Based on my test results, I may have a cause, or simply another correlation.

Recap: Last night when I measured voltage, after a very short drive, its was:

12V – Engine off
14V – Engine on


This morning:
12.25V – Engine off
14.5V engine on

Conditions were perfect: Rainy and cold. After startup, tapped the accelerator to trigger the ASR fault, and it delivered. Voltage held steady at 14V. Shut off, restarted, no change in voltage.

(Note: The relay ping which I previously said was behind me may be an acoustic illusion. Depending on how I tilt my head, it sounds like its also coming from behind the passenger dash, where the OVP lives.)

While driving, voltage held rock-steady at 14V. Pulled over into a nearby shopping center to idle while the car warmed up. Still, voltage held at 14. Headlights were on the entire time. Switching on seat heater and radio barely moved the needle. However, turning on rear window defogger and blower motor dropped voltage to about 13.25. Turning them off returned it to 14V.

After about 8 minutes, coolant was at 82/83 and voltage had dropped to about 13.1V Engine-off was still 12V. Restarted, no ASR fault. Took a 30-mile road trip and voltage held at 13.1, with a possible increase to 13.25 or so on the highway, but I did not take my eyes off the road long enough to verify that. Upon arrival at my destination, voltages were:

12.1V - Engine Off
13.0V – Engine On

When MBDOC mentioned a spike I assumed a momentary one, but it would appear I experienced a spike lasting about 10 minutes, beginning the moment I started the car.

So: Appears I need a new voltage reg, correct?

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black