BenzWorld Junior Member
Date registered: Dec 2012
Vehicle: 1985 190D 2.2, 1990 300SE, 2008 E350 wagon
Location: Los Angeles
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Smoking Battery 190D - Overcharging?
My car problems seem to come in waves. I can be problem-free for months on
end, and even years on end, and then all of a sudden everything breaks at
once. And this time it was with my 1985 190D 2.2.
My wife calls, stuck in traffic, and says there is white smoke coming from
under the hood. She pulls over, and the smoking battery is hissing and
doing everything it shouldn't be doing. She is only five or so miles from
home, so it is AAA to the rescue, and it gets towed to our garage.
Meanwhile, the AAA guy said he has seen many a car with a battery that
caught on fire so it was good she called.
The gel battery was seven years old, so for starters we bought a new
Mercedes gel battery. Before taking the old one out, however, I put my
voltmeter across the terminals to see if I could detect an overcharging
condition. I could not. And the battery held its charge overnight. I was
not able to reconcile any of these facts, but felt ok changing out the
seven year old battery and taking it from there.
I put the new one in this afternoon, and after starting it up, I put a
voltmeter across the terminals and it read 15.5v or so at idle and higher
when I gave it some throttle. It was higher than the 14.2 I was expecting
to see, so I shut it down and took out the voltage regulator mounted on the
back of the alternator. I then visually compared it to a used but good
Bosch spare I had, and noticed a couple of things: firstly, the used one
was a Bosch and the current one didn't have a name; secondly, the Bosch had
a green resistor or diode or similar soldered to its exterior, and the
no-name one did not. I'm thinking it may have been there once upon a time
but came off somehow. I then installed the used Bosch, and upon
restarting and checking with the vom, I received the expected 14.2v.
Thinking back, I was given the cheapo voltage regulator last time I had the
alternator rebuilt. Next time it happens, I will make sure that a new
Bosch is used, even if it's a few more bucks, and better to avoid the
knockoffs. I now see how they save some money on the rebuild but causes
I also see how if a battery is toast, voltage readings across the terminals
may not be accurate. Only when I had a new battery installed did I notice
an overcharging condition. If anyone has anything to share along these
lines, I'd like to hear about it.