The vehicle died while being driven. The charge light had come on some time before. I tested the voltage output of the system at the battery. At idle with a discharged battery, engine running I read 12.5V, at speed the voltage topped at 13.4. I assumed the voltage regulator was working because it limited. The voltage seems low. I took the alternator to be tested and it showed that the alternator put out too much voltage something like 15.75 and the limit was 15.5V. I'm not sure I trust the operator of the machine as he had to try several times to move the ground just to get the test to run. 220,000 miles....any other simple tests....For $8 and some bruised knuckles I can change the brushes and re-establish connections to make sure corrosion is not the issue. Any other thoughts.