Thanks for your help, I think this might be the answer, nicked this off The Samba:- Assuming the ignition switch isn't leaking current somehow in the 'off' position (not impossible, but highly unlikely), then the only way you can get power to the alternator light with the switch 'off' is (excluding bizarre harness shorts, or something else completely off-the-wall) from the alternator itself. As it isn't turning when this occurs, then power is somehow getting from the alternator B+ terminal to the D+ terminal - see schematic: And the only way that could happen is to have failed one of the big diodes connected to the B+ terminal inside the alternator - see schematic: This will also cause the battery to discharge overnight. I would like to think I'm wrong on this, but I have that sinking feeling that I'm not... Rule Of Thumb: ALWAYS disconnect the battery before using a charger to charge it. The big brute force ones are usually unregulated. They will put out serious voltage, and can damage all kinds of electronics, including alternators. The little 2 amp trickle chargers are not a problem, as they generally can't overdrive the battery to the point of doing damage. If this is what you were using, then I guess it was just your lucky day... Diagnostic: (1) Disconnect the battery, then disconnect the regulator and disconnect the blue wire from the red wire at the regulator. Now reconnect the battery so that there's power to the alternator B+ terminal. Connect a test light from the alternator D+ terminal (red wire at the regulator) to ground and see if it lights up. If it does, bad alternator. If it doesn't, then the problem lies elsewhere (and you likely save some money...) (2) Reconnect the regulator, but leave the blue wire disconnected. If the ALT light still glows, then the problem isn't the alternator / regulator, but in the harness or ALT light connections. Post test results. One other thought: if you've added a big stereo, it's possible, with an internal failure, to get feedback power from the 'keep alive' wire (full time +12 volts) to the switched power (+12 volts from the ignition switch). Disconnect all stereo power wires and see if the problem goes away.
Not exactly sure I follow his reasoning in (1). I'd expect a popped diode to go open-circuit rather than short. You can see from the diagram tho' that by connecting an external battery the wrong way round (-ve to B+, +ve to "ground"), all of the main diodes in the pack will be forward biased and driving a zero-ohm load, so they'll be destroyed . Try a Bosch dealer for a replacement.
The point being its doing the wrong thing, so if you're confident your wiring setup out of the alternator is correct them you've fried something within it. Looks like @snotty has pointed out what though! Any local auto electrical shop worth its salt should be able to repair it for you. I've had dead type 4 alternators fully rebuilt before for £40.
