Understanding Alternator Exciter Bulb/Circuit

[JHZR2]

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!

 

[97K15004WD]

I'm not seeing fuse 23, where is it on the top picture?

 

[97K15004WD]

to answer your question, if the last picture has the alternator as G2 then you have a three phase bridge rectifier in the diagram there. That will only allow the proper voltage (plus or minus) based on the configuration. The output on D+ will be positive voltage only as the alternator outputs it. I can't see the picture very well, but I found F23 on the bottom picture.

Okay, making more sense of the question. You are asking about field excitation below 2000 rpm correct?

What is the part labeled A1?

Have you checked the capacitors as indicated on the circuits in G2? I can't see the diodes well enough in G2 to determine direction they are facing.

 

[mk378]

D supplies power to the voltage regulator and field coil (brushes and rotor) in the alternator.

With the key off, D is at zero volts as otherwise those parts would draw current and drain the battery. Terminal B stays at battery voltage at all times. When the alternator is stopped the three large diodes on B are reverse biased and no current flows out of the battery.

When the alternator is running normally, the three small diodes connected to D rectify some of the stator output voltage and bring D up to approximately battery voltage. Meanwhile the three large diodes connected to B supply power to the battery.

At startup, battery voltage from the key flows through the bulb and into B. Most of the voltage drops across the bulb, causing it to light up. The small voltage on B is enough to "bootstrap" the power generation to the diodes that produce voltage at B when running. When that happens there is no voltage across the bulb and it goes out.

It is important to use the correct bulb and not replace it with a LED, since the current through the bulb is important. It looks like the B circuit is also used to drive some "engine running / not running" logic such as an electric choke.

I think you are overthinking this and the real issue is just that the alternator brushes are on their last legs.

Does the light come on when you turn the key on but don't start?

 

[JHZR2]

D supplies power to the voltage regulator and field coil (brushes and rotor) in the alternator.

With the key off, D is at zero volts as otherwise those parts would draw current and drain the battery. Terminal B stays at battery voltage at all times. When the alternator is stopped the three large diodes on B are reverse biased and no current flows out of the battery.

When the alternator is running normally, the three small diodes connected to D rectify some of the stator output voltage and bring D up to approximately battery voltage. Meanwhile the three large diodes connected to B supply power to the battery.

At startup, battery voltage from the key flows through the bulb and into B. Most of the voltage drops across the bulb, causing it to light up. The small voltage on B is enough to "bootstrap" the power generation to the diodes that produce voltage at B when running. When that happens there is no voltage across the bulb and it goes out.

It is important to use the correct bulb and not replace it with a LED, since the current through the bulb is important. It looks like the B circuit is also used to drive some "engine running / not running" logic such as an electric choke.

I think you are overthinking this and the real issue is just that the alternator brushes are on their last legs.

Does the light come on when you turn the key on but don't start?

The light never comes on. I only get 14.3 v and current out of the alternator after the engine has been taken to 2000 rpm or higher, and there’s enough residual magnetism to get it to produce power. Everything is fine for hours after that, until I shut it down.

The bulb is good and correct. I see 12v on one of the two terminals when the key is turned on.

I’m not sure if the D+ wire is bad, regulator is bad, etc. that’s why I’m trying to understand that circuit. It acts as a gate, right? So when one puts the key to “on” it is conducting through, the D+ terminal is shorted to ground, right? Then when the alternator starts producing, the voltages are high enough that there’s no real delta across the bulb and it goes out, unless the leads are shorted or there’s a substantial voltage delta.

Am I getting it right?

 

[mk378]

You could unplug the D wire at the alternator and jump it to ground, the light should then definitely come on when you turn the key on.

Voltage on only one side of the bulb and it is not lit means that the bulb is bad.

 

[Donald]

There are self exciting alternators. They only need to have one wire output. A regular alternator will have a sense, excite and a power output. The self exciting alternators will not put out any power until they are rev'ed up to about 2000 RPM then they will put out power even at idle.

 

[JHZR2]

You could unplug the D wire at the alternator and jump it to ground, the light should then definitely come on when you turn the key on.

Voltage on only one side of the bulb and it is not lit means that the bulb is bad.

The bulb is 9.5 ohm the same as the other idiot light bulbs in the cluster. Makes me wonder if the cluster harness is bad. Or the D+ wire. Im thinking I need to verify D+ wire for continuity.

 

[JHZR2]

to answer your question, if the last picture has the alternator as G2 then you have a three phase bridge rectifier in the diagram there. That will only allow the proper voltage (plus or minus) based on the configuration. The output on D+ will be positive voltage only as the alternator outputs it. I can't see the picture very well, but I found F23 on the bottom picture.

Okay, making more sense of the question. You are asking about field excitation below 2000 rpm correct?

What is the part labeled A1?

Have you checked the capacitors as indicated on the circuits in G2? I can't see the diodes well enough in G2 to determine direction they are facing.

A1 is the battery dummy light in the cluster. And there’s an SCR hanging off that connection too.

The sequence of events I guess is what I need to understand.

Is the D+ terminal shorted to ground so when the key is turned the battery light comes on? Then after the alternator is excited the scr is then rectifying?

Or is the D+ terminal somehow seeing battery voltage? Because if it’s seeing battery voltage, how is the alternator light coming on? Just because of minor variations in potential?

 

[mk378]

Those are transistor symbols not SCR.

The sequence of events is that a current flows through the bulb and the D wire until the alternator starts generating. This current lights up the bulb to tell you there may be a problem (or perhaps the engine is just not turning). Once the alternator makes its own voltage at the D terminal, there is little potential difference between it and battery voltage. The current through the bulb stops and it goes out, indicating normal operation.

Do my test where you disconnect the D wire and ground it. If the bulb does not come on, troubleshoot the car wiring. If the bulb does come on, the problem is in the alternator.

 

[JHZR2]

Those are transistor symbols not SCR.

The sequence of events is that a current flows through the bulb and the D wire until the alternator starts generating. This current lights up the bulb to tell you there may be a problem (or perhaps the engine is just not turning). Once the alternator makes its own voltage at the D terminal, there is little potential difference between it and battery voltage. The current through the bulb stops and it goes out, indicating normal operation.

Do my test where you disconnect the D wire and ground it. If the bulb does not come on, troubleshoot the car wiring. If the bulb does come on, the problem is in the alternator.

You’re right, transistor. But I guess that’s where my curiosity lies, what does it do? Im assuming the graphic shows it there as part of the voltage regulator… it doesn’t play a part in the excitation sequence at startup?

 

[Snagglefoot]

Now to try get rid of my headache.

 

[JHZR2]

OK, so here’s the deal I’m facing:

My alternator will excite and charge the battery after I get to 2000-2500 rpm.

I do not have a battery light under any conditions.

When I check with engine running or with key to on, I get 12v on both pads of the bulb socket. Pardon the lousy pic:

With the key on, I get 12v on one side, 8-11v on the other. The bulb is good. 9 ohms.

If I short the D+ wire to the alternator case, the alternator light does not turn on.

If I put the key to on, I do not see 12v on the D+ wire when I disconnect it. I didn’t check the D+ terminal on the alternator when operating at full voltage, but I’d assume I would see 12v there.

I think this all points to there being a short in the cluster wiring, and a broken D+ wire. That said, I’m not entirely sure that’s correct. The very top two schematics in my OP show the other items connected to what I take to be the top and bottom pads of the bulb socket.

Any recommendations on how to better evaluate this? The one thing I know is that the other two bulbs that I think are powered from the D+ side, the coolant level and washer fluid level bulbs do not illuminate either. But I don’t know if that would matter.