Ok my son had the 420 and on his way home it stalled, could not get it to fire and he said would not roll over. It took me a couple of hours to get to the car and when I put the volt meter on it it had 12.26 volts, I continued to look for trouble spots found none. Hit the key and it rolled over a few time then fired up. I drove it home and noticed that the tack and speedo switch posistions. after a few blocks they were ok but when I placed my foot on the brake they dropped quickly, as if you had jamed on the brakes real hard. The next thing I noticed was the SRS. and the ABS. light and the lamp out lights had a very faint glow to them. I am going to hook up my meter to see if I am dropping in my voltage from the battery, this should tel me if I am dropping power from the alt. or I hope it will.

Sounds like a failed Over Voltage Protection Relay.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Checked the fuses two in my four year old unit and power is good Now it may be bad, but I just wonder why it is the first thing people pick out as the cause of trouble?

Sounds like alternator acting up , check the brushes in regulator on the rear of the alternator, could be voltage got too low while driving, then stalled, 12.2 volts seems low, hook up voltmeter while driving should hold up to 12.7 to 13.5 volts at idle

after driving home no trouble except that the gauges kept acting up put it in my shop and put a meter on it the findings are that the battery begins to recover the instant you stop the drain, sutch as trying to start it. thats what happens when you use a realy good battery. The one in this car is as heavy as the battery in our big rig. I think it may be strong enough to start that 600 hp diesel. The completion of the story will be posted after the alt. arives and I have tested the system. as of now the battery is on slow cook at 2amps.

The Alt. was the trouble with the stalling and poor performance of the dash board gauges. hope this will help other people who are having troubles like this. JNT