Heading to work this AM and all of a sudden the VLT gauge on ScanGauge II drops from 13.7 down to 12.1, 12.0, and 11.6. Stays in that range for a few seconds, then pops back up to 13.5. Rinse and repeat several times until it drops to 11.8 and stays there. At this point I'm dead stopped in 6 lanes of traffic (in the middle of course) and I shut the headlights and AC off. Comes up to 12.1V. Since I was not moving, I ran through the PIDs and FVP and FLP were both down to 11.0V where they are typically 13/14V. FICM was still 48.5V. Stop and go for about 20 minutes and no change. Get back up to highway speed and after about 3 minutes, back up to 13.6V.

At lunch, started it up to check charging and enter an EGR voltage PID in the ScanGaugeII. After a couple of minutes, up to 13.7 volts. OK.

After work, hop in, start up, after about a minute, battery light comes on in the cluster. Went up to 13.7 about 5 minutes later, then down to 11.5 and stayed there with the battery light going on and off for the rest of the trip. Lucky to make it home - headlights were very dim.

Truck has dual alternators. Both Ford dealers it's been to (Raleigh NC and Hampton NH) have remarked that batteries will need attention in near future. I've had them load tested at two places and both were over 700CCA and charging system passed.

Guess the Ford guys were right but I'd like to make sure that one or both alternators are not bad. So question, if one alternator fails, will the other continue to charge the batteries? As I recall in a dual battery system, that if one battery is bad it tends to eventually take the other one with it, particularly if they are the same vintage.

 

Once the engine is running the alternator supplies the running voltage.

With dual alternators, the PCM won't command both on unless the system amperage supply demands it.

Possibly why you'll see 13+ (lower alt is commanded on) and then low 12s (the lower alt was commanded off for the time being)

Have the upper alternator tested, make sure to replace with the 140 amp version, which is stock for dual alt setup.

Josh

 

Remove and bench test? Are the alternators different amperage? And not to sound dumb (new to daughter's Ford), but upper meaning the one that is more on top physically? OMG, this engine is complicated compared to my Cummins...

Thanks,
John

 

Yes, remove the upper (physical) alternator and have it tested at a chain store.

Might as well have the batteries test while you're at it.

Josh

 

Test both batteries separately.
A shorted cell could also be a problem
But then again a lot of things can happen
Test the batteries and if you can also
check with a hydrometer on the cells.

Sean

 

If one battery fails replace them both.

 

Josh and Sean hit the nail on the head, but remember to let the batteries cold soak overnight before you take them to be tested.


I had a similar problem with the batteries on both my trucks. I made this mistake several times thinking that I could disconnect a battery and be fine for testing....No! Driving the truck creates enough of a chrage to cover up a defective battery.

The problem started with the 6.0. It would never consistently crank with the authority I expected it to. Sometimes the starts were weak, but it started and sometimes it would start with some moderate authority. I pulled the batteries, let them cold soak overnight and one turned out bad. I ended up getting two new batteris and now the 6.0 turns over with the authority I expected.

With the 7.3, I was basically working with 1.5