Had the following problem start up the other night...... Went and started my truck and was warming it up. All of a sudden charge and brake light came on idle increased 500 rpm. Note very cold -20c. Drove to work volt meter showing 12v. Lower than normal. Came home checked system with reliable meter 12.5 v at 2000 rpm. Charged batterys (running two batteries) Figure its alt. buying time until the weekend to change. Now tonight coming home lights and heater on volts started dropping fast, tach doing crazy things, lights dimmed etc. Limped home, engine missing, 5 volts on meter. Let it sit 3 hours figuring I'd have to boost it to get it into garage. Nope, fired right up 12v on the gauge lights working everything normal except charge and brake light on. (realize volts low). Both batteries are new and equal size 850ca. Even with pooched alt why low voltage and dead in the water then 3 hours later she starts perfect and 12v on the gauge. This situation does not compute with me.