89 240DL Not charging

My daughter pointed out that the battery charging light was glowing very
dimly with the engine idling. The battery voltage measured around 11.7V with
the key in ignition position ll and with the engine running the battery
voltage dropped to 11.4V. Furthermore in with the key in position ll only
the oil light comes on and it goes out with the engine running. The car
starts and drivers normally except that it revs up a bit before it settles
down.

 

The lack of the battery light coming on during start like the oil light does
makes me suspect the alternator brushes are worn out. The brushes are part
of the regulator, which is separately and easily replaceable. Last time I
bought one it was $35 US, probably closer to $50 US by now.

Mike

 

In addition, check the wiring connections on the alternator as well as
the battery.

 

Good call... I've run into both the brushes issue and the braided
ground line to the engine block being charging issues.

Bill

 

I re terminated the alternator output connections but I didn't check the
brushes nor the grounding. I am somewhat puzzled that the alternator failure
would cause the warning lights not to come on, except the oil light which
comes on in position ll and goes out as it should when the engine is
running.

 

That's what makes me suspect the brushes. It is a problem that I believe
only exists in cars built in and around the '80s. The circuit seemed so
serendipitous: the alternator lamp is essentially in series with the
regulator's ignition input. When the ignition is on but the alternator is
not charging, current flows through the lamp, the regulator and the brushes
to energize the rotor and light the warning lamp. When the alternator is
working, rectified output from the stator feeds the regulator, putting 12
volts on both sides of the warning lamp and extinguishing the lamp. Great,
huh? But if the brushes wear out there is no path for current to flow
through the lamp so it never warns you. The only hint (until the battery
goes dead) is that the battery warning lamp doesn't light in position II.

Don't feel bad - it happened to our '85 765T and an '84 Nissan I had. I
noticed the drop in the voltmeter in each, but wouldn't have noticed the
warning light out in time. More modern alternators have regulators have a
separate terminal for the warning light.

Mike

 

The design of the system requires current to be able to pass through the
brushes (and the rotor) for the light to come on. So only stator, drive
belt, and some regulator failures will turn the light on. All other
failures, you're on your own, although the clue is the light not coming
on before the engine is running.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

Is the charge failure light burned out? That could cause these symptoms.

 

The bulb is ok. It was glowing very dimly. I turned out to be the
alternator itself. Replacing brushes didn't cure it as the commutator was
too worn out.




Regards,

Boris Mohar

Got Knock? - see:
Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

void _-void-_ in the obvious place