Scottwizer

A little history on my truck... It is a 2006 f-150 5.4 liter with 142432 miles on it. About two months ago my alternator went out so I replaced it. When I got the new one in the battery light continued to stay on but seemed to charge. So I took it to a reputable mechanic and had them look at it. They said the battery and new (rebuilt) alternator was bad. I had both replaced and the light went off and everything was fine. Now two months later the light came on interminantly. I checked voltage and it was back and forth between 14 and 12 volts. The light would come on when it wasn't charging and go off when it was. Finally, it didn't want to charge and the new alternator died. I replaced it again with a rebuilt alternator from ford because I am tried of this. Anyway, after I started the truck the light came right back on. I fully charged the battery overnight and it is still on. I checked voltage and get 13.18 volts with the truck off and 14.28 with the engine running at an idle. I checked all connections on the alternator and wiring including the fuseable links. I receive continuity on all wires from the alternator plug to the wire end. The only wires I can't check are the wires at the plug after the alternated harness which are green/red and green/orange. I am not sure where these wires terminate... Looks to be behind the firewall. I checked every single fuse and none are blown. Any help with this would be great. I know it is charging even with all components on... I hate the battery light being on and I want it to be perfect. Thanks for any help....
Jason

Mod (Ret.)

The light green/red and light green/orange connect to the PCM (Powertrain Control Module) which is located on the passenger side firewall.

The orange/blue and black/orange connect to the battery positive post, via the three fusible links that you have already checked.

At this point, I suspect a bad generator (alternator, although the Ford schematics refer to it as a generator) yet again, or the wiring to the PCM, or bad/loose connectors at the PCM. It is C175B; there are 70 pins within this connector.

I'd check it's connectivity, make sure it is dry and tight. Each PCM connector has a "sliding locking tab"; slide it down to remove the connector to do the continuity test with the ohm meter. Make sure the connectors are clean. It would not hurt to use the ohm meter to check between the alternator (generator) to this connector; pin 22 of connector C175B is the light green/red (generator communication lead), and pin 23 is the light green/orange (generator monitor).

I cannot rule out the PCM itself, but the fact that the truck runs fine makes me think (and hope) it's just a bad wire between these two to the connector, or a bad connection of C175B to the PCM.

There are other connectors to the PCM, but C175B is one of two that have 70 pins, theres another with only 50 pins (smaller).

And hats off to you for doing your homework! Not many will have known to check the fusible links, and identify color codes to post here.

Glad to have you aboard!

Scottwizer

Thank you for the kind welcome. I will try checking your suggestions, I hope that is it and not a PCM. Also I did perform the dealer test, by holding down the trip/reset button while I started it and the voltage readout came back with 14.1 Volts while running with AC and radio on... It did fluctuate at idle a little but stayed constant at a cruising speed. I wasn't sure if this tells me that PCM functionality is good... Due to the fact that I am not sure that the cluster gets it's readings from the PCM. Thanks again. I will post my findings as hopefully it may help others.
Jason

Scottwizer

Ok... Just check for continuity to the PCM connector. It has good continuity and extremely low resistance. The ohms were at .8 on both wires. Any ideas on what next? Or is a PCM, or quite possibly my third bad alternator..... Thanks Jason.

Mod (Ret.)

Your alternator output readings are within spec; this indicates the alternator is charging.

Why the dash generator light is on indicating otherwise, I'm at a loss. What does the charging "guage" show in the cluster? Mine always has the needle just slightly past the middle point at all times. The fact that you are seeing 14 volts at your meter tells me all is good with the alternator. I'll assume the headlights stay at a constant brightness; they do not increase when the engine is rev'd, or dim at idle with this output.

I suspect something within the PCM is thinking the alternator is not putting out a high enough charge (pin 23, generator monitor) and causing the idiot light to illuminate. This may be one for the Ford shop, in order to test the PCM.

Let us know what you find out.

Scottwizer

I am at a loss I guess. The volt Gage is just above half, where it always has run since new... Anyway, I am going to either put it on a scanner or take it to the dealer when i get a chance. I hate to throw more parts at it when I don't know exactly what it is....

rick-from-upper-mich

I wanted to share this information:
My 2008 F-150 had charge indicator that stayed on but charged. I replaced the battery and alternator but the light did not go out. It wasn't a big problem because it always started. After a few months I noticed the alternator did not charge after starting until I reved the engine to excite it. Today I switched the alternator with another I knew was good. Same problem.
I checked the 3-wire plug on the alternator. One wire went to battery voltage and two went to the ecm. I removed the center plug from the ecm and found the corresponding pins. When facing the plug the pins were on the second row down, and the fifth and sixth pin from the right. The center wire on the alternator plug was "open circut" to the fifth pin on the ecm plug. This wire supplies a pulse to the alternator every 5 seconds to set charge rate. Started unwrapping harness from the alternator and when I got to the battery tray I found the problem. In between the 2 white clips that hold the harness to the battery tray, where two harnesses wye off, there was an obvious break in the wire. The insulation was still intact but bent at a sharp 90 deg. anlge and copper inside was broken. Too much stress. Crimped on a butt splice and everything is perfect again. This may have been an intermittent problem in the begining but it got worse.
I'm sharing this because I found a lot of similar problems posted but I couldn't find the fix. Hope this helps someone out.

PerryB

It's amazing what you can find with a little systematic investigation. Think what you'd have spent paying the typical dealership to diagnose and fix this problem.

F150Torqued

I'd like to extend a positive thanks @rick-from-upper-mich for posting a useful update to five year old dead thread.