electrical problem, on three hr trip head lights and cab lights dimmed an flickered especially when fan was turned on (a/c or heat). One headlight failed, then a/c coil (compressor clutch)failed, then alternator failed. Replaced alt with after market and in one day regulator failed. Took to dealer, they found the day light running light module had shorted, after it was replaced and a new alternator was installed the truck runs great.