'91 420SEL stalls at cruise, electrical issues
 

At 55 mph '91 420SEL w/178,000 miles stutters, finally stalls. Tach goes above 3000 at idle, speedometer bounces from 0 to 25 or more with tran in park and foot off accelerator.

Radio had quit a week before. Power seats and windows quit after stall.

Old battery tested 10.5 volts. Used jumper cables and car started, all electrical worked. Installed new battery and car performed perfectly for about 100 miles and radio quit again. Tach again appeared to indicate higher than engine speed. Parked the car in park and let idle. Same fluctuation reappeared in speedometer.

Any help appreciated.

So what voltage is the current battery?

'91 420SEL stalls at cruise, electrical issues
 

New battery tested 11.8 volts after second incident. Tightened alternater belt; tested 13.3 volts while at idle.

All interior electrical systems came back on.

Ready for a test drive to see if alternater belt was the only problem.

So the test failed. Same pattern, eventually car dies.

Put fully charged old battery in and drove 30 miles with no problem. Measured voltage after 30 mile; 11.8 while at idle, 13.2 after turned off engine.

Could it be needs new alternator brushes, or what?

Thanks.

Sure sounds like the alternator isn't putting out.

Problem solved. Old alternater's woodruff key was entirely absent. Pulley must have been slipping on the shaft at speed. Installed new alternator; no problem so far.

Thanks for your help.