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Old 06-27-2002, 12:01 AM
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Join Date: Mar 2000
Posts: 567
'92 300 E - No Instrument Cluster Warning Lights at Startup

1992 300 E, 125K miles - had voltage regulator replaced recently as part of preventive maintenance (brushes on existing regulator were worn).

Several days later vehicle failed to start, apparently due to low battery charge. Prior to no-start condition, vehicle exhibited symptoms of low battery voltage - ABS, SRS warning lights illuminating while running, poor performance and stalling.

Cleaned battery terminals and + cable clamp; replaced negative battery cable, due to excessive amount of corrosion present. Corrected battery electrolyte level, recharged battery and had battery (approx. 9 months old) checked at dealer. Battery tested as good.

Now, with recharged battery installed, only SRS and ABS instrument cluster lights illuminate at startup and the ABS only for less than a second before extinguishing. All bulbs check out as good.

Suspect the battery is not being charged and obviously need to check alternator output. Battery warning light does not illuminate with the engine running.

In the interim, is there anything else to check that might account for the lack of instrument cluster warning lights at startup, despite having a fully charged battery on board?

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Old 06-27-2002, 09:42 AM
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Join Date: Mar 2000
Posts: 567
Checked diodes in alternator using a Fluke 88... when connected to alternator's "B+" and ground, obtained a reading of approx. 0.384 volts. Same test on a new rebuilt Bosch alternator yielded 0.949 volts. Additional diode testing using the Fluke's beep mode confirms the presence of diodes that are shorted (or have resistance of less than 4 Kohms), which also explains the concurrent loss of the instrument cluster warning lights.

So the question is now... did the replacement of the voltage regulator precipitate the diode failure or was it merely fortuitous the diodes failed shortly after the regulator's installation?

The instrument cluster warning lights were functional for a period of time after the regulator was renewed. In a perfect world, should one scope the alternator's output, prior to regulator renewal, to check for spurious wave forms that might indicate impending diode failure?

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