Voltage regulator issues - Page 2 - Mercedes-Benz Forum

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post #11 of 12 (permalink) Old 10-21-2011, 01:34 PM
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Date registered: Nov 2006
Vehicle: 1995 W124 E320, 1982 W123 240D, 1991 W126 560SEL, 1993 W201 190E, 1978 C107 450SLC, 1992 W140 300SD
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Had similar situation on my 88 W126. The brake indicator was on and battery light was on as well. Turned out the burned diodes in the alternator. If you look at schematics the "blue" thin wire from the alternator goes to the instrument cluster and connects to battery light on one side. There is also a wire from the battery that connects to the same light.

What happens is when the voltage from the battery is greater than the voltage from the alternator, said voltage "pushes through" the blue wire voltage and to the ground thus lighting the lamp. When the alternator voltage is higher it holds the voltage from battery "at bay" and thus the lamp does not light up.

There are supposed to be diodes that prevent the current from jumping from main wire from the alternator to the battery to the blue line. However, if the diodes are busted you'll have a current leak into the blue wire when car is not running. Because of that all kinds of lights can light up if there is current going from blue wire into the instrument cluster because there are no diodes there and they share some connections.

I've written up my findings in http://www.benzworld.org/forums/w126...t-tracing.html

Hopefully all above makes sense.

Now looking for sewing lessons
1982 W123 240D "Turtle"
1995 W124 E320 "Rogue", 1991 W126 560SEL "Pegasus", 1992 W140 300SD, 1990 W201 190E 2.6, 1978 C107 450SLC, 1973 R107 450SL, 1999 W140 S500.
Gone but not forgotten:
1988 W126 560SEL - got rear ended, 1993 W124 500E - parted.
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post #12 of 12 (permalink) Old 10-24-2011, 01:45 AM Thread Starter
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Date registered: Jun 2004
Vehicle: '82 W123 300D, '93 W124 300D, '01 W220 320 cdi
Location: Portugal
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Originally Posted by augapfel View Post
.....Part number???
The part is aftermarket and i don't think Mercedes sells that part. Il try to find out the brand and I'll get back to you.

@ larrypayne - I don't know why the wear is unequal but it is consistent with what happens also on our W123 300D.

@ Zeitgeist - Thanks... ridiculously big for this job though.

@ russianwol - Thanks for the extra info. Fortunately, the diodes seem ok. What a dream of a car that 126 of yours btw!
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