just put a meter on the battery while its running. I remember when my 420 sel had a bad regulator the dash lights were super bright so I put a meter on the battery and I had something like 15 volts to the battery.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual