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Old 12-15-2008, 09:19 PM
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Join Date: Oct 2002
Posts: 67
1988 300e battery light on and off

My battery light has come on and goes off occaisionally but when I measure what's happening at the battery I get a 13.7 volt rating when the engine is running and a 12.8 when the engine is off... there is no variation...

what else could be an indicator when the battery light goes on...

aren't these numbers pretty much where they ought to be?


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Old 12-15-2008, 11:01 PM
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Join Date: Oct 2004
Posts: 3,687
You need to visually inspect the voltage regulator brushes. If they are worn out they can go intermittent before they buy the farm. The other possibility is that a diode is failing, but before you worry about replacing the alternator check the regulator.

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
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PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff
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Old 12-16-2008, 10:49 AM
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Join Date: Jan 2003
Location: Dallas
Posts: 790
Yeah, the regulator is a 5-minute job, and a cheap repair.
I had the opposite problem; regulator/alternator failed, and the light never came on. I only discovered the problem when the lights dimmed to almost nothing.

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Old 12-16-2008, 11:03 AM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,574
Brushes are easy to check. Safely elevate front of car enough so you can slide underneath. Alternator will be more to passenger side. Back side of alt. has 2 phillips screws. Remove both and out comes the volt. reg.

When new, the brushes on the volt. reg. are 1/2" long.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
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Old 12-19-2008, 07:12 PM
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Join Date: Oct 2002
Posts: 67
Checked brushes

They were good.... still getting consistent 13.7 reading with engine running but the light on the dash mostly stays on and only on occasion will it go off for a few seconds....

Anybody have any other thoughts?

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Old 12-21-2008, 09:34 AM
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Join Date: May 2003
Location: bullhead city, az hot as hell
Posts: 301
check all the ground straps

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