JHZR2
Staff member
My 1993 300SD charges fine after I take the engine up to over 2000 RPM. Before that the alternator doesnt put out.
My understanding is that this means that it isnt getting excited by the D+ circuit, which is supposed to come from a switched source. The battery light (9 Ohm 1.5W bulb) comes on when there is a potential difference between the operating alternator output, and the battery itself. In my car the bulb is good, and I see 12v on one of the bulb terminals when the key is switched on.
The wiring diagrams show the following:
You can see the battery light connects the line that is coming from D+ (bottom graphic) and fuse 23 (top image).
The battery comes through this way.
What Im not sure about for diagnosis is, as long as B+ is connected, should D+ be hot? So, then I should always see 12V at the D+ terminal on the alternator (as long as the battery is connected to the alternator main stud), and at one side of the bulb socket in the cluster? I don’t think this is what the graphic below shows. I think all paths back are blocked by diodes.
If I see this above image correctly, a 70mm conductor goes from the battery to a terminal block, a 10mm white and red goes to B+, but is blocked by a diode from going back on the 0.75mm blue conductor. So Im not sure if I should always see 12v at the bulb socket even when the vehicle is switched off (I don’t), or if I should see continuity to ground from the one side of the bulb circuit. The ref lines and fuse 23 determine whether or not there is 12v or an open curcuit on the other side...
Correct?
Or does the bulb ground through the scr circuit? The 12v switched source is the gate for the transistor? Is there a good source for the principles of excitation through the battery light, and how to diagnose?
Thanks!
My understanding is that this means that it isnt getting excited by the D+ circuit, which is supposed to come from a switched source. The battery light (9 Ohm 1.5W bulb) comes on when there is a potential difference between the operating alternator output, and the battery itself. In my car the bulb is good, and I see 12v on one of the bulb terminals when the key is switched on.
The wiring diagrams show the following:
You can see the battery light connects the line that is coming from D+ (bottom graphic) and fuse 23 (top image).
The battery comes through this way.
What Im not sure about for diagnosis is, as long as B+ is connected, should D+ be hot? So, then I should always see 12V at the D+ terminal on the alternator (as long as the battery is connected to the alternator main stud), and at one side of the bulb socket in the cluster? I don’t think this is what the graphic below shows. I think all paths back are blocked by diodes.
If I see this above image correctly, a 70mm conductor goes from the battery to a terminal block, a 10mm white and red goes to B+, but is blocked by a diode from going back on the 0.75mm blue conductor. So Im not sure if I should always see 12v at the bulb socket even when the vehicle is switched off (I don’t), or if I should see continuity to ground from the one side of the bulb circuit. The ref lines and fuse 23 determine whether or not there is 12v or an open curcuit on the other side...
Correct?
Or does the bulb ground through the scr circuit? The 12v switched source is the gate for the transistor? Is there a good source for the principles of excitation through the battery light, and how to diagnose?
Thanks!
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