Sometimes the battery wires and grounds get corroded and won't carry the current properly. Give them a hard look. The corrosion can spread up underneath the insulation on the wires, so if you see signs go ahead and peel it back a bit.

A voltage reading on the batteries and at the dash might show something. Check it engine on and off. Keep in mind when starting cold the glow plugs will be on for a while, and that will suck down the voltage almost a full volt until they turn off.

 

 

Replacing the alternator... without first fully charging the batteries (with AC charger), dissipating the surface charge, and then load testing the batteries to verify ampacity, or replacing the batteries altogether... is a sure fire way to replace the alternator again.

A new alternator cannot charge depleted batteries. Especially a new single 110 amp alternator that only puts out 50 amps at idle.

 

 

At least you have a battery light. I've now had two alternators in my e99 and I never get a light when they go out. I think the early PCM don't have them . Just your gauge to show voltage in the system.

 

If your batteries are 5ish years old. It's time. clean every connection from A to B and you should see drastic results with a fresh bank. I roll with Motorcrafts 850cca. I live in frozen hell for 5 months. I don't plug in and I have had good luck.

Denny

 

 

I believe the PCM monitors will detect any weak link in the system. Your Battery light is telling you that someone in the circle has left the building. May be Alt, maybe a battery bank? I think you'll find when you match batteries and clean all pos and grounds all the way to the starter...........You'll be grinning for years.

Denny

 

In the Underhood Distribution Box there is a fuse for that circuit. The Third wire on the Alt Plug controls that circuit. The PCM does nothing with the Battery Lamp - Independent circuit - -That's where your issue is.

 

 

In some vehicles this might be true, especially newer ones. The trouble is, there is nothing in the OP's thread, signature, or user profile that indicates what year the OP is working with. The user profile says 2000 F-150, but one has to assume if glow plugs are involved, and the post is placed here, then maybe he has a 7.3L??

For a 2000 7.3L, the PCM is out of the picture in terms of the battery indicator light being on or off. The instrument cluster is in the picture, which is where the 470 ohm resistor is located. See green box in photo below:



It is rare, if not unheard of, for there to be a problem with this resistor.

I think the OP is on the right track looking at wiring. First wire to look into is that Light Green wire with the Red stripe, Circuit 904, aka the "I" circuit. If it is on when the engine is running, that suggests the wire could be shorted to ground, which is exactly what the OP already plans to investigate.

With the key in the ignition set to Run, and the engine off, the indicator light should be ON, and there should be between 1 to 3 volts measured between that LG/R wire at the alternator harness (harness side, not alternator side) and ground.

If instead you are finding zero voltage, there is an open in that wire.

If you are finding 12+ volts (or whatever volts your batteries are currently putting out) then take away the meter, plug the harness back into the alternator, and start the truck, turn on the headlights, and run the blower motor up to high. If you have a high idle tune, then set that up, or have someone step on the accelerator pedal to bring the engine to 2000 rpm steady state and hold it there, which will bring the alternator to 6,000 rpm full tilt maximum electrical output. The headlights and blower motor fan on High are necessary to create a load to induce the alternator to put out the current to satisfy the load, for purposes of the next test, which is the difference in voltage between the batteries and the alternator at full tilt.

Now, with all that blaring away, go back under the hood and measure the voltage drop between the Black with Orange Stripe B+ alternator output terminal, and the positive battery post. This can be done by attaching the negative lead of the meter to the alternator B+ terminal, and the positive lead of the meter to the positive battery terminal.

If the voltage drop is less than 0.5 volts, the problem is inside the alternator.

If the voltage drop is greater than 0.5 volts, then there might be some corrosion in that Black with Orange Stripe wire, or maybe one of the two fusible links is fried.

Now, going back to the Light Green/Red wire... what if the battery indicator light is OFF when the key is on, and the engine is OFF? Try grounding the LG/R wire between the harness at the alternator and ground. Now did the light go on? Then that would be another indication to change the alternator. Or, you can try changing the voltage regulator instead, because the fault is more likely in the small circuit board of the voltage regulator.

For additional information see Diagnostic PinPoint Tests J and L, Charging System, Section 414-00-28, Ford Workshop Manual, F-Series SuperDuty

 

Frick Y2, why not do the redneck approach and see if a matched battery and clean connections do it.

Sounds pretty straight forward to me. Then we can get out the tasers, lazers and Phazers,


Denny

 

The bulb is lit. It's not burned out. JS

 

Is your truck equiped with dual alternators? My old work truck did and had the same problem as yours, mine turned out to be a damaged fuseable link, leading from the secondary alternator to battery.