2002 4.0L charging problems

[Glen66]

I've read a lot of posts on this and other sites regarding testing and troubleshooting my Ranger's charging problems but I'm stumped.

What happened and what I did:

The truck sat for a few weeks and when I last drove it, the battery light was on and the voltmeter was way below normal. I turned around and went home.
Battery voltage and alternator output were around 8 volts. I assumed it was the alternator which was 3 years old and 3 months out of warranty. To Napa's credit, they replaced it at not charge. Still no go.

I checked the cables, checked for voltage at the voltage regulator (the ISA 3 prong connector) and everything seems ok. I check just about every fuse in both panels and all were OK. I charged the battery and tried but it ran down and nearly stalled within minutes of starting the truck. I tried the battery out of my escape and it seems fine for a few minutes but then the battery light started to flicker on and the truck acted as if it was going to stall. The voltmeter on the dash was reading low. I put that battery back in the Escape before I ran it down.

While the truck was running on the Escape's battery, the alternator was only putting out 11.75 volts whereas I think it should be in the 14 V range. The Ranger's battery is exactly one year old and the Escape's is about 3 years.

My next step is to try to find a shop that can bench test the alternator and the battery.

Is it possible that the alternator is putting out 0 zero voltage and the reading I get by measuring the volts from the alternator post to ground is just the battery voltage being fed back to the post?

Based on my experience (although limited) a fully charged battery should run the truck for at least 30 minutes.

Is it possible that the battery AND the new alternator are shot?

 

[RonD]

I think you are over thinking things

Ford Alternator wiring is pretty simple.

First check battery voltage, say it is 12.2volts.
Make sure key is off for next tests

On the alternator there will be the B+ terminal, it can have 1 or 2 wires connected, usually Black with orange stripe wire, this connects directly to battery positive via a Fusible link(or fuse), these are the larger wires on a terminal with a stud and nut.
Test this terminal/wire, it should have 12.2volts(see above), or whatever you battery voltage was when you tested it, this means connection/fuse is good.
B+ terminal is the alternators output voltage to charge and maintain battery while engine is running.

Then alternator will have a 2 or 3 wire connector, this is the internal voltage regulator connection, unplug it from alternator.
There will be a Yellow wire, test it, it should have 12.2volts as well(battery voltage), it also uses a fusible link(or fuse) and is connected directly to battery.
Voltage regulator uses this wire to monitor battery voltage.
There will be a Light Green wire, test it, should be 0volts
Now turn on the key, engine off
Test Green wire again, it should now have 12.2volts(battery voltage), with key on, this is a very important wire, it is the ON/OFF switch for alternator.
It is the battery light circuit, the green wire comes from the battery light in the dash, it has a fuse in the cab fuse box, 7.5amp.
When key is on voltage from this fuse goes to one side of battery light bulb, the green wire is connected to the other side, and goes to voltage regulator in alternator.
With key on, engine off, the alternator/regulator is a Ground, so battery light bulb is on, 12volts on one side, 0 volts the other side, means power is passing thru the bulb so it lights up.
When engine is started and alternator is producing voltage, say 14volts, then one side of bulb has 14volts and the other side has 14volts, so no power is passing thru the bulb and it is off.

Battery light being on when engine is running means two things, first thing is that the 7.5amp fuse is good, second thing is that the alternator is producing less voltage than the battery has, if battery was 12volts and alternator was producing 0volts to 10volts then Battery Light would be on because power is passing thru the bulb with 12volts on one side and say 5volts on the other side.

So alternator wiring is pretty simple, and simple to test.
B+ should have battery voltage, 24/7, if not then fuse or fusible link is blown
Yellow wire should have battery voltage, 24/7, if not then fuse or fusible link is blown
Green wire should have 0volt with key off, and battery voltage with key on.

If all voltages check out then replace alternator.

If regulator has 3 wires then 3rd wire will be a White wire loop, it is also important so check that it is not loose or frayed, it connects alternator and regulator together.
This was made an internal connections in later years.

To check battery hook up voltage meter and leave it connected, write down voltage, then check it in 30min. and then an hour, if it is steadily dropping then battery probably has an internal short so is self draining, this can be caused by alternator overcharging the battery or just the battery's age, 5 to 7 years is normal battery life.

And an alternator can fail in a way that draws extra power from battery, so even after charging battery engine can die in a few minutes when battery has drain down.
An alternator is basically an electric motor used in reverse, so it can "act like" an electric motor and draw power FROM battery vs producing power FOR battery.
This is also why alternator needs an ON/OFF switch, the green wire, if left ON with key off it would drain the battery.


A Fusible link is a "slow blow" fuse, it is a 6" to 8" wire with a smaller gauge than "protected wire", it can absorb voltage/amp spikes and heat up a bit without blowing like a fuse would, so these are popular to use with alternators since amps often change quickly.
Fusible link can get hot enough to separate(blow), it is made to do this without dripping hot insulation or melted metal that could cause a fire.
Alternator fusible links will located on the Battery end of the alternator wiring.

 

[Glen66]

Thank you RonD for your detailed response. I'd already done some of the procedures you described but your explanations of how things work and why will be very helpful.

My next step is to get the alternator tested so I'll Know for sure.