Lately my charge indicator light has been partially illuminating when both the engine rpm is fairly high (20+ mph in 1st, 70+ mph in 4th) and the voltage is low (~13.25 V).

That the charge light comes on when the voltage is low is not a surprise, of course. I'm mainly curious as to how the voltage gets that low.

Checked the voltage regulator; brushes seem fine, although that doesn't rule out some internal problem there. The alternator works (the charge light is registering a voltage difference between the low battery and the alternator voltage), but not well enough to keep voltage up with the headlights on (see below).

Voltages measured through cigarette lighter during a 20-mile drive:

Car off, 13.5
Idling voltage, 14.25
At 70 mph with nothing electrical on, 14.5
At 70 mph after having headlights on for some time, 13.25 (charge light illuminated here)

Being unfamiliar with charging system performance in these cars, I'm curious whether any of this is substandard. I've read that the system should provide 14.4 V or so while the engine is running...but how much of a charge dip is within spec with headlights, radio, &tc. on? And what does the problem here sound like?

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'Betsy the Getaway Car' : 1978 MB 240D, 582k
'The Mistress of the Sea' : 2001 Giant Boulder, 10.30k

With what did you measure? My 85 SD alt is rated at 14 volts & I forgot how many amps. Output near 14 is what you want. Loads such as glow plug circuit will pull measured voltage output down. In-dash guages are frequently not accurate. Are you having problems with discharged battery? Battery could be bad or battery or alternator connections could be corroded or loose. You want to look for voltage drop at connections to test them. (ie less than battery voltage) Battery can be tested with load tester, hydrometer or chain store equipment. Alternator components can also be tested at the chain store without removing alt. Take advice from $5/hr store cleark with grain of salt. Confirm results before you buy expensive parts. Sometimes they mess up the test.

how are you doing 20MPH in 1st gear? also, 70 in 4th would not be very high RPM's in either of my MB drivers. what car is this in? also, at high RPM's you could be slipping the belt, perhaps a new alt belt would fix it?

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Junkman: For the measurements in my original post I used a multimeter connected to the cigarette lighter. I conducted similar tests a couple of nights ago with the multimeter connected directly to the battery; I didn't make notes of particular numbers then, but I recall seeing similar behavior to what I observed this time around. Any pointers on how to check the alternator output with a multimeter?

vstech: Starting from a dead stop with the accelerator full down, the car accelerates to around 20 mph before going into second. It is a 1978 240D with an automatic transmission, the car in my signature (transmission shifting is probably sagging by now, but, if so, that's a problem for another day). When I replaced the alternator belt a month ago I had high hopes this problem would end, but not so; the sound of the belt slipping was very noticeable then, but I cannot hear anything of that sort now.

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'Betsy the Getaway Car' : 1978 MB 240D, 582k
'The Mistress of the Sea' : 2001 Giant Boulder, 10.30k