'91 300SE electrical problem
 

Driving down the highway in rush hour and my car starts acting odd. Radio freezes and the car starts to hesitate. A few moments later the ABS and SRS lights come on, the lights are dimming, and the heater fan stops. I turn my blinker on to move to the right lane and the engine dies. I immediately turn the blinker off and it it runs a little longer, then completely loses power. There was no alternator light so I assume a dead battery (It's -6 degrees F here in Minnesota !). So, a few hours later I put a new battery in and it starts right up as expected. I drive most of the way home (about 20 minutes) and the same thing happens. I make it to a parking lot just as the headlights are dimming out of sight and the fuel pump is barely pumping. In total frustration I sit in the car waiting for a ride. About 20 minutes later (by now I'm freezing my buns off) I make an attempt to start the car knowing it's a waste of time. Surprisingly it starts right up. Lights are bright, etc. So, I drop it in gear and head home as fast as I can. As I pull into the driveway the process starts to repeat and as I stop in the garage the car dies. Any suggestions ? I am taking a shot at it and have ordered a new OVP relay.

Yeah, sounds like a bad/weak battery. Alternator may be failing.

Most likely (90%) the alternator.
This happens when the brushes of the voltage regulator are worn/defective, easy to replace. Use only Bosch, other brands don't last very long.
Check the voltage when the car is running, it should be around 14,5 volt, if lower the regulator is defective.

second this month
 

Jump to page 7 on 1 Dec and you will find the same topic from free300e. It is titled alternator issues? Easy fix and that is always nice to hear.
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Chris

microtoad, definitely check the condition of the voltage regulator brushes before doing anything else. The voltage regulator mounts to the back of the alternator with 2 allen head bolts ... a simple DIY replacement.

Thanks for all the feedback. I'll check the regulator brushes this weekend. I would have thought the alternator light would come on if it wasn't putting enough voltage out ? But, simple to check.