[Jonoj]

Hi,
My 96 s320 alternator isn't making any voltage at its output terminal, even after changing the regulator unit.
Could a lack of energize signal from the ecu be the cause, or would it still make decent voltage without the signal?
Thanks
Jon

 

[jal1224]

ECU is not involved. It's a old style alternator, no PWM.

Check the alternator light in the cluster, engine not running. If it's out, D+ won't get energized after you start the engine, and the alternator won't output.

Jon

 

[TimeCapsule]

If it's out, D+ won't get energized after you start the engine, and the alternator won't output.

Would you be so kind to walk me(us) through this mess (I am still learning how to read the MB electrical diagrams):

G2 is the alternator, A1 is is instrument cluster. I see a fuse and a diode that could possibly affect D+, is my reading correct?

Thanks.

 

[jal1224]

The fuse provides power to the D+ line and lights the alternator light and excites the alternator to initiate charging. When the alternator provides power it overrides the fused power to extinguish the light; basically, the voltage is the same on both sides of the light once the alternator puts out power. The transistor symbol simply says the D+ goes to the instrument cluster circuit board. According to the diagnostic chart, D+ also tells the car that the engine is running.

Compared to modern cars, the charging system on the W140 is very old fashioned. Just an alternator and regulator. No PWM telling the alternator how much power to put out, no need to reprogram the car if you change the battery, simple to diagnose.

Edit: the D+ line is the key to the alternator starting to work. If the alternator light is burned out, the alternator never gets voltage on D+ and it doesn't start working. D+ literally kick starts the alternator charging each time you start the car.

Jon

 

[TimeCapsule]

Thanks! And apologies for mistakenly writing diode when I meant transistor.
So, light out = no charge. Light out could be both burnt bulb or fuse, correct?

 

[jal1224]

Yes it could. And you're welcome.

Jon

 

[roadhouse]

You should be getting 12v on the thin blue exciter wire on the alternator with the ignition on. If you don’t, the alternator won’t charge.

 

[Jonoj]

Thanks all, great info here.
I had 12v at the D+ and threw a cheap reman on there which is working. Hopefully I can fix the original.
Where would I find that wiring diagram?

 

[jal1224]

w140 Documentation – Mercedes Restoration Enthusiast

zsmaster.com

Jon

 

[JHZR2]

Can someone walk me through the operation of the light? I think, when I turn the switch to on, I get voltage on the one pad of the bulb socket in my cluster.

I think that 12v is from F3 in the above image. But how does it work in the alternator? Is the other side of the circuit that ends inside of G2 via the blue wire initially shorted to ground, to give the (-) side for the battery/exciter light in the cluster? What occurs to give a full circuit to let the bulb illuminate, and then what gets enough voltage to make it go out? How does the SCR shown by the exciter in g2 change connection once the alternator is working?