'83 300D. Battery light glows more brightly as engine is revved. Very little increase in charging voltage. Changed regulator, no difference. What is making the light glow more brightly if the alternator is bad? How to diagnose before the PITA job of changing the alternator? Thanks!

That could be a lot of things but sounds like a diode issue in the alternator itself (not part of the regulator) but I’m no EE.

What is the voltage output at idle? and under load?

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1981 240D 4sp manual. Ivory White.

At a fast rev, the voltage goes up to about 13

With the headlights on, and fully charged battery, the voltage rises only a few tenths above the battery voltage when the engine is revved. When i changed the voltage regulator, the new one had longer brushes (less wear) and there was a slight groove on the armature where the brushes ride. Belts are tight.

The odd thing is how the battery light, which glows dimly at idle, gets quite bright when the engine is revved.

Odd, yes, but not mysterious.

Your alternator is not making nearly enough current. You should be in the ballpark of 13.8v+

If you’re confident your replacement brushes are of good quality and your grounds are all clean of corrosion, then it sounds like your alternator needs replacing due to an internal fault (could be diode, could be in the windings, not my area…).

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1981 240D 4sp manual. Ivory White.

If the dash light goes out when the alternator is unplugged something is wrong inside the alternator. Start the car, confirm the light comes on, turn the car off, unplug the alternator and restart the car. Light on indicates a wiring problem, light out indicates an alternator problem.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

This!

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84 300D, 82 Volvo 244Gl Diesel

More basic stuff. Check you drive belt tension.

Remove the cables from your battery and clean the terminals and the inside of the cables. Take a peek under the insulation of the batter cables if you can and see if they are corroded away.

Remove the negative battery cable from the chassis and clean both down to the bare metal and re-install.

There is a chassis to transmission bell housing ground cable about where your feet would be while driving but under the car. Remove that and clean the contacts down to the bare metal.

See if after that the voltage has not gone up a bit.

Does your battery have enough liquid in it is one that needs that?

While the engine is not running disconnect the electrical connector at the alternator. What you are looking for is corrosion inside of the connector. The plastic part of the connector can be taken apart and you can see if where the wires are crimped if it is badly corroded.
If they are corroded it is possible with care to bridge over the crimp with solder restoring a good connection.

While that remove the 2 screws holding the voltage regulator on. Inspect the length of the brushes and get a strong flashlight and look inside to see of the slip rings are badly worn out.

The screw through area on the passenger side of the alternator (right side) is where the voltage regulator makes contact with the alternator aluminum frame. That one needs to be down to the bare metal.

It is common for where the alternator mounts to be rusted. Rust does not make for a good ground. Connect a single jumper cable to the Negative (-) terminal on the batter and find some bare metal on the aluminum alternator frame/casing and apply the batter cable to that. If you voltage goes up your alternator is poorly grounded where it bolts up to the engine. Some have run separate wire from the negative battery terminal to the aluminum alternator frame/hosing.

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84 300D, 82 Volvo 244Gl Diesel

In post 14 there is some pictures. There is another contact area inside of the alternator that can get corroded.
The 2nd picture shows a corded one and the 3rd shows a nice clean one.

Alternator Excitation Circuit

There is a test to determine believe it or not the AC voltage of your alternator. It tells you if one of the diodes is having issues. I cannot describe it so someone else will have to do that.

If your battery is not fully charged the alternator will not charge properly. So, test the alternator voltage maybe 30 minutes after the batter has been fully charged.

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84 300D, 82 Volvo 244Gl Diesel

Your alternator has nine diodes. Six of them rectify current to feed the electrical system and charge the battery, the other three produce 12V+ to feed the regulator only. This group of three is called the "trio" When you start the car, the regulator receives power from the battery, fed through the dash lamp. Once the alternator is running, it gets current through the trio.




What's happened here is that the trio is either disconnected, or has a bad diode. The alternator still works, because the regulator is receiving battery current through the bulb. The faster the engine turns, the higher the voltage and the brighter the bulb glows.

Sooooo. The connection between the trio and the regulator is via a terminal inside the alternator. When you install the regulator, it makes contact with this terminal. It's possible that the terminal is corroded, and can be salvaged by cleaning. Or the terminal or diode may be irreparable. Your best bet might be to replace the entire alternator.

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Thanks. A few things to check.

I tried the test Sugar Bear recommended, sure enough, the light was on with the connector attached to the alternator, but with the plug pulled, no lite.

I got another alternator from a junk 79 300CD that i will bring to Autozone for a bench test.

I am hoping i can take the front off one of the dead alternators, and put it on the working one with the ovalized holes, Has anyone tried this?