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?