Hi, I have a 1988 300SEL and I replaced the voltage regulator and when I first start up the car, it charges a little over 14, which is great. Once the car warms up it starts to charge less. Sometimes it stays around 13.3 but as the car warms up and has been driven a little, at idle it charges at 12.3-13.1 and rarely goes above 13.3 after fully warmed up and driving around. Not sure what it should charge at at idle, nor what it should charge at normal driving. Any help would be appreciated. I did order a new (rebuilt) alternator but am waiting on a pulley I ordered for it. Worst case I'll just drop this one in, but does anyone know any other electrical reason why the charge may be varying that much?

My '88 W201 doesn't have a voltmeter. Does your W126 have one as OE? If not, how are you measuring system voltage.

As the battery approaches fully charged after starting, voltage can drop. System voltage will also vary with load and engine speed and alternator temperature. The higher the load and the lower the engine speed the lower system voltage. If there is full load on the system at idle voltage may drop to battery voltage because the alternator cannot maintain normal system voltage of 13.5-14.5 volts.

It's not clear to me if you have a problem especially with a new regulator. If after driving around especially if you make multiple stops and restarts, measure batter voltage after you get home. With the engine off if it measures close to 12.5 volts at room temperature, the alternator is probably okay.

Give it some time.

Duke