I'm working on a vw with a potentially failing alternator. (Well, the issue is its a diesel engine swap into a vehicle with a rough ripped up engine harness, and I'm trying to figure out if I have a failing alternator, or wiring issues/confusion.

Anyway, my basic question is on the old mb diesels, with the alternator unplugged and the engine running, can I probe the back prongs with a voltmeter and see 14ish volts with no other wiring attached?

It should be putting out voltage just grounded at the bracket and spinning fast enough, right?

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This post brought to you by Carl's Jr.

No. This is alternator, not dynamo. If I am not mistaken, you need an excitation voltage to the alternator to generate electricity.

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ah-kay is correct. An alternator won't generate (remember "generators"?) current on it's own. Instead of big magnets, it has the big windings, once they are energized it creates the field that generates the current. And they need the wiring to energize them.

The easiest way to troubleshoot the alternator is connecting a VOM across the battery.

With the VOM in DC Volts:
Start the car and watch the voltage. It should go to ~ 14 Volts DC and then slowly decrease as the battery charges. This verifies the alternator and regulator are doing their jobs.

With the VOM in AC Volts:
With the engine running the meter should read less than 500mV AC. This verifies the internal diodes are rectifying the AC to DC voltage.

Low DC charging voltages usually point to the regulator.
High DC charging voltages usually point to the diodes and the AC voltage test will usually verify this.

(Note: It has been found that a burned out "charging" indicator bulb will also cause a no-charge situation.)

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Sam

84 300SD 350K+ miles ( Blue Belle )

Actually if you are trying to isolate the alternator, the easiest thing to do is to remove it and take it to an auto-parts store. They have a machine to test it and test the regulator at the same time. If works well, then you know you have to deal with the harness.

Good luck.

X2

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84 300D, 82 Volvo 244Gl Diesel

Thanks guys!

This points me toward wiring mistakes rather than a failed alternator. I am planning to pull it off anyway and have it tested, but the problem really is there are 5 posts, and 4 wires. One wire is missing.

Just from process of elimination, I've properly plugged in 3 of the 5, but I think I am missing that all important energy coming in wire.

The alternator has two wires that go direct to the battery via the starter solenoid hot post, and what I think are a tach signal wire, and an idiot light sinal wire.

There is one spade connection with a little + sign on it seperate from the others, maybe this is power in.

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This post brought to you by Carl's Jr.

Very possible. The field current (the current that goes through the rotor wiring creating the magnetic field needed for power generation) isn't very much. I've seen it supplied through a small spade connection on more than one application.

We are talking about an internally regulated alternator, right?

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88 Jaguar XJS V12 94,xxx miles. Work in progress.

Yes in theory you are both correct!! (~90% of the time)

In practice, some times there is a small amount of residual magnetism in the rotor. This can provide the necessary magnetic field to get the alternator going. It doesnt always happen, but when it does, it causes confusion as the alt charges without the idiot light working. If you are seeing charging probs, like others have said, testing is the only way to go .

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1980 300D now parts car 800k miles
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1987 250td 160k miles English import
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1968 Ford F750 truck. 6-354 diesel conversion.
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Keep in mind that the older VW diesel alternators do not turn the excitation signal on until a certain RPM (above idle) is achieved. So you can start the car and let it idle for days and the alternator will not put out current. On mine the "Charge" light stays on until I blip the throttle (maybe 1,200 RPM??)

I understand that they did this to lower the load on the starter but that's just speculation I think.

Eric

Got it all figured out. Turned out to be a bad stator on the alternator. The wiring I was playing with was just the tach signal, and not the B+ input. That was coming from one of three wires that were correctly hooked up.

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This post brought to you by Carl's Jr.

keep in mind also, that having the alternator tested can show you what wires to connect to get the alternator working.
pay attention to the wires they hook up for testing, and you can figure out what is needed to connect your particular alternator.

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1987 300TD
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Sadly, this is not always possible to do, because the newer testers have pre-made cables. I watched mine be tested by attaching " Cable Harness B2 " .

It appears that the alternator and regulator on my 240D are ok, but it does not seem that the alternator is getting excited, so the charging voltage is never above 12.20 volts.

Now we just have to find which instrument panel bulb or circuit is causing the problem...

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1983 380SL
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3 x Citroen 2CV, down from 6

Added: The below applys to Alternators with 3 female connectors where the smallest slotted Connector goes to the Charging Light and the other 2 larger slots go to fat Red Wires.
The Blue Wire that goes on the Alternator Connector is the Wire that goes to the Charging Light Bulb.
With the Key off you should get no Voltage there; with the Key on you should get slightly less than the Battery Voltage.
On source I read said you need a 3 Watt charging Light Bulb in order to excite the circuit.
After that the Alternator needs to be well grounded.

The 2 larger Slots on the Alternator Connector that have fat Red Wires should always have Battery Voltage.

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84 300D, 82 Volvo 244Gl Diesel

On my 92 VW diesel, the charge light in the cluster is an LED, not a bulb. I am not sure how the excitation is provided to the alternator, maybe it is a power resistor. You don't really need to know all that to troubleshoot the VW charge system. As long as you get a little under 12 V at the excitation wire at the alternator when the ign key is in the on position, you are most likely getting excitation. There should be 3 wires at a VW diesel alternator, the big fat wire goes to battery + terminal (on some cars via the starter motor terminal), a small W terminal wire to drive the diesel tach, and another small wire for the excitation. Ground is via the alternator mount.

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