meter and clean the contacts.
Wiring/relay board is suspect. Once F9 and GEN light are tested ok, with D+/DF shorted at the VR contacts, remove the connector to the alternator and short DF to ground. The GEN light should come on if the wiring and relay board are ok. If the light doesn't come on, there is an issue with the wiring or relay board.

With this setup, if GEN light is on with engine off, start the car and measure the battery voltage. It should be more than 15V. Do this test quickly as this test applies maximum stress on the alternator and the battery.

If you get more than 15V with the engine running and D+ connected to DF, then chances are your alternator is ok and your VR is suspect.

If you don't get 15V, your alternator may be suspect.