First two paragraphs are just backstory, diagnoses starts on third paragraph.

Just got back from an 1,800 mile trip. About halfway through the trip, after a refuel of the biological units, the engine wouldn't turn over, just clicking. The car took to a jump ok and we found what was likely the only available correct battery in International Falls, MN on a Sunday morning. Slapped it in and went on our way. About 650 miles later after we crossed the Missouri border and the sun had set, with the A/C blasting, two cell phones charging, Garmin guiding, radio cranking, headlights burning and wipers slapping time with a sudden cloudburst, we lost the headlights. They're LED and without the correct voltage they just shut off entirely, no warning.

So with a quick bit of thinking I shut down everything electrical we didn't need and the headlights lit back up. It was obvious at that point that the alternator either wasn't charging, or at least wasn't keeping up with the electrical demands we were putting on it. I pulled off the highway, with three hours of driving left to go I stripped the car down of any electrical loads we didn't need to keep moving, aux fan disconnected, radio off, no A/C, marker light bulbs removed, one tail-light bulb removed, license plate bulbs, you name it, if we didn't need it to move forward, it was off or removed. Thankfully the rain subsided so the wipers were not necessary any longer. The car limped the rest of the way home, rolling in to it's home berth at roughly midnight with the headlights flickering. The next morning it was dead, the clock had stopped at 9:30am. I put it on the battery charger, 6amp charge, and went about reading up on charging problems here.

After my Peachparts Mercedes charging system 101 education I went about checking common issues/symptoms, here's what I found:

• Battery light (and brake wear warning light) is not illuminating on key-on.
• Electrical connections at alt appear in good shape, making good contact
• I've got battery voltage on two leads to alt
• I've got 0 voltage on blue wire lead to alt with key-off
• I've got battery voltage minus about .6v on blue wire lead to alt on key-on.
• Bypass blue wire lead at alt to neg terminal on bat results in battery light (and brake wear warning light) illumination on key-on.
• 0.01ohm resistance reading from neg terminal on bat to chassis
• 0.01ohm resistance reading from engine block to chassis
• 0.01ohm resistance reading from engine block to neg terminal on bat.
• Car has Bosch style alternator, not Motorola.
• Voltage regulator brushes are in decent shape, move freely
• Voltage regulator is clean and making good contact with alt body
• Voltage regulator was getting very hot while charging battery with external charger
• Alternator makes a rhythmic rattly banging noise during operation (only with wires connected)
• Battery voltage (somewhat still discharged) @ disconnected from vehicle was 11.8
• Battery voltage (somewhat still discharged) @ engine off, no key was 11.8
• Battery voltage (somewhat still discharged) @ idle was 12.3v (took reading after g-plug relay clicked off)
• Battery voltage (somewhat still discharged) @ high RPM was 12.7v (took reading after g-plug relay clicked off)

So I'm pretty sure I need to replace/rebuild the alt regardless. The noises it's making are bad wrong like a bearing is shot. But what I don't get is how a shot bearing is preventing good charging. Perhaps is has that bearing problem plus another? I also still don't get why the batt and brake wear warning lights aren't illuminating at key-on, and why they never illuminated when the battery was nearly depleted and the alternator not charging/keeping up with electrical demand. Those conditions make me think perhaps the charging incident is caused by a problem elsewhere somehow. Any thoughts?

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Own:
1970 Mercedes 250c, 1976 International Scout II, 1982 Mercedes 240d, 1997 Honda Accord Wagon, 1998 Acura Integra Type-R, 2009 Mazda5.
Owned:
1974 International 100 pickup, 1975 International Scout II, 1981 Mercedes 240d, 1985 Mercedes 300SD, 1988 Honda Accord, 1988 Ford Thunderbird, 1991 Honda Accord, 1996 Geo Metro, 1996 Ford F150, 1996 Dodge Grand Caravan, 1997 Honda Accord, 2003 Acura TL-S, 2003 Yamaha V-star.

If the brushes are worn enough and the bearing worn enough, it can impede contact between the brushes and armature.

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Now that's a good thought but if that were the case, would I not feel the spring tension when re-installing the voltage regulator? I had to push it and hold it in place to set the screws in, otherwise the springs from the brushes would push it down and out.

Spring tension is likely felt from the leaf spring contact point and not so much from the brushes. At any rate, it sounds like you need to go through the alternator...

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Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Fair enough. If I were to simply buy another alternator, should I buy another Bosch AL80X 55amp alt, or is it worthwhile and ok practice to buy the Bosch AL117X 65amp unit? I'm asking because I always like to upgrade if I can, and I like having a spare alternator around to tinker with and/or shove in a car in a pinch.

Make sure your glow plug relay isnt staying on. I went thru battery and alt only to find my relay wasnt releasing.

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1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."

That's all normal.
I never thought to check that when I charged the Battery.

I am curious what an Auto Parts Store has to say when You take the Alternator for the Free Test.

If the Rotor (it has the Field) is hitting the outside Steel Parts (Stator Core) of the Alternator due to a bad Bearing it would be grounded when it is not supposed to be.

I suppose it is also possible for something to happen to the Slip Ring Connections if the Bearing on that end is shot.

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84 300D, 82 Volvo 244Gl Diesel

This is good advice.

Have you checked for AC voltage across the battery terminals with the engine running? Set meter for AC voltage range higher than 60VAC to get a good reading - if necessary, work your way down in voltage scale switch.

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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)

You must find out why your battery light isn't illuminating. Have you pulled all the fuses and checked them all? If the battery light does not come on, the alternator will not charge the battery. Pull the cluster and check the filaments of the bulbs. Clean all the ground contacts too.

Its a little confusing, because you say when the car is idling the volts on the battery is 12.7v....that means its charging, but if the battery light does not illuminate....how's that possible?

Has someone been messing with your electrical system? What about these led lights? Is the stereo aftermarket? With subs and an amp?

I am wondering if your just over running your charging system. These cars aren't generators.....typically if you plan to use all your house hold gadgets and every accessories in the car....you really need to upgrade the charging system, thicker wires...beefer alternator....dual pulley alternator....maybe even a solar panel...

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To give closure, alternator replacement appears to have cured my issues. Battery and brake warning lights now illuminate at key on and shut off after engine start. Reading 14.4vdc @ idle after the GP relays click off.

Curiously, after charging the battery and parking the car for a couple of days while I awaited a new alt, the battery discharged again. Now I had the old bad alt hooked up still so perhaps it had some internal fault (short) that drained the battery. Well, that's what I'm gonna tell myself for now. With everything all repaired now and the key off and battery discon I see no voltage drop upon connecting the battery, so I don't think I have a parasitic draw anywhere.

About the LED headlights, they're plug and play 7-inch round units manufactured by Truck-Lite. They're very low draw, probably less than the brake lights, however they're not tolerant of voltage drops, so the internal circuitry will cut them out once levels drop below tolerance.

The rest of the car is stock, except LED dome lights and license plate lights. I'd do the marker lights but the LED units I've found don't seem to play well in the housings, resulting in poor illumination/visibility. Same with the brake and tail lights. The signal lights are similarly a no-no and have the added problem of screwing up signal operation, as the draw isn't enough to operate the flasher units. To correct this one must wire a resistor in line, which defeats one of the major advantages of LED, which is low power draw.

The radio is all stock.

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Own:
1970 Mercedes 250c, 1976 International Scout II, 1982 Mercedes 240d, 1997 Honda Accord Wagon, 1998 Acura Integra Type-R, 2009 Mazda5.
Owned:
1974 International 100 pickup, 1975 International Scout II, 1981 Mercedes 240d, 1985 Mercedes 300SD, 1988 Honda Accord, 1988 Ford Thunderbird, 1991 Honda Accord, 1996 Geo Metro, 1996 Ford F150, 1996 Dodge Grand Caravan, 1997 Honda Accord, 2003 Acura TL-S, 2003 Yamaha V-star.

I've heard that if the rectifiers, one or more, are shot then leakage back thru the alternator could occur. That was one of the reasons I wanted you to check for an AC voltage reading at the battery; the other is the battery charges poorly or not at all with rectifier failures in the alternator. Generally, a rectifier failure requires replacement of the alternator anyway.

Glad to hear you are up and running!

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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)

Yeah I didn't read your reply till after the new alt was installed, but I do remember checking AC voltage accidentally, it was reading 78, I accidentally had the multimeter set to VAC instead of VDC and was really confused at the reading.

Sounds like bad rectifiers. Replacing the alternator was, evidently, the correct course of action.

Good job! And happy motoring.

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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)

Well, the no-charging problem came back. I got one fifty mile drive and that was it. Went to start it and no battery warning light, no brake warning light again. Really frustrated, going to go back through the testing again, but I may be pulling this brand new alternator.


Not sure what else to do. Really frustrated now.

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Own:
1970 Mercedes 250c, 1976 International Scout II, 1982 Mercedes 240d, 1997 Honda Accord Wagon, 1998 Acura Integra Type-R, 2009 Mazda5.
Owned:
1974 International 100 pickup, 1975 International Scout II, 1981 Mercedes 240d, 1985 Mercedes 300SD, 1988 Honda Accord, 1988 Ford Thunderbird, 1991 Honda Accord, 1996 Geo Metro, 1996 Ford F150, 1996 Dodge Grand Caravan, 1997 Honda Accord, 2003 Acura TL-S, 2003 Yamaha V-star.

No battery warning light and no brake warning light sounds like a possible problem with the electrical section of the ignition switch.

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Sam

84 300SD 350K+ miles ( Blue Belle )