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Old 11-16-2007, 09:05 AM
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Question W140 Electrical Gremlins!

Started my 93 S500 yesterday after work and the alternator light immediately lit up. I decided to try to limp home with everything shut off except the headlights (it was dark). Got halfway home and one by one the warning lights started coming on. The Headlights and the dashboard lights started to go dim so I pulled the car off looking for a place to park. Then all of a sudden the warning lights went out, the headlights and dash lights got bright and everything went back to normal!

I'm going to go ahead and bite the bullet and have to alternator replaced before I leave to go on Thanksgiving Holidays but I wanted to know if this is a common symptom on the W140's when the alternator goes bad? I've heard that they tend to go berzerk when the voltage drops below a certain level but why did everything go back to normal. Could the bearings in the alternator be going bad.
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Old 11-16-2007, 09:46 AM
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Most likely the brushes on the voltage regulator WORN out..either replace IT or the complete alternator.

Make sure that the wiring going to the alternator isn't showing bio-degradable insulation.
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Old 11-16-2007, 10:20 AM
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DC, check the condition of the voltage regulator brushes by removing the 2 screws that secure the voltage regulator to the back of the alternator. It's likely that the brushes are worn and not making good contact with the armature. You can buy a new voltage regulator online for less than $30 ... probably no need to buy a new alternator.
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Old 11-16-2007, 01:11 PM
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Well, I talked to my mechanic and was told that on the V8 W140's checking the voltage regulator requires removal from the engine bay because the back of the alternator is not accessable without removal- a one hour (read $185 labour for remove and reinstall) process. The rebuilt alternator is $150 as opposed to the $38 for the voltage regulator. $335 for the whole job as opposed to $223 for the regulator only.

I decided to go ahead and replace the alternator. With my luck he'd replace the voltage regulator and the alternator would go bad the next day.
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Old 11-16-2007, 03:19 PM
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DIY write up for alternator work on the V12

Here's my alternator bearing replacement write up. Voltage regulator is covered as well. Obviously, this is for the V12, but the alternator is identical in construction. Just the location will be a little different.

http://v12uberalles.com/Alternator_Bearing.htm

Brett
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