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Charlescrown wrote:From my experience little goes wrong with the rotor. I have seen one go short to ground easily picked up by measuring resistance from a slip ring to the rotor shaft. Usually around 3-4 ohms resistance between slip rings for a lot of alternators. Even if the regulator is faulty (which is my guess) the rotors resistance will controll the current and prevent damage occuring. As prevoiusly mentioned a look at the colour and a smell should tell you if there is a problem. I did have a regulator that only went short to ground after the engine was turned off and that was obvious by a hot regulator when it should have been cool.

Yeah, the rotor looks OK, yet its 2.0 ohm resistance across the slip-rings is considerably lower than the 3-4 ohms that you have experienced, and that is what is causing me concern. Unless I can find the specifications for this particular alternator, and they correspond to about 2.0 ohms for the rotor field coil resistance, then I'm afraid that the RE72 regulator that I've got on order isn't going to fix this thing for long.

Joe.

I can only go on older cars. The later ECU controlled alternators are probably different. Is your multimeter aaccurate? It's getting harder to find specs for newer cars because of the throw away mentality. I do know that in many cases it's better to repair than to replace with the cheap crap.

Charlescrown wrote:I can only go on older cars. The later ECU controlled alternators are probably different. Is your multimeter aaccurate? It's getting harder to find specs for newer cars because of the throw away mentality. I do know that in many cases it's better to repair than to replace with the cheap crap.

No, this isn't one of those newfangled ECU-controlled alternators, it's a traditional Aussie-made Bosch from ca. 2000 (I once had a document for decoding Bosch date codes, but can't find it now, when it would be useful to use it).

I absolutely agree about avoiding cheap clone crap. Some years ago (unrelated to the present issue), I bought an RE72 clone regulator for the alternator in my VS (different model alternator, 85A rather than 100A). Here's a key paragraph I sent to the vendor, requesting a refund : "I fitted this regulator to my car on Saturday and have been driving around to get my battery charged up. However, on doing some checks on Sunday, I found that all is not as it should be. This regulator (which looks quite different to the item picture) has extremely poor regulation! At idle, it gives 17V at the battery terminals, while at 2000+ rpm engine speed, only about 13V5. This will adversely affect battery life!". This clone regulator worked perfectly on the bench, but had some strange stability issue when fitted in the car.

Joe.

PS. I too had been wondering about my multimeter's accuracy. As a "sanity check", I found a 5.1 ohm 5% carbon-composition resistor in my junk box and this measured 5.6 ohms on this multimeter. As carbon-composition resistors absorb moisture over time when not in use, their resistance goes up (yeah, I know, you'd think it would go down) a few percent, so this reading was about as expected. Which means that my multimeter is indeed "sane".

When I started this thread, I had a faulty alternator that had flattened my battery quick smart, despite the car being off (except for an initial 10 minutes or less during a short drive home). Finding that the regulator had a shorted transistor that drives the rotor field coil, and a resistance of only 2.0 ohms across the rotor slip-rings, I feared that the faulty regulator had fried the rotor.

However, the general opinion here was that if the rotor field coil were fried, it would look and smell like it (whereas this one doesn't). In addition, I had made a mistake in calculating the power dissipated in the rotor field coil under the fault condition, so instead of 72W, it would be significantly less, about 32W. So based on all this, it would seem that the rotor is actually not fried.

So then there is the question of why the rotor field coil resistance seems lower than I would expect, at just 2.0 ohms? Maybe a short-circuit had developed between windings of the rotor field coil, and this in turn had led to the failure of the regulator transistor? (So cause and effect would kinda be the reverse of the original hypothesis.)

However, Gareth has provided information that means that a rotor field coil resistance of 2.0 ohms is indeed possible for this alternator, albeit at the lowest end of the range. So crossing my fingers, when my replacement RE72 regulator eventually arrives (bad time of year for ordering and receiving parts, I guess), hopefully it will not only fix the problem, but not suffer the same failure as the original. I'll let you know how things turn out.

Joe.

I really doubt there is a problem with the rotor. I used to always carry spare regulators due to the failure rate. They usually stop charging and the red light starts coming on but yours is another problen with it continuing to provide current to the rotor with the ign off. I had that once on my own car and it did turn out to be the regulator.

Charlescrown wrote:I really doubt there is a problem with the rotor. I used to always carry spare regulators due to the failure rate. They usually stop charging and the red light starts coming on but yours is another problen with it continuing to provide current to the rotor with the ign off. I had that once on my own car and it did turn out to be the regulator.

Thanks for that info., particularly that you've experienced this same fault. FWIW, I too have experienced a number of regulator failures in cars of my own and of friends and family, and prior to this, they were all of the benign "stop charging" variety.

Anyway, the replacement regulator arrived today, and this afternoon it went in the car and all seems well for now. Fingers crossed it continues that way.

Joe.

Well, it's about 6 months on now, and the new regulator seems to be content with its life, no repeat failure. So it's time to call it. The original regulator must have failed (shorted output transistor) of its own accord, and not due to the surprisingly low field coil resistance (which must therefore be within normal limits for this alternator).

Thanks to all who contributed their knowledge and experience here, it saved me having to buy a replacement alternator!

Joe.