Is this alternator dead?

[Jetx]

I have a 2002 Volvo D2-55 with stock Valeo 60A alternator, charging house and start banks through a diode isolator (old tech, I know, but not ready to replace just yet). Alternator recently stopped charging.. confirmed with clam ammeter which shows 0 amps on alternator output cable.

Voltage on the field excite wire coming from the engine panel -- when disconnected from the alternator -- is 12V, as it should be. But when I reconnect it to the D+ terminal on the alternator (before starting the engine), the voltage on the D+ terminal is just ~3v. It stays at 3v after the engine is started.

Does this indicate a dead (internal) regulator, dead alternator, or something else?

[Wotname]

First, disconnect the field excite wire from the D+ terminal and connect this wire to a small load (light a small 12V lamp) and measure the volts on the wire again. It should remain at 12V but if it drops significantly then you have a problem that is external to the alternator.

Or run a temporary wire directly from another 12V source (say the battery +ve terminal) to the D+ terminal and measure the volts (engine OFF), if it drops to ~3V then the internal regulator is bad.

[smac999]

It could be something wrong with the alt pulling too much current. And droping the voltage. Or something wrong with the wiring. You should Clamp the d+ wires and see what it’s drawing.

Likly it’s feed through a dash light so dropping voltage is normal if too much current. If so it also won’t feed another load. As mentioned above.

If you do connect another 12v source. Make sure it’s fused. The d+ can ground that wire. Ideally it should be fed through a resistor. (Ie the light bulb). So it can safely ground that wire

[Jetx]

Thanks for the suggestions. I think the alt (or regulator?) may be pulling too much excitation current. Found the Volvo wiring diagram, and it indicates that the excitation lead is fed through a 33ohm resistor. So with a 12v source, it could only supply very small fractional amps to the alt excitation circuit. I also took some additional measurements as suggested:

• D+ current, switch on but engine off: 0.3 amps, 2.8v on the stud
• D+ current, engine running: 0.3 amps, 3.6 v. (Voltage increase is due to a second alternator that has begun charging. The voltages seem consistent with 33 ohm radiator at the currents measured.)

I then tried feeding the D+ direct (but fused) from battery, resulting in:

• D+ current, switch on but engine off: 2.6 amps, 12.5v on the stud. I think this is more current than it should be drawing?
• D+ current, engine running: 2.5 amps, 13.5v (current in this case i believe is flowing from D+ to the battery).
• Revved engine up to 1800 rpm, at which point alternator began charging, but only at 10 amps.

Finally, since I have two identical alternators on this engine, I tried swapping the excitation leads. In this case, my “good” alternator still makes 40-50 amps as usual (so no problem with the excitation lead coming from the engine panel), and my “bad” alternator still does not charge at all. So either the alternator or regulator is toast. At this point I think I’ll just replace with a new Prestolite unit and be done with it.

Thanks again for the helpful suggestions.

[Wotname]

Thanks for the update Jetx.