For the last week or so I've had my alt. light glow dimly after start-up until the engine has run for at least 10 minutes.
checked voltages at battery and the alt. is diffidently charging. idle v. is 13.2
high rpm v. is 13.75 and this is while alt. light is glowing dimly.
After running for 20 miles to work the light will sometimes glow at idle before shutting down.

Check your belts, clean the connector on the alternator and get a voltage regulator.......its telling you the brushes are getting short on the voltage regulator....or your belt may be loose....

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I'd like to have you check this:

Put your voltmeter on AC Volts with the leads on battery terminals and see what you get with the engine running.

As cooljjay said, it very well could be the alternator connector issue being dirty, loose or defective - especially if issue is intermittent. Also, make sure battery terminals are clean and tight.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

X2 On the brushes !

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

I had previously cleaned the connector, checked belt tension, and made sure the brushes were making good contact with the commutator.
It just seems strange that with batt. voltage so high that the alternator light would be glowing.
What exactly makes the light come on?

Bad voltage regulator.....

The brushes wear down, they aren't made of titanium or some magical elf powder like people think the starters are....here's what they look like new....vers old....

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My understanding of what makes the light come on is derived from an article on Pelican Parts (before the acquisition) regarding how alternators function. And a very informative article it was too - kudos to Pelican Parts.

When the engine is not running, the light comes on because voltage is across the bulb and current is flowing thru it and thus thru the alternator energizing circuit.

When the engine starts and the rotor begins to spin, the alternator, if in good electrical condition, begins to produce voltage and current. This voltage counteracts the voltage across the bulb and equalizes allowing the light to go out.

When the alternator and/or it's voltage regulator are defective, this equalization of voltage degrades allowing the light to come on while engine is running.

Did you measure the AC voltage across the battery with the engine running?

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)