Time for new brushes? - M103

[Mike Murrell]

Automotive text books claim that a 6 cell automotive battery produces 2.1 volts when healthy. 2.1 x 6 = 12.6 volts - the sign of a charged and healthy battery.

My '91 300-SEL has a yr. old AC-Delco battery designed for this car. After sitting overnight, a multimeter indicates a reading of 12.3 to 12.4 volts. After driving the car 'til good and warm, it reads 13.25-13.30 volts at idle. I see the same reading if I rev and hold the engine speed at 1500 rpm.

I've read elsewhere that I should be seeing 13.4 - 13.9 volts at idle. Are the readings I'm seeing an indication that the brushes are growing tired? Thanks for your time.

Mike Murrell

Need to check charging system via amps using a load rather than volt meter. The best results I have experienced is using MB batteries only. Aftermarket will often give problems, especially corrosion and obscure voltage readings.


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Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture
2000 SL500
1995 E320
1989 300SE

Forgot one thing, take the 2 screws out of the regulator and look at the brushes. easiest way of telling wear.


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Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture
2000 SL500
1995 E320
1989 300SE

[Mike Murrell]

MBZ.Ian - thanks for your help - Mike Murrell

Mercedes-Benz does not make automobile batteries. Save your money and get an Interstate battery, or even a Sears battery(and Sears does not make batteries either)
There are a lot of parts other than batteries that Mercedes uses on their cars that they do not make:
Brakes and brake parts(most are ATE)
Diesel injection pumps and diesel turbos(I have two MB turbos and both have turbos made by Garrett located here in the USA. I just rebuilt the turbo (a Garrett)on my 1982 300D-Turbo.I got the parts from a place in Tx. Cost including shipping:$22.00. There really isn't much to a turbo. Bill Lewallen Lex.Ky.

Just a quick note on the last post, no MB does not actually make the batteries, they are made by other company's to Mercedes-Benz specifications. Basically over the past 20 years in the dealership level I have seen what after market batteries can do to a wireing harness. Stick with MB, prices are very competative. They also have the correct vetilation for the tube to vent the gasses away from critical components and wireing. I have replaced many a harness caused by aftermarket batteries. Penny wise, pound foolish. stick with MB.


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Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture
2000 SL500
1995 E320
1989 300SE