Help with Battery voltage problem

[Flim]

I recently had the alternator warning light on my 250 SL illuminate, albeit dimly, on idle. Curiously it would illuminate like this after long driving. There are no start issues, with starter cranking as normal, but the only other electrical clue I noticed were the slight dimming/brightening of the driving lights at night whilst driving.

I tested the battery voltage, and to my surprise idle voltage read 15.5-15.7 (!) volts, and with the engine turned off 13.5 volts (?).

I suspected the voltage regulator, however substitution with another (old) unit left reading unchanged.

Does the problem lie solely with the alternator, and if so why would the circuit overcharge rather than not produce enough voltage? Has this excess voltage damaged the battery cells?

Car currently has original type Bosch 35Amp alternator.

Any help/advice much appreciated.

Charles

[ejboyd5]

As all the readings seem a bit high you might want to verify with another voltmeter.

[Flim]

As suggested I re-checked the battery voltage with a different (new) multimeter this morning.

Battery voltage reads: engine off 12.78V and engine idling 15.44V

Has my alternator charge damaged the battery/voltage regulator?

Thanks
Charles

[450sl]

looks like your charging is ok , you may check on higher revs , where voltage should be 15 - 15,5 volts also.

[Iconic]

I agree, check at higher revs. I do believe 15.44 is too high at idle though.
Also, are you checking the battery voltage right after turning the car off?
If so, wait a couple of hours to see what the battery voltage settles down to.
If it drops, the battery is bad and maybe your voltage regulator is overcompensating.
All of this advise is not very expert (and it is free) ... just something to try to get more clues.

[Flim]

So at higher revs (2000-2500 RPM) voltage tests at 15.11V (@2000 RPM) and 15.05V (2500 RPM)

Voltage taken immediately when engine turned off is 13.41V

I'll check again in a few hours to see what reading I get: Voltage after 12 hours rest (engine off) 12.88V

Does this sound as if the battery is bad?

[114015]

Charles,

Most likely
your voltage regulator has gone bad
and causes the over-charging of your battery while the engine runs.
This kills sooner or later your battery. :'(
Since you have already got a low 12.8 V voltage reading with engine of, this implicates that your battery is already dying.

You can check back, your battery should really become hot while charging (engine running) - good indicator.

Replace your voltage regulator ... and most likely your battery as well (perhaps a bit later, depending how quick the battery now looses voltage while engine off).

Had the same issue a couple of years back.

Good luck

Achim
(absolutely no expert)

[Jonny B]

The new voltage regulators (at least the one I got a couple of years ago) are solid state. Bosch recommended changing that when I had to change the alternator. If looks are a concern, the new regulator is a bit shorter than the old ones, but the base is the same dimension. You can carefully pry the top off the old one, get some yellow electrical tape at the local home store, and voila, looks right, and should be utterly reliable.