I've blown three alternators in as many weeks. That doesn't often happen, and it never happens to me. In each case, the problem is that the rotor is open, perhaps burned out, perhaps the victim of some mechanical fault. The first photo shows a burned out rotor. The unattached end is visible if you examine the coil carefully. The insulation remains, but the wire is gone. I recovered the missing length of wire (second photo). It doesn't give much clue as to what happened, one end is sheared where it must have been flung away after the other end came loose. But it isn't clear to me whether the other end is burned or broken. In any event, it was beaten around for a while before I pulled it out.

I could understand a burned stator or diode, it's easy to picture that happening. But while I'm not sure what killed the first alternator, two burned out rotors in a row is just weird. The only thing I could think of is a short between the B+ and D+ wires, keeping a constant field current active after the engine is stopped. But after a full day of tracing wires, I'm satisfied that there are 16 million ohms of separation between the circuits. The only explanation I'm left with is that there are a lot of mechanically weak rotors out there. But that can't be right. Anyone have a theory?

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Have tested/had your battery tested? It could be toast and overdrawing on your alternator. That's the only thing I can think of.

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

On my 1990 w124 the alternator wiring came up the inner wing to the brake servo area (right hand drive UK ) and there was a plastic box bolted down where they connected to other wires that went to fuse box etc.
The plastic lid just unhooks and peels open and then I think it was 8 or 10mm nuts holding the connectors down.
Could be something gone bad in that area letting them short out .

There's a junction box on the right fender where both B+ wires and the D+ wire are terminated. I think that's what you are describing. It's a good place to test the charging circuit. It's also a logical place to look for shorts, but no joy. I also disassembled the alternator connection, and pulled back the first six inches of the sheath. No obvious problems, and no shorts. The D+ circuit has a few interconnections. One goes to the anti-lock brakes, the other to the dash pod. Inside the pod, it seems to power some of the indicator lights. But all of that was ok.

Battery is good, but even if it was bad, it doesn't explain a bad rotor. A shorted battery would burn the diodes or stator, as would a weak battery. The rotor carries at most 14v, usually half that. It's about 5 ohms, so figure 3 amps, tops. It would have to be fully powered, without the engine turning, for hours, maybe days, before there was enough heat built up to cause damage.

The more I think about it, I'm leaning to mechanical problems. But three in a row (the last a Bosch rebuild) is like winning the lottery. Meanwhile, my original 380,000 mile alternator is back in the car, working fine...we'll see if it lasts. All this started because I thought the slip rings were too worn for comfort. Should have left well enough alone.

The field can be left on for long periods of time and not be damaged ( think leaving the ignition key on overnight ) .

The loose piece shows mechanical damage. Maybe over speed / something at the back of the alt dragging ( long bolts in the end frame )

The starter motor carries a permanent live connection as well , nothing going on there ?? Or dodgy solenoid ?.

If the starter motor stays engaged and is then driven by the engine (is this possible ? ) could this do the damage ?.

The starter solenoid has power always available, but not engaged unless the key is turned. And once again, anything that draws power would damage diodes and stator before the rotor was damaged.

The broken wire was loose in the case, no telling what happened to it. I suppose the only explanation other than physical defect is some sort of inductive effect. But that would require a loose connection somewhere, and I found no problems. Three bad rotors would say simething about the future of rebuilts, if it could be proven.

Were they all from the same re-builder? If so, I'm betting it's a common problem due to mishandling of a tool or something similar.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Just something I found in my notes.

How the Bosch Alternator; charging system works
Porsche 914 Alternator Troubleshooting | (1969-1976) | Pelican Parts DIY Maitenance Article
In the Bosch system, the size of the charge warning lamp bulb is critical. Too low a wattage bulb will not supply enough field current for "bootstrap" operation to be reliable. The Bosch book that I have states that the lamps must be at least 2 watts for 12 volt systems. If you have replaced your charge warning lamp recently, then too small a lamp may be your culprit.

I don't know what happens when the chargingl light is over a 2-3 watt bulb or someone has changed it to a LED.
I going to speculate that it is an issue with the field not being well grounded. Or one or more of the other wires is not well connected as someone else said.

One of those times when the lift time warranty pays off.

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84 300D, 82 Volvo 244Gl Diesel

One of our electrical type Members can tell you if Bosch alternators are B circuit type alternators. I believe they are.
Diagnose Starter & Alternator Problems

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84 300D, 82 Volvo 244Gl Diesel

Cross that write up off your notes. The Porsche 914 uses a different type of alternator, more like a Lucas externally regulated alternator than a "modern" internally regulated Bosch. I've never changed the charge indicator bulb. In any event, an open rotor can't be caused by the bulb.

As a point of interest, an open rotor is one of many conditions that can't be detected by the indicator light. If the light can't find ground through the rotor, it won't light. Another reason to install a voltmeter.

Internally regulated Bosch's are A circuit. But what difference does it make? An open rotor is an open rotor. This problem is diagnosed, the question is the root cause. If the only possibility is a physical flaw in rotor construction, all rebuilds are suspect.

What year and model vehicle is this alternator on as that info was not in your first post?

It is not impossible for 3 rebuilds to have the same failure but the odds are very small of that happening.

The only commonality that the 3 rebuilds shared is they were put on the same vehicle.

Please post what fixes the issue.

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84 300D, 82 Volvo 244Gl Diesel

May not be an external issue as the original alternator did not fail. Now that the original is back in service I would wait and see if it survives.

If all rebuilds got a new rotor as obviously the slip rings would be worn on every rebuildable core coming in. You can change the slip rings and machine the new part once installed. Perhaps bosh possibly has taken a different route with inferior replacement aftermarket rotors. I have the two common sizes of replacement slip rings themselves in my parts inventory.

There are mountains of poor aftermarket starter and alternator parts out there even years ago and it may be getting worse. I might try to even contact the bosh company to see if it is a known issue to them.

Of course if they lie you will have no way of knowing. Yet better than trying to find the cause if it is not there other than being inferior rebuilts.

Another thought perhaps is if the exciter circuit was getting too much voltage one would think the alternator would seem to be working audiably very hard. I might monitor the battery voltage because the regulators on those replacement alternators you got may have have issues.

On the other hand what do I know. I would ask the wife but she is out.

If it makes a difference, this is my 190DT.

I installed a digital voltmeter in my dashboard some time ago. It's come in handy a few times, this is one of them. While I don't drive with my eyes on the gauge, I check it pretty frequently. I've never seen it over 14.1v, but it could be that there is some transient condition that's too quick to catch.

After the second time, I made a point of opening the alternator and examining the rotor before installing it. It looked to me as if it was a recycled rotor to which new slip rings had been applied. In fact, if you look at the photo in my first post, you can see a length of white insulation sticking out from the slip ring area. Immediately below that, you can see the remnants of the original insulation stuck in the epoxy coating. As you can see, the new insulation wasn't attached with lacquer or epoxy, and I suspect that allowed it to break free at higher RPM's.

The rotor on the prior alternator had a break deep within the rotor winding, so that was a different problem.