[tonksie93]

I have a 2012 F30 330D.

When driving the vehicles voltage will drop from 14.7v to 13.8v (i have a volt gauge installed) and then shoot back up. I am assuming this is my alternator going bad.

This morning when attempting to crank my vehicle the whole ignition lost power then came back on.

Battery is in good health and terminals are tight.

Could a bad alternator cause the symptoms i’m having? Thinking there may be a bad diode in the alternator.

Thanks in advance.

[Wires]

I'd load test the battery that would rule out the alternator. The alternator wouldn't have anything to do with starting unless it wasn't charging properly. Both 14.7V and 13.8V are "ok" charging voltages though, so I wouldn't use that as a reason to point at the alternator.

Is it logging any codes?

[Billfitz]

This is symptomatic of excess current draw in the starter circuit. It could be a bad starter, a bad starter cable, a bad ground strap, even a loose battery terminal.

[tonksie93]

Thanks for the replies. I forgot to mention when the voltage drops my headlights dim and heater blower slows down so something is not right with the charging circuit.

[im_an_alien]

Also check the ground strap between the chassis and transmission, I recall someone having similar issues which pointed to a bad ground

[Billfitz]

Originally Posted by tonksie93

when the voltage drops my headlights dim and heater blower slows down so something is not right with the charging circuit.

Not necessarily. The same fault that's happening when you're starting the car could be causing the intermittent voltage drop. The only way to know for sure that the alternator is wonky is to measure the voltage at the alternator output, removing any other problems from the equation...like the ground strap.

[Wires]

Originally Posted by tonksie93

Thanks for the replies. I forgot to mention when the voltage drops my headlights dim and heater blower slows down so something is not right with the charging circuit.

If your battery load tests fine, then anything you see during starting has nothing to do with the charging system. I'd be looking at ground straps like the others had mentioned.

[tonksie93]

So i got my vehicle running with a jump. Drove completely fine but as soon as i switched off the engine i lost all power to the ignition, which then came back on straight away.

I will load test the battery and clean up all my grounds. After that what else should i be looking at?