I have been getting the battery light in my dash intermittently for the last week or so. SCT X4 has shown voltages while running of 10.5 to 13.5 with and without the light.
Current situation: batteries show +/- 13.7v each (I disconnected cables and tested with mutimeter on terminals). SCT shows 10.5v and truck won't start (no real surprise if it's only seeing 10.5).
Terminal connections are tight, cables look good, but maybe I missed something. I've been thinking that the alternator was going out, but now I'm completely confused.
Thanks in advance for any help.

 

Vid will be redone, ignore the first 1/3.

https://www.ford-trucks.com/forums/1...l#post17100783

But typically when the light is on the alternator is the problem.

 

Update: still showing 13.7 for batteries, test for fusible link cable to alternator is good (value matches). SCT now showing 9.1v and multimeter in dash power socket showing 10.1v.
If my battery voltages are that high (perhaps they are too high?), where could I be losing 4+ volts?

 

With family in So Jersey so won't be having a quick response.

If both batteries individually are showing 13+ volts (and will be that high right after charging), you may have a ground issue. The main grounding cable is the one between the pass neg batt terminal and the fender right beside it. It's 8 ga.

Secondarily there are two bonding cables. These are braided cables, one is above the heater box going to the motor, and the second is under the pass floor area, from the frame rail to the bottom of the cab.

The other thing to check is the connection from the drivers positive. It the 4ga cable connecting from the stud and goes to the distribution box. You really can only check for contact at the battery terminal.

 

You see 13.7 at the batteries while running? and see 9.1v at the batteries with the truck off?

 

Jmoen - truck wouldn't start. 13.7 was at the terminals, key off. I know my multimeter is a little off, but I was still seeing an almost 4v change from the battery posts to the power outlet in the dash, it was showing 10.1 on the same multimeter.

TooManyToys - Thanks for the additional troubleshooting steps. I will look into them shortly.

I have since jumped the truck with my wife's car. SCT voltage went from under 10 to 12.3 immediately upon connecting the jump cables (I let it charge to 12.7 before starting), so I am on my way to get new batteries and will continue to troubleshoot if voltages are strange or battery light reappears in the dash. With any luck it was just bad batteries and they didn't manage to kill anything else as they were letting go.

 

Sounds like a possibly intermittently bad regulator.

 

It may be as the battery light reign of terror is limited to the alternator.

Matt, didn't mention it but if the batteries posts read 13+, the system harness reads less, and the attachment of jumper cables immediately brings system voltage up, are you sure the terminals and posts are clean?

Getting the batteries tested is always a good step.

Back to the meal.

 

Terminals were quite clean. Batteries are now new, but I will be keeping a close eye on voltages to evaluate for alternator failure and checking cables for continuity and voltage drops after I put a new 9v in my multimeter.
Quick check at the parts store said the alternator was good, but I have seen plenty of posts where they passed testing but were failing slowly.

Thanks for all the advice and ideas. I will update if my future observations turn up anything significant.

 

Parts store can only test if it charges. Find an electrical shop that can run the alternator at different rpms and with varying loads. That will show you what the alternator actually puts out with consumers and when it's hot. You'll also see what voltage it puts out as the load increases and maybe you'll see the regulator drop out proving it's the issue.

 

SCT showed max voltage of 13.8 on the way to work this morning, with most of the time spent below 13.4 and the battery light on 50% of the time. I will be getting a new alternator on the way home today. On the plus side, new batteries have the truck starting better than it has in a couple months and my FICM voltage has stayed in between 48 and 49 through all this. Still need to go through all my cables to make sure I have good grounds and connectivity.

Thanks to all for their help.

 

13.4v isn't bad. Others have indicated that the amperage output can fall even though the voltage looks ok and that could be why the light comes on.

 

No the light is connected to your key power on one side, and the alternator's field power on the other. In normal circumstances that's equal voltage on both sides which is why it doesn't light up.

If the alternator's field doesn't turn on, or the voltage to it sags low enough it behaves as a ground for the light which is why it comes on.

The only reason the "battery light" ever comes on is because voltage at the alternator field is too low. 9 times out of 10 that's a failing voltage regulator, which is internal for our alternators.

However since this vehicle is measuring large voltage drops, I'd start with with the grounds. The alternator might be okay but there's a good chance it isn't. There's also a good chance a new one will fail early if the voltage drop isn't addressed.

 

And to add insult to injury... that same failing alternator charging voltage is killing the FICM input voltage. Add $700 to the tag if you let it go long enough to damage that bad boy.