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Old 04-25-2015, 04:15 PM   #1
BurnedOut OP
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1992 R100GS alternator rotor test

My alternator light stays on, so I did the "alternator and regulator quick test" described in Haynes where I unplugged the voltage regulator and connected the D+ and DF terminals on the plug with a piece of wire. Then I started the engine and revved it, and the alternator light didn't go out. According to Haynes, that means my alternator's f'd up.

Now, I've been told that to check if my alternator rotor's good, to slide a piece of paper between the brushes and the copper rings, and then measure the resistance between rings. Then I was told that if the rotor's good, the reading should be 3.5 ohms. Clymer says the reading should be between 3.4 and 3.74.

Mine is 4.0.

I was also told to measure the resistance between the tips of the leads first, and then subtract that from the above reading. When I crossed the leads, I got 0 resistance, so the reading's still 4.0.

Is my rotor toast? Or OK and something else is wrong?

Please help. I'm still very new at this stuff.

Thanks.

BurnedOut screwed with this post 04-25-2015 at 04:44 PM
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Old 04-25-2015, 06:19 PM   #2
bmwrench
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A reading in that range should be fine. If the GEN light is bright with the ignition switched on and motor stopped that generally rules out a bad rotor. Since you've done the "quick test", you've also probably ruled out the regulator. This leaves the diode board, stator, and wiring. Of the three, the diode board is the mostly to fail. You can find tests for it in numerous places, including, I suppose, The Haynes manual. Or you could just swap in a replacement.
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Old 04-25-2015, 08:43 PM   #3
Big Bamboo
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Here's Jeorgs excellent article on testing the diode board: http://jhau.maliwi.de/mot/r-elec.html#diodes
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Old 04-26-2015, 05:20 AM   #4
photomd
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Your reading maybe OK, maybe not. Sometimes they open up with heat. Mine tested fine cold: 3.5 ohms. I tested it 3 or more times, then got the idea to heat it with a heat gun. When I did that, the resistence would open up. I'd try heating it and see if that reveals your problem.
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Old 04-26-2015, 11:37 AM   #5
BurnedOut OP
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Quote:
Originally Posted by photomd View Post
Your reading maybe OK, maybe not. Sometimes they open up with heat. Mine tested fine cold: 3.5 ohms. I tested it 3 or more times, then got the idea to heat it with a heat gun. When I did that, the resistence would open up. I'd try heating it and see if that reveals your problem.
Please go easy. I'm still new.

I initially tested the alternator rotor when it was hot, and that's when I got the 4.0 reading. I just measured it cold, and it's 3.5, so everything's fine there.

That said, I believe I located the nature of the problem: one of the three soldered wires from the stator to the housing connector is broken. So now I will try to fix that.

Two questions (remember, I'm new):

Is the best way to remove old solder to hold a soldering iron to it and melt it away? And then use the iron to re-solder the wire?

Also, will an alternator (housing, stator, rotor, brushes, the whole deal) from a 1990 R100GS fit a 1992 R100GS?

Thanks.
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Old 04-26-2015, 03:15 PM   #6
Warin
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The 'old solder' can be reused! A little heat to melt it and some fresh flux should give you a good joint. If you must .. heat to melt it and then let it flow on to the soldering iron and remove the soldering iron .. then tap the soldering iron onto, say, a wooden block.

Components are compatible over many years, certainty 1990 to 1992 .. I think from 1982 to 1996 ... but would need to check.

--------------------
If re-soldering does not fix the problem then suspicion should now be on the diode board.

The diodes on the 'board' are in two sizes .. large and small. You test each one.. oh .. disconnect the board before testing...

To test a diode you need to test it in one direction (red lead to one end , black lead to the other) then revers the direction (put the red lead on the other end and the black lead back to where the red lead was). The two readings .. one should be 'low' the other 'high' the actual values don't matter .. it is the difference between the two that you want.. if they are both close together then it is faulty. How different should they be? Say at least a factor of 10 .. that is the 'high' reading should be 10 times the 'low' reading.
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