2006 F150 SC Lariat 5.4 81,000 miles
Original alternator failed 9 months ago and I replaced with oem reman. No problems for 7 months until 3 weeks ago. On my way home from out of town headlights and dash lights brightened. Gauge needle was one needle mark above normal. After 30 seconds needle and lights back to normal. This repeated every 20 miles. Next day I took off alternator and Ford parts warrantied. 250 miles later needle moved to 3/4 mark and back to normal in 30 seconds (would not repeat). Next day had to go out of town and 40 miles into trip needle jumped to 3/4 mark and back to normal in 30 seconds. 15 miles later jumped up again and I was able to pull over and checked with volt meter 15.52 volts and needle was one needle mark above the norm. Drove 20 miles to Ford and I took off alternator and was warrantied again. 380 miles later needle jumped to 3/4 mark. At this point everyone is assuming the alternator is not my problem. Motorcraft 850cca battery is a year old. No problems starting. All cables and post are clean. No wires rubbing or signs of damage.
Thanks for your time and help,
Scott

 

I have a similar situation replaced alternator about a month ago with a Ford reman and have had the volts jump to 15.1 a hand full of times. My battery is also only a couple of months old. My negative terminal was corroded and found more corrosion a few days ago so I'm going to replace the ground wires. My truck is a 08 FX4 5.4 with 75000 miles.

 

It can be completely normal for a PCM-controlled Gen 4 charging system to operate as high as 15.5 VDC. It's controlled by the reading from the IAT sensor [edit] ... and other inputs to the PCM.. Just because the voltage is over 15 volts is not a definite indication that the system is malfunctioning.

See applicable workshop manual for explanation and the correct diagnostic routines.

 

I agree totally with you projectSH089 regarding the upper limit on voltage from the alternator, but I don't think it should be fluctuating like he says. This behavior seems to me to indicate poor connections in the charging circuit, possibly removing the battery at times from that circuit. Since the battery acts as a "buffer", to smooth out and limit charging voltage, I would say it is being cut out, then reconnected.

- Jack

 

Did anyone bother to scan for non-MIL DTCs?

Is the charging fault light in the cluster coming on or not?

Keep in mind that the charging circuit is no longer restricted to the generator/regulator and battery, it now includes other inputs to the PCM and the PCM is what "sets" the desired charging voltage.

 

You raise good questions that obviously need to be answered. And, I must admit to a degree of ignorance regarding the role of the PCM in the charging process, as well as other tasks.

Still, would sudden large changes in charging voltage be consistent with PCM control?

- Jack

 

Hi and thanks to all who have replied!!!
I am sure the volts were way over 15.5 when the gauge was at the 3/4 mark (dash and headlights were super bright). I was only able to catch and test the volts one time and the gauge was only one needle mark above the normal (normal is half a needle over the 1/2 half mark on gauge) dash and headlights were still bright. No check engine/service light. I took to dealership yesterday and no intermittent overcharging occurred. The tech let the truck idle for several hours with the lights, a/c, etc. on and had a volt meter on the battery. The meter fluctuated constantly from approximate 13.8 to 14.3 volts. Since no intermittent overcharging occurred the tech can't say for sure if its the PCM, sensors, etc. I have a 2000 mile round trip next week and don't want to melt wires or be broke down on the interstate.
Thanks again for your time and help,
Scott

 

Buy one of these, then you don't have to guess: http://www.walmart.com/ip/Equus-Inno...nitor/15137663

 

Thanks projectSHO89, I ordered the Equus monitor today. I was driving by another dealer today and decided to have truck check again, hoping something would show up. Two year old Ford 850 cca battery tested bad, only 500 cca. Dealer warranted and replaced battery. Tech did a charging system check and complete scan test. All good and no codes. Both techs that tested my truck this week stated its the PCM but could not say for sure due to not being able to duplicate the overcharging. I left the dealer today and drove 50 miles down the interstate to visit a friend. On the way home the needle moved up one needle mark for about 10 seconds twice within 5 miles and never duplicated again. I couldn't tell if headlights got brighter due to daylight. At least I will have the voltage monitor for my long trip next week.