I have an '08 F-150 Supercrew XLT with the 4.6l Triton V8. Driving down the highway today with radio and a/c going when the radio and dash gauges (including tach) quit working. A few minutes later the a/c quit and then I was able to pull into a parking lot when the engine quit. A good Samaritan gave me a jump and the truck started right up but died shortly after removing the jumper cables so I called a tow truck and got towed home. Once home, the tow truck driver gave me a jumpstart so I could put the trans in neutral then left. The truck started right up and continued to run with everything now working. I put a voltmeter to the battery and got a reading of 13.55 volts before killing the engine. Went out and restarted the truck, turned on all of the lights and accessories and checked batt voltage again with a reading of 14.18 volts, put high beams on and voltage dropped to 13.91 volts. I'm wondering if I have a bad relay, maybe a bad alternator or battery. Any ideas?

 

you should have a dash light come on if alternator / battery charge is low.... If you have the original battery , I would change that now... its time... Other than that, I would check all the cable connections to battery, alternator, relays on fire wall... and ground cables . Clean / check corrosion and reinstall....... then monitor success.

 

alternators have an "AVERAGE" life of 150- 160K........ what have you got ?