I'm good with mechanical but diagnostics is not by strongest talent. Driving at night 4.0 auto 4wd 97 Ranger and it shut off while at speed about 45 or so. Won't restart. I cannot hear the fuel pump running at ignition on. Starting fluid gets it to kick but won't stay running. Inertia switch isn't tripped but bypassed it anyway. No effect. Tried checking codes and the tool says link error. Its a relatively new tool and works on my other vehicle.
in tje past few days I hace noticed some odd things going on. Battery lamp coming on then going out and intermittently instrument panel bulbs will seem to go out in different locations in fuel guage then the speedo but on restart again all are illuminated.
Trying to eliminate anything possible before taking the bed off for a fuel pump. Any ideas? The link error on the code reader is pretty curious.
thanks much.

UPDATE I found the 30 amp PCM fuse blown. Didn't have any but wondering if the unusual instance of the battery light coming on while running is an indication of an internal short in the alternator. At the time because the alternator showed charging on yhe gauge I shut it off and unpluged the alternator and plugged back in. The light was off and stayed off but still I wonder. No fuses for replacement so it has to wait till after work. Any ideas? Truck runs strong but thats why no fuel pump sound. No PCM power. Just don't know why the fuse blew.

 

Welcome to the forum

The alternator, charging circuit, has no direct connection to PCM Fuse or PCM Relay, so it would be a separate issue if ONLY the PCM fuse failed, if several fuses failed then there could be a reason connected to alternator, charging circuit

A fuse blows when there is no or low resistance in the circuit, in this case limited resistance allowed close to 30amps to flow between battery positive and negative terminals on the battery and the 30amp fuse melted/blew

Resistance is the "throttle" in electrical circuits, a gasoline engine with throttle plate removed would start and go to MAX RPM and then BLOW
So engines and electrical circuits both need "throttles" to limit MAXIMUM flow

PCM fuse and relay powers several devices thru several wires, the fuse is 30 amp, but the relay is rated as 40amp, so fuse should blow before relay contacts melt, should being the key word
I might also get a new PCM relay

Wires of course can "short to ground", allowing unlimited amp flow because there is no resistance, no throttle
Devices can also short internally

Below are the wiring diagrams for 1997 4.0l
PCM Fuse/Relay is at top of 1st page
2 Red wires go out to all the devices it powers with Key on

I would use an OHM Meter before replacing the 30 amp fuse or get several 30amp fuses
OHM meter Measures resistance in a circuit, 0 ohms is no resistance so would be bad, somewhere a wire or device is shorted to ground

PCM relay should be Mini-type, slots look like this: https://www.gtsparkplugs.com/images/...iring-840x.jpg
30 and 87 carry the 30amp load when connected
Pull out the PCM relay
Put in the PCM fuse, Key OFF
Use volt meter to test which slot has 12volts, 30 or 87
Usually 30 but can be either one
The other slot, that doesn't have 12v, is the slot the 2 Red wires are connected to, and this is where you would hook up OHM meter and then other probe goes to battery Negative, or any good ground
If there is a "dead short", fully grounded, then new 30 fuse would blow instantly(key on), if its lower OHMs then it would take some time to blow
With OHM Meter connected and visible move wiring harness's around see if OHMs change indicating an issue in the harness or a connector

You can also unplug devices using the Red wire(PCM Fuse), one at a time, to see if OHMs change, they often will since each device will have its own unique resistance/amp draw, but you need to access if the resistance is out of spec for that device

 

Awesome information
thans so much Ron. Lots of information and very in-depth response. lots to check and minimal time working seven days a week but I have a lot of good information. I was afraid the pump had failed and was dreading the idea of pulling the bed off but wiring is just as bad.
I appreciate the time you spent in reply and will update with what I find. I will get fuses tonight and another relay. I would have thought yhe PCM would control the fuel pump relay as well. When I turn the key I can hear a relay click from the box but can't tell which one it is. I may have to eliminate the heater relay to determine what is clicking. Thanks again. Appreciate you all being there.

 

If you follow the red wires from PCM relay you will find that it does indeed provide 12v to the Fuel Pump relay's coil, so computer could not turn on fuel pump unless its relay had 12v, but.....................
Computer(PCM) is not on anyways because PCM fuse also powers the PCM which activates the fuel pump relay by grounding its coil

Fuel pumps themselves are not likely to fail while running, electric motors tend to run once started, their failure mode is not starting up when they fail, like starter motors, they work fine IF they start to spin, but one day they just won't start to spin, no warning usually, just "worked fine yesterday but won't work/start today"
Fuel pumps fail that way as well, one day they just won't start up

And your OBD2 reader couldn't connect to read codes because the PCM was off, no power because fuse was blown
And that fuse blowing was why engine stalled while driving

 

Further diagnosis
forgot to add the HEGO fuse under the hood also has no power either leg. I assume it may be powered off the PCM fuse as well. Physically the fuse looks good but neither side is hot. Not sure if it is supposed to be.

 

Yes, 15amp O2 heater(HEGO) fuse is powered by the PCM fuse, its on the diagrams, 3rd page top

 

Further diagnosis
forgot to add the HEGO fuse under the hood also has no power either leg. I assume it may be powered off the PCM fuse as well. Physically the fuse looks good but neither side is hot. Not sure if it is supposed to be.

 

Diagnostics
I pulled the PCM relay and installed the fuse
87 has 12.6 volts so 30 was open. Turned the key to run and checked ohms. 46 ohms. Disturbing wiring had no effect. Disconnected get and otjer items with no effect. Disconnected the alternator and the ohms went to .5
is it possible the alternator is shorted and feeding back? I can't locate the red wire you referenced

 

Alternator has 3 wires connected to the vehicles 12volt system

Larger wire on B+ (stud/nut terminal) runs directly to Battery Positive cable on the Starter relay(solenoid)

The 3 wire plug on alternator has a yellow wire it runs to a 15amp fuse in engine bay fuse box, Fuse 6
White wire is just looped back to single tab terminal on alternator
Green wire runs into the cab and connects to the Battery Light bulb in the cluster

Not sure how these could effect PCM fuse and not all the other fuses?
Is fuse 6 near the PCM fuse?
I don't have a 1997 Ranger to look at

 

Found it.....sorta
Ron I have to give many thanks for your assistance. Not only was your diagnosis spot on but your procedure for isolating the problem was straight forward and practical. The frustration level was intense but I hung in and followed the procedure. Connected the alternator again as well as all the other items I disconnected. I still had the PCM disconnected so the large harness behind the engine was kinda loose. I grabed the harness in the middle of the firewall and lifted it repeatedly and the meter went stupid. I released the harness and the meter went to zero and I could not get it to go back to 46 ohms. Even though I know it is in that harness I could not make it show itself again. $ It needs some attention back there but with the meter reading zero I installed the relay and started the truck. I know where to look but its in the worst possible place. Still I am thankful that you took the time to share your knowledge with me. The solution was not out of reach once I had the tools you provided me with. The repair does not seem insurmountable now and once again the Ranger lives. Opening the harness behind the engine or removing it will be a genuine task but at least I have the idea the repair is achievable. Thanks to you and The Ranger Forum.