A very knowledgeable tech told me whenever a 124 comes in with problems, the first thing he does is replace the OVP relay. it is located aft of the battery, takes 10 minutes to replace, and costs $100. It affects a large number of devices and circuits.

This is a good thing to consider. Much easier than replacing the alternator.

To add to the story a bit. I broke our the multi-meter and tested the battery leads with the engine running. The Voltage is dropping even while revving the engine.

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Current
2006 E320 CDI, 57K bought at 67K "Liesl"
1986 190E 2.3-16, 198K bought at 56K "Brigitta"
1987 300TD (Chuggin Along), 292K "Friedrich"
1995 E320 Wagon, 200K "Louisa"

Past
1987 300SDL, (sold) 125K, bought for 1$ "Kurt"

I would question to a certain extent a knowledgeable tech who immediately replaces a $100 part when a car comes in "with problems". Not trying to be snarky but yeeesh people.

Anyways, what I would do first is yank out the voltage regulator to look for worn out brushes. At that point, if one or both are worn down, you would probably be better served replacing the regulator. Other course of action is to get the battery charged up and take it someplace to get the charging system checked. This is what the next step would be if the brushes look good. back when I was a tech the first step was to check the charging system and if it was low output (less than 13.8 or so) check the brushes. A poor mans way of testing would be to check batt voltage with the engine running, again it should be 13.8 to 14.4v when running. To be fair make sure the major electrical consumers are off, AC and blower fan, headlights, etc. The charging system or battery makes way more sense than a the OVP given the symptoms.

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Well using this reasoning, fine, replace the oil cap first but it won't fix the problem.

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