My van will not start without a jump pack but once started will run forever I can drive miles but as soon as I stop it is completely dead and when running the battery reads 11.75 so it seems to work enough to run but not charge ?
Put a voltmeter on the battery. Watch to see if the battery drops in number. If it does, get a new one. Sounds like it will. Start reading should be 13.8 or so.
Your generator or alternator isnt doing anythibng. 11.75 volts is a battery in torment. If its actually an alternator and the wiring to the warning light is faulty or the bulb is loose or blown, you will have this fault. .. Or the brushes have worn out or the regulator is dead. You dont need to take out the original Bosch alternators to change the brushes. And ones with internal regulators can be changed if you have three screws and a cover plate on top. If its a generator ( cylinder with big plastic covers near the pulley ) you can change the brushes in situ. And the regulator will be external too. Although generators can need repolarizing if they havent been used - there is a procedure documented in manuals .. run it as a motor for a bit to build some magnetization , so it can self excite and start producing power. Alternators need the warning light working to self excite and start producing power.
if it is a dynamo there are two terminals on the back, one small one is the field connection the other is the main output. Disconnect both cables and connect a test lamp or voltmeter to the large terminal and a good earth point then run the engine tickover is fine and with a screw driver or a short piece of cable short the field and the output together. If the dynamo is good it will show a voltage or light the light you can increase the speed to see if your dynamo is putting out a decent voltage. Done it loads of times it is only like bypassing the regulator.