I have one of those cigarette lighter gauges. normally, the voltage reads 4.1-.2 but today it was 14.7-.8.


isn't that a bit high? does this especially cold weather have anything to do with it?


is it time for a new regulator?

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Ben
1987 190d 2.5Turbo

It should be around 12.8V - 13.2V depending on RPM and whatever load you have on the system (i.e. lights, AC, heater, radio, etc.).

Dkr.

The regulators in these cars seems to be sensitive to temperature, likely due to a simple zener diode as their voltage reference point. I've seen as low as 13.4V when really hot, and as high as 14.5V when cold out of mine. Granted, I don't get as cold here as a lot of people do.

Rev the engine and see if the voltage follows it up. If it does, the regulator is toast. If it stays put, the regulator is regulating at that setpoint for whatever reason. Typically you want to see somewhere around 14.2V at the battery with the car running. 12.8-13.2V as stated in the previous reply will NOT charge the battery (12.7-12.8V is considered "charged").

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Voltage will vary widely with temperature. If you see exactly 14.2 all of the time, that's really more unusual. The attached graph is from the Bosch manual. Also check out the service bulletin:

http://www.peachparts.com/shopforum/attachments/diesel-discussion/143025d1498964603-troubleshooting-charging-system-bosch.pdf

Attached Thumbnails

 

We don't know the accuracy of the test instrument, it might creep with temp.

A charging voltage of 12.8 - 13.2 is low. However older ( pre maint free battery) cars had a lower charging voltage with 13.2 - 13.8 being more common so perhaps that is what he was thinking.

Cold batteries can tolerate / need a higher charging voltage, an alternators voltage rise on lower temps is by design.

I beg to differ. I've driven many tens of thousands of miles on a few different diesel W123s with a voltage reader in the cigarette lighter, and that is exactly what you will see as being normal. These are in cars that experienced low freezing winter temps and did not have battery or charging issues. The alternators in these cars are weak compared to newer cars and if you charge your phone or run accessories, the voltage will drop accordingly. In addition, many of the alternators we have are old and not quite in the best shape that you would get from a new alternator.

Dkr.

Be aware that the cigarette lighter tends to read slightly lower than actual battery voltage. Mine reads 13.5V or so when the battery is running 14V. The cigarette lighter is fed from another circuit that has a slight amount of load on it, this causes a voltage droop on that circuit. Just something to be aware of.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Right, there is a difference. But, my answer is in response to the OP as that is how he is measuring voltage.

Dkr.

So, the bottom line is; measured at the cigarette lighter, 14.7 volts indicates a bad alternator voltage regulator...

Unless you are running LiFe-Po batteries...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Not the same car, but I have a 2012 Versa with a Scanguage installed, and it usually reads 14.1-14.2. With this cold weather, when first started it reads about 14.5 for a short period of time.

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96 E300d

All normal. If it ain't broke, it's not even worth posting, much less fixing.

First verify the voltage with a good meter. Second replace the regulator on the backside of the alternator with a new one. $15-18. Two screws. No need to remove the alternator but disconnect the battery negative first. The alternator will come with new brushes which is great. Now check voltage again. 14.8 is indeed too high . Your battery may be the problem. Have a drawdown test done at autozone or wherever. A fully charged and new battery should have a "float" voltage of 13.0-13.4 without a load on it. Charging voltage should run up to around 14.5 or so depending upon the regulator . It varies with different cars and different battery types like AGM , gel and sealed batteries. My comments apply to old school flooded acid batteries. I almost never ever buy reman alternators. I rebuild them by just replacing the brushes which is an easy job with W123 Bosch units but difficult with some GM and Toyotas without special tools.

Just curious how you found and replied to a three year old post?

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1981 240D 4sp manual. Ivory White.

My 300d peaks over 14 sometimes after startup but once warmed up and
driving down the road it's usually in the 13.6-13.8 range.

Symptoms of a failing regulator. Voltage should pretty much be consistent no matter the temperature. Your battery will not be charged at 13.8v and under.