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'89 s124 300td
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Discussion Starter #1
Hi, my headlights dim whenever the a/c (compressor) turns on. Is this more of an alternator (regulator problem) or additional grounding that I have to look into? Perhaps someone have already experienced this. Hoping for your help

thanks!
 

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2001 E320; 2002 ML500
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As a quick test, connect an known good GND to your headlights and it should tell you the answer. Measure the battery voltage when the AC compressor is ON and see if drops substantially.
 

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About a dozen 1988, 1989, 1990, and 1991 sedans, wagons, 4Matics and 1 coupe
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Haven't heard of that one on a 124 but check battery voltage. Engine off with battery fully charged should be 12.6V and with the engine running voltage should be 13V or a little higher immediately after a start.
Also measure voltage between the battery post and the battery cable clamp to see if oxidation or corrosion is present. Any more than 0.1 or 0.2V present and some cleanup is necessary.
 

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my altorator is putting out 13.4/13.5 volts most of the time, but is then spiking up to 15 and down to 7/8 randomly, what could cause this? i am running megasquirt ignition and fuel, and these spikes are causing havok with the ecu fuel calculation, i need to sort it or i will loose a piston to lean out
 

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About a dozen 1988, 1989, 1990, and 1991 sedans, wagons, 4Matics and 1 coupe
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Nick,

First thing I'd check is the alternator brush length. It's easily done by unplugging wires from the back of the alternator and removing 2 screws to pull the regulator/brush assembly. You can even leave the battery connected for this check.
Very worn brushes or a deeply grooved commutator ring could cause a make/break connection and cause spiking.
Any friends with 124s that would do a temporary alternator exchange to see if that fixes it?
 

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will try, thanks
 
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