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Old 07-25-2010, 04:14 PM
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Location: Pharr, Texas
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Battery Light On - Charging System Checks OK

I have a 1985 300SD with 218K miles that I recently purchased. I've had the instrument cluster out to repair the odometer and now that it is back in, the battery light comes on and stays on very dim when the engine is idling and glows brighter as the engine speed increases. I've checked the voltage across the battery terminals and it shows 13.1 volts at idle and climbs to 13.8 volts as the engine speed increases. I had the battery checked at an auto parts store and is was putting out more than rated CCA (880). Also, the ABS light is on all the time now. I haven't head any problem starting the car, so it seems like the battery is charging and is holding a charge.

Could I have done something to the instrument cluster while I had it apart or do I need to dig further into the charging system? Before I got the car, it had set for over 6 months without driving. The PO purchased the battery new in February.

Rick

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Old 07-25-2010, 04:18 PM
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My money's on a worn voltage regulator -

http://catalog.peachparts.com/ShopByVehicle.epc?q=&yearid=1985@@1985&makeid=63@@MERCEDES+BENZ@@X&modelid=6209:ED|10000134:MBC|1510@@300SD&catid=240798@@Engine+Electrical&subcatid=240870@@Voltage+Regulator&applicationid=W0133-1714889&mode=PA

Sixto
87 300D
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Old 07-25-2010, 04:21 PM
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Sorry, I should have mentioned that I have already replaced the voltage regulator before I started working on the instrument panel. Prior to that I was barely getting 12 volts across the battery.
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Old 07-25-2010, 04:29 PM
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smoke gets in your eyes
 
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Location: Eastern TN
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You might have not replaced the round connector properly since it's easy to miss the clocking key. You'd probably notice other things amiss with the gauges if it's not properly in place, though.

Make sure the cable between the alternator and the distribution block is in good shape and the ends are securely fastened to their terminals.

Sixto
87 300D
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Old 07-25-2010, 05:08 PM
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Bad ground somewhere in the instrument cluster, one of the metal strips that connect the three main units together in the cluster case?
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Old 07-25-2010, 10:09 PM
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Battery Light On - Charging System Checks OK

I took the instrument panel out, cleaned all the ground contacts and put everything back together, making sure all of the connectors were properly in place. I took the new voltage regulator off and carefully replaced it. Removed and cleaned all of the connections on the terminal block where the battery and alternator are connected, and reconnected everything, but the battery light comes on like it did before.

With the engine running, I turned on all of the lights, the blower fan (AC compressor doesn't work yet), the rear defroster, etc. and checked the voltage across the battery - 11.97 volts at idle, increases to 12.8 with the engine revving.

I'm thinking that I need to remove the alternator and have it tested at a parts store. Any suggestions?

Also, for what its worth, in my struggles with the instrument panel, the turn signals and flashers don't work, and the glow plug light doesn't come on (that may be normal since it is in the high 90's here in deep south Texas - the engine starts right up every time)

Rick
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Old 07-25-2010, 10:14 PM
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smoke gets in your eyes
 
Join Date: Jun 2000
Location: Eastern TN
Posts: 20,841
Less than 12V while idling seems low. 12.8V at revs seems low too. I'm used to seeing closer to 14V at revs. If it's not the regulator, it must be the alternator.

The turn signals worry me but that might be the relay. IIRC wiring to the glow light is independent of the gauge cluster so that's a separate problem. Could be as simple as you didn't plug the bulb into the glow window socket or it fell off in your struggles.

Sixto
87 300D

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