I've noticed a few of us seem to have the issue whereby our alternator's fail to generate until we blip the throttle.

I always assumed this was normal. It is not.
In most threads people are steered toward new alternators or regulators but I do not believe that is the issue.

The circuit seems to work as follows: three wires going into the alternator, two red 12v, one blue exciter wire. The exciter wire is part of a circuit between 12v and the idiot lights in the cluster. When the idiot lights are on, exciter wire serves as a ground, illuminating the cluster lights. When the alternator is generating, the exciter wire becomes 12v (positive) turning the lights off.

In theory, if the lights are coming on, 12v should be making its way to the alternator and exciting it, engine turns over, alternator makes power, fin.

In practice, this doesn't seem to be happening. Instead, by blipping the throttle, we are manually "bootstrapping" our alternators into generating.

I removed the alternator harness today and checked it with my multimeter.
I got battery voltage at the two 12v terminals, but only 11.7 volts at the exciter wire. I don't know enough about this stuff to know whether that's sufficient to excite the alternator though it would appear not. Current thinking is additional resistance is dropping that voltage.

Who has encountered this?



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1981 240D 4sp manual. Ivory White.

The only times the Alternator Light should becoming on is whey you first turn on the Key and and when that happens all of you "idiot lights" should come on and that is to check and see of the Light Bulbs are working.

Not sure on your description but the Alternator/Charging Light provides a resistance to start that alternator charging. Depending on what info you read you need a 2 or 3 watt bulb to proved sufficient resistance.

But at the same time you also have to have the alternator hosing well grounded and the contact area between the Voltage Regulator and the Alternator housing has to be good and there is also a metal spring/finger inside of the Alternator that has to have good contact with part of the Voltage Regulator.

Also the Alternator Electrical Connector can be taken apart and the connectors inspected and cleaned. It is not as easy to clean the terminals where the connector contacts on the alternator but they should be clean enough to conduct electricity well.

Then there is slipping alternator belts and poor cassias to engine ground and poor batter to chassis ground.

If all of the above is just a little bit off it conspires to cause a larger issue.

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84 300D, 82 Volvo 244Gl Diesel

Yup, I got all that hence the post.
A few of us have to blip our throttles to get our alternators going.
If it was a grounding issue, the alternator wouldn't work at all, or only marginally.
Once it's been "bootstrapped" everything is normal. It's simply not turning on via key.

I don't know how much voltage needs to be coming through the blue wire, so I don't really have a baseline. Seems to me like it should be at least 12v.

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1981 240D 4sp manual. Ivory White.

My Alternator was charging OK but needed a bearing. What I found inside was my Alternator Brushes were worn and one side of the slip ring was worn and extremely rough.
After fixing those I got a sightly higher charging rate of about 14.3 volts.

Don't remember where I got the info but this is from my notes: Explanation of how No Charging Light works.
“The field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which

goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped” found at: (this may be the site the info came from) http://www.yourautonetwork.com/alternators.html

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84 300D, 82 Volvo 244Gl Diesel

Explanation of how No Charging Light works.
“The field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine is running and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which

goes out. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped” found at:
http://www.yourautonetwork.com/alternators.html

On the older AC Delco Alternators where there was a Volt Meter the Alternators have a Resistor added to take the place of the wattage of the warning light bulb.

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84 300D, 82 Volvo 244Gl Diesel

When you turn the key and the Charging/Alternator light goes on it is less then the battery voltage.
As you noted the 2 red wires eventually end up being connected to the positive terminal. So you disconnect the Alternator Connector and use one of the red wires to see what the battery voltage is at the connector. Then you turn the key on and check the voltage at the smaller slot which is where the blue wire goes to and you will get less then the Battery Voltage (because the resistance of the light bulb reduces the voltage just a bit).

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84 300D, 82 Volvo 244Gl Diesel

I agree.

It should be much closer to battery voltage as that little bulb could not be responsible for more more than a .7 volt difference.

Something in the circuit is creating excess resistance.
I suspect that is the case for the rest of us having this issue.

Possible the bulbs are old? Not sure, but don't think it's normal.


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1981 240D 4sp manual. Ivory White.

There is also this test to see if there is a diode issue: AC voltage across battery
VAC across battery should be under 100 mV; excess AC voltage on the DC line means one or more of the rectifier diodes in the bridge are leaking or shorted. Such a problem is relatively rare but unfortunately means replacing the entire alternator.

I don't know if all Pep Boys have the same equipment but I had an alternator tested at a Pep Boys and they gave me a small print out saying what was wrong.

I guess I was lucky with mine. When I got the car it had a charging rate of about 13.7 volts. Over time cleaning contact areas and changing my Batter Ground cable it it increased to 14.1 volts. Now that I changed the regulator bushes and the slip ring it is 14.3 volts. But I also cleaned the contact area between the regulator and the alternator casing and the alternator connector connections.

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84 300D, 82 Volvo 244Gl Diesel

Yeah something is fishy.

It was like hitting my throttle made something charge up and just turn on and the lights went out. Battery voltage was consistent with the lights. Before I blip, 12v, after blip 13.5v and instantly lights go out.

It all went away with the new 120A alternator I put in yesterday.

Maybe it has to do with the diodes? Do they need a little extra current to start rectifying? Or maybe the magnetic field has to set up? I too don’t know what is exactly happening.

All I know is my SD did not do this for ten years and then it started a week ago. Now with a new alternator it’s good again. Voltages are higher too.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

I wonder if the design of your alternator is different.

I’ve read of enough people throwing new alternators and regulators at the problem to no avail. I’ve tried two different alternators and three different regulators.
I get between 13.8-14.2 when running. It’s a healthy alternator.

This is a pretty common problem across the Internet. Some have solved by jumping the exciter wire with an additional lightbulb, or running a new exciter wire to the dash.

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1981 240D 4sp manual. Ivory White.

My Alternator has a sticker on it that said rebuilt by Bosch North America on it.

Alternator part number from sticker- 005 154 7492 80

Actual Voltage Regulator on my Alternator Bosch 1 197 311 003 EL14V4B Made in Germany

There is also pulley size and and the engine idles speed that effect the low rpm charging rate.

The only different alternators I have seen over the years is 3 members turned out to have Motorola Alternators. The Motorola Alternator has a different sized Alternator Connector and the Regulator is on the Alternator but different.

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84 300D, 82 Volvo 244Gl Diesel

I’ve also a bosch (standard part number), but I was referring to the single terminal alternators. Internal, self-exciting.

Honestly though, there are so many variables that could determine why Ykobayashis new high capacity alt solved the issue.

Hopefully a few of the electrical engineers show up! In any event, I’ll continue to troubleshoot and update this thread with what I learn.

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1981 240D 4sp manual. Ivory White.

My '79 w123 does this.

100% of the time.

Once the RPM's are raised above idle for an instant.

Full charging mode and the dash indicator light goes out.

When I made the Alternator Test Cord I used a 3 watt Bulb. Has anyone ever pulled the charging light bulb out and gone an bought a 3 watt bulb to replace it and see if it changes things?

In the attached picture of the Alternator you see the yellow arrow pointing at a contact spring inside of the alternator that makes contact with the voltage regulator. The one pictures is corroded. The corrosion/oxidation does not conduct electricity well (not my Alternator).

The last alternator picture is mine and the yellow arrow points to the inside contact spring and the red arrow points to the other area on the alternator that the voltage regulator makes contact with which I sanded down to the bare metal.

I have had my car since 2007 so it is hard to remember the details of what I did over time. But, for sure most of the time I had the Car it did not charge 14 or above volts; I remember 13.7 something volts.

Over the years I had cleaned the Ground Strap and Battery to Chassis Ground and the Battery Terminals more then a few times got no difference in Charging Voltage from that. But they were never badly corroded to begin with.

When the Battery finally gave up I did not by one that was supposed to be for the Car but one I bought locally that had the terminals on a different sides. That caused me to replace the Battery to ground Cable. After that I got 14.1 charging volts.

When I rebuilt my starter I removed the the original positive Cable and I cut through a slot through the thermal clamp and spread it a little so I got the cable out of the socket it was in and I cleaned off any corrosion and inserted it back in and got a propane torch and solder and soldered it in. I don't recall any voltage change after that.

After replacing the Brushes and Slip Ring on my Alternator and cleaning all of the contact areas I got 14.34 charging volts.

Attached Thumbnails

         

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84 300D, 82 Volvo 244Gl Diesel

Exactly. If you get a moment, I’d love to know what your voltage is at the blue wire in your alternator harness. Measure when key is in position 2.

Are you referring to getting a good charge from your alternator, or the issue I’ve described above?

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1981 240D 4sp manual. Ivory White.