On my way home from Lake Mary a few weeks ago, my Ranger started behaving irratically; losing power, resuming power, running bad, running okay. This was on the interstate at about 55 mph.
When I got to my exit, it all out died. Zip, nada, gone. I put the hazard lights on, but there was no power. When the power sloooowly resurrected, the hazard lights were there.

Well, a neighbor got it jumpstarted for me, and I was able to drive it fifty yards before it died again. Since then, it's been towed home, where I've been waiting for it to magically 'fix itself'. When that didn't work, I'd put a battery charger on it, and then, afterwards, watch it shut down again when I tried the ignition.
When the power resumed, even using the horn would shut it down.

I haven't looked at the obvious culprit (crank position sensor) just yet. But what would drain the battery so badly?

 

No, CKP(crank position) sensor generates it's own power, and in any case CKP wouldn't be even close to causing your problem.

Your issue reads like alternator has failed to provide enough power to run the system.
Put voltmeter on battery, after charging it, you should see 12.2v or higher.
Start engine, you should now see 14.5v or higher at battery.
That's the alternator working.
If still at 12v then alternator is not working.
Remove it and have it tested.

You have been running off battery power and when that power is gone so is spark and fuel so engine dies and lights and "horn" don't work either.


And just as a heads up your battery may be bad now, or at least on it's way out, draining a car battery is the worst think that can happen to it, they are made for quick discharge(starting engine), the quick recharge(14volts for 2 minutes), any long term drains shortens their life.

 

Thanks, RonD!

The alternator (I replaced 10 years ago) sounds like the problem if there's no power, or if the battery's being drained. But would it just 'shut everything' down while the Ranger's in motion down the interstate?
And would it be the issue if I turned on the ignition, and, again, everything just dies?

 

Most newer cars use relays to power everything, when voltage gets low enough then relay 'opens' and all power is cut instantly to everything on that relay.
So yes, it could just die, when for example the EEC relay opens, it powers fuel injectors and computer.

Once power drain stops battery will regain some voltage over time, so when key is on some systems come back up(relays close), but as soon as you add any extra power drain the relays open again, so they go dead.

Also remove battery cables and clean them, this type of thing causes corrosion on the terminals.

 

I replaced the alternator with one from a '91 Ford Mustang, 4 cyl 2.3l.
Identical engines!
Still nothing...where's this EEC relay?

 

What is the battery voltage before you start the engine?
And what is the battery voltage after you start the engine?

Another alternator won't fix a wiring problem.

Battery should have 12.3 to 12.8volts just sitting
With engine running you need at least 1.1 volt higher, 2 volts for a few minutes then a drop to 1 volt higher
SO engine running you would see 13.4 to 13.8volts, 14.5 volts right after starting then it would drop to 13.6volts

If you are not getting the 13+ volts then alternator power is either not reaching the battery or alternator is turned off, yes alternators have an on/off switch, it is internal, it needs to get 12v from the battery to turn on and stay on.

 

Checked the battery, and it has 12.7 volts without the engine running.
Everything powers on fine; Horn, cab light, headlights, radio...
The moment I turn to key to crank it- DEAD!

Although, the fuel pump (Which isn't even a year old) gauge started giving me a bad read a little more than a month ago; When I cranked the engine, the needle would swing quickly to FULL, before slowly sliding back to where the actual fuel level was.

If this is a fuel pump inertia switch issue, would it keep the engine from starting? I only ask because the erratic behavior before it died sounds similar to other problems I saw online.

Still mystified here...

 

In most vehicles when key is turned to START position the power is cut to accessories, most people don't notice it or think it's the voltage drop from starter motor drawing amps.
This is done to give starter all the power available.

So your issue may be that starter is not being "told" to start.
On a '90 there are four possibilities.
If you have a manual trans the there will be a Clutch Switch, clutch pedal must be all the way down to engage the switch, which then passes the 12v from the key switch to the starter Solenoid/relay which closes and pass battery power directly to starter motor.

If you have an automatic trans it will have an NSS(neutral safety switch) on the transmission shift linkage, it is like the clutch switch and will only pass the 12v from the key if trans is in Park or Neutral, these can break or be out of adjustment.
NSS also does reverse lights on most models.

Ignition switch is located under the steering column, when you turn the key the cylinder rotation pushes a bar that slides the ignition switch into different positions making and breaking contacts to pass power or cut power to different systems.

Starter motor or its wire is bad.

On the fender near the battery you will see the starter relay, follow the large red battery cable, it will go to the starter relay.
You will see a smaller connection on the starter relay, a post, with the label "S".
If that post gets 12volts then starter relay will close and if the wires and starter are good then engine will crank, start turning over.

MAKE SURE trans is in NEUTRAL OR PARK!!!!!

Use a jumper wire from battery positive to the "S" post, have key on so engine will start if other systems are OK.

If you don't hear the starter relay click when you apply 12v to "S" post the starter relay is bad, so I guess there are five things, lol.
If starter works then that part is OK and you will need to find the break in the "S" wire.
If starter doesn't work then bad wire or connection.


Gas gauge going to Full means a bad ground in most cases, or a loose connector.
1990 Ford gas gauge reads full if OHMs are higher than 160, no connection = infinite OHMs , super high number, a short would be 0 OHMs and gauge would be below Empty

 

*sigh*!

I replaced the starter in a convenient store parking lot seven months ago?
But I can't understand why the starter would cause the truck to act like it was running low on fuel/power while it was in motion down the interstate.

 

It wouldn't, I based my answer on the last question, "The moment I turn to key to crank it- DEAD!"
I assume you mean starter is not turning and other systems lose power.

Locating the problem of why the starter is not working may led you to the reason other systems are effected.

On a '90s vehicle some wiring insulation will be getting brittle and some connectors can get moisture inside, and your symptoms do read like a wiring related problem.

One thing that is often left disconnected when engine work is done is the ground strap from engine to firewall, usually from drivers side head to firewall.
This is one of the main grounds for all the cab systems, make sure it is hooked up and clean and tight.

Fuel pump power comes from a fuse in the "engines power box", power goes from this fuse to the Fuel Pump Relay, the computer controls this relay, when relay is Closed by the computer the power goes from the relay to the inertia switch, then from the inertia switch to the fuel pump.
Inertia switches do not become intermittent, but the connectors can get corroded.
Also near the inertia switch is the ground point for the fuel pump and fuel gauge, if this gets loose or corroded then pump and gauge would be effected.

The grounds wires should be black and orange.

Inertia switch wires are Dark green/yellow IN from relay, Pink/Black OUT to fuel pump.

 

When I turn the key, I hear the fuel pump engage ("Whiiirrrrr...dunt").
I'm going to take a rubber mallet to the starter (Assuming even newer ones can go bad), and look at the fuel pump inertia switch under the dash, next to the transmission hump.
And will also look at the fuses in the "engines power box". If the fuse for the fuel pump is going bad, then fuel isn't being sent to the engine...?

 

If you hear fuel pump for 2 seconds when key is turned on then everything is working as it should, EEC relay, fuel pump relay, inertia switch and fuel pump.
Although i would still check the ground wires near inertia switch.
Fuses don't "start going bad", they are either good or blown.

I would jump the starter solenoid/relay(as said above) on the fender well first, that's the usual issue if starter is fairly new

 

After I got the Ranger back to the house, I DID clean the posts and battery terminals.
Then I got a little more when I tried to crank it (Before it everything shut down), in the form of a simple 'click' from the starter solenoid

 

UPDATE:

After banging on the starter a few times, and some adjustment to the positive battery terminal, I stripped the end of negative cable, cleaned it, installed a new positive cable, and installed two new terminals.
VROOOOM!!
The Beast cranked up! Just some dirty connections.
"And the recipient of this year's Darwin Award is..."

 

Thanks for the update, good work