Thread: Alternator dead -300tdi

Hi Oberon,

I'm no auto electrician, but my understanding is the dash warning light is part of the the field winding circuit.
So if the lamp is blown, or the circuit is open, the alternator won't charge.

Tony

Originally Posted by TonyC

Hi Oberon,

I'm no auto electrician, but my understanding is the dash warning light is part of the the field winding circuit.
So if the lamp is blown, or the circuit is open, the alternator won't charge.

Tony

Pretty sure that's correct on a pre "smart" alternator, pre CAN BUS car. Replace the globe and see what happens.

Yes, a 2 wire alternator of that vintage needs a battery voltage to excite the field windings. That voltage source is via the "ignition" lamp on the dashboard. It works in a similar way to the old "dynamo" DC generators.

Thanks all for your replies, very helpful info. Have swapped the bulb with the one in the high beam slot. Still no charge or battery light illumination, but the high beam indicator doesn't illuminate anymore, so obviously the battery light bulb blown. Not sure what's happening but I'm still hopeful the alternator is fine and there's some other simple fault in the circuit

Thanks all for your replies, very helpful info. Have swapped the bulb with the one in the high beam slot. Still no charge or battery light illumination, but the high beam indicator doesn't illuminate anymore, so obviously the battery light bulb blown. Not sure what's happening but I'm still hopeful the alternator is fine and there's some other simple fault in the circuit stopping excitation of the stator.

On some of those alternators the field connection terminal (the small terminal), can develop a high resistance. It is usually a bad crimp joint, or corrosion. Carefully undo the nut on the terminal, remove the lug and make sure that the lug is tight on the wire and clean.

Ah, your explaination is a bit confusing? Are you now saying that the high beam lamp does not work anymore? If that is the case, then there is either a ground fault on the field wire, or the alternator has an internal fault, which blew you test lamp.
How are you testing the output to know that it is not charging, or does it have an ammeter?

Scroll down a bit:

add link: Alternator Exciter Wire Light | Electronics Forum (Circuits, Projects and Microcontrollers) (electro-tech-online.com)

Originally Posted by Old Farang

On some of those alternators the field connection terminal (the small terminal), can develop a high resistance. It is usually a bad crimp joint, or corrosion. Carefully undo the nut on the terminal, remove the lug and make sure that the lug is tight on the wire and clean.

Ah, your explaination is a bit confusing? Are you now saying that the high beam lamp does not work anymore? If that is the case, then there is either a ground fault on the field wire, or the alternator has an internal fault, which blew you test lamp.
How are you testing the output to know that it is not charging, or does it have an ammeter?

Scroll down a bit:

add link: Alternator Exciter Wire Light | Electronics Forum (Circuits, Projects and Microcontrollers) (electro-tech-online.com)

Thanks for the tip regarding the field connection wire, I'll check that out.

Regarding the dash lamps, I was saying that I had swapped the high beam bulb (has always worked) for the battery bulb (stopped working shortly before the battery went flat, no charge since); after doing this, no high beam light on the dash when high beams are on, but also still no battery warning lamp.

Now with all this being said, today the battery light came on, and then went off after a few moments of higher rpm - just as it should. I put a multi-meter on the battery and it was charging normally - sitting at around 13.5V, dipped to 12.8V or so when I turned the lights on but then came back up to more than 13V after hitting the throttle (although I suppose the voltage should come back up at idle too?). Then when I started the car this eveing, no battery light at all. So my thinking at this point is that the alternator is fine, but there's an intermittent fault somewhere. Perhaps the field conmnection terminal?

Bit of a confusing one, but again, appreciative of all your responses. Cheers