So...the truck just quit in the middle of the road downtown 4thbof of July weekend. Wiggling the wiring harness got it going again a few miles, but the 20A fuel pump blade fuse blew. And blew again and again. Matter of fact, it's still blowing. I've run a circuit breaker in there are not found any shorts in the wires, when I take out the fuel pump relay the 20A blade fuse does not pop. Getting a reading of 12v and 10.6v across the fuel pump relay receptacle with key on and fuse intact. This happens also when all wiring harnesses are disconnected too. My only guess is its in the power distribution fuse block but I also have a feeling that if I drop $200 on a replacement the fuse will keep blowing.
Like I said, the wiring harnesses have all been disconnected, I replaced the fuel pump and sending unit too.

 

Any help?

 

Welcome to the forum

What year Ranger and what engine?


Fuel Pump power runs like this

Battery--------20amp fuse----FP Relay------------inertia switch--------------------Fuel pump---Ground

20amp fuse has 12v all the time
When FP Relay closes(key on) that 12v is sent to inertia switch and to fuel pump

Inertia switch is in passenger side of cab under glove box area or in passenger side kick panel, depending on the year
Disconnect inertia switch and see if load drops

A fuse blows because too many amps are being pulled thru it, in this case more than 20amps, this is often caused by a wire shorted to Ground, so full battery amps can be pulled

If incoming volts is 12v and you are see 10.6v on load(output) then that is close to 40amp draw, so fuse should blow.

A bad Ground on fuel pump can also cause larger amp draw


If you post year and engine I will provide wiring diagram with the color of the wires to check

 

Thank you. I thought it was in the signature.
1993 4.0 V6

The fuse blows with the inertia switch unplugged, and the fuel pump unhooked. And I mean the entire wiring harness leading to the rear unplugged up in the engine compartment. A visual check shows no frayed wires and I used a FLIR infrared camera to look for shorts to ground with the key on and 20A circuit breaker in but saw nothing, maybe the sensors on the exhaust showed hot for some reason?

 

O2 sensors have heaters built in

So engine fuse box or wire to computer or inertia switch has short.
You will need to lift up engine fuse box
There will be a TWO dark Green/yellow stripe wires connected to the under side of Fuel Pump Relay socket
One goes to inertia switch
The other goes over to the computer(PCM), it changes to a Light Blue/orange stripe wire, computer monitors if fuel pump relay is closed, i.e. 12volts on this wire

One of those is shorted, could be at computer end
Computer is drivers side engine bay down next to firewall where it meets inner fender
60 wire connector on it

1994 will be the same as 1993

 

Thank you, I am going to check the inertia switch with the multimeter now

 

Ok it might have sounded like I know what I'm talking about up until this point, but I unplugged the inertia switch and the fuse blows. I followed the wires and located the light blue/orange one which goes into the engine block and disappears. The multimeter on the wiring harness pin of lt blue/orange wire on supply side didn't show any voltage with key on. I'm going to check the pcm module for shorts visually, not sure how to check it with the meter. If I remove the cover, can I use the meter on the powerr wires to see if there is more than 12v draw?

 

You disconnect battery first
Then disconnect the 60 pin/wire connector by taking out the bolt in the center, pull connector off

Look here: Ford EEC-IV

That has the pin numbers for the connector
#8 is the one you want to test, should see light Blue wire connected to it, from the Relay

You should be using OHMs when looking for a short
I assume you have a Multi-meter, so tests voltage and has an OHM setting
Set it at 200 ohms if it has that setting, or the lowest ohm setting it does have.

Touch the meters probes together, should show 0 ohms, thats a Dead Short, what you do not want to see when testing a wire.
With inertia switch and computer unplugged
Put one of the meters probes on a Ground, any bare metal in engine bay
Put other probe in FP relays socket slot that goes to inertia switch
If you see 0 OHMS or close to it then you have a short to Ground

If its high OHMs then plug computer connector in and test again, if now at 0 ohms the short is in the computer, unlikely but could happen, I would just cut that wire

Anyway you have narrowed down the short to relay socket or one of those two wires so test away

 

OHMs is the resistance in a device or a wire

Electricity flows at the rate of the resistance and the draw of the device being powered

The AMPs are the rate of that flow

A toaster works by having little resistance so high flow of AMPs, and the coils of wires inside get very hot because of this.

A wire for the most part should have no or very very little resistance like a toaster coil
So it you test both ends of a wire it should show 0 OHMs, no resistance

In a vehicle the Battery Negative is connected to the metal of the engine, and body and frame.
Most of the wires carry the Battery Positive(12v) power needed to power devices.
If one of these wires is chafed or otherwise damaged it can come into contact with bare metal which is the Negative Battery terminal, so it is like putting a wire between Positive and Negative terminals on the Battery, and that wire will get very hot, like toaster wire does, because there is no resistance, low, 0, OHMs.

Fuses are just smaller gauge or softer metal wires inside a case, so instead of the wire becoming like a Toaster wire the fuse heats up and separates, saving the wire with the short.

Thats where OHM meters come in, a 12v wire should always have resistance, so no direct connection to a Ground/battery negative, never 0 OHMs

Electric motors like heater fans or fuel pumps can be tricky to test with ohm meter because like a toaster they have to have low resistance to work, but still needs to be above 0 ohms

 

Ohms! That makes a difference. A reading of 1 all around would indicate it's the fuse block, right?

 

All around what?
And yes 1.0 OHM is virtually no resistance so a short
If fuse connector measures 1 ohm to ground/metal then it has the short

However, ignition working coils measure .8-1.8ohms, but they do get hot and are not connected full time, lol.

 

I meant with the inertia switch plugged in read 1 and with the computer plugged it read 1. No zero readings.
Is this the kind of situation where I can hook up a test light between negative cable and terminal on the battery and unplug things until the light goes out?

 

No, you would hook up the test light to the two fuse terminals, and then try to make it go out.

The test light works as a fuse but HAS resistance, so like a simple light circuit.
One terminal of the fuse will have Battery Positive connection
Other terminal is called the Load terminal, it would normally connect to a device that needs power(12v) but in this case it is hooked to a Ground(battery negative) so fuse would blow, light bulb just lights up
When you remove the Ground/short light will go out

But in your case the relay would be open so may not work, so connect test light to 20 amp Fuse 12v terminal and then other end to the Relay socket terminal for inertia/computer wires, the ones with the short

 

OK sounds simple enough. If the light stays on then the fuse block has the short?

 

Deleted

 

I cut the wire from the relay to the computer and the fuse does not blow any more.
The truck doesn't start though. I'm going to try to jump the battery and see if that makes a difference.

 

That wire is just to monitor voltage doesn't effect operation

The OTHER wire to the computer closes the relay so WOULD effect operation, it is Light Blue/orange stripe all the way to pin 22 on computer

So you cut the Dark Green wire at the relay end right?

 

Dark green on the relay socket, yes. And the light blue/orange on the computer end, so wrong one huh?

 

Pin 8 in the computer connector is monitor

It changes from green to blue then back to green, so green at both ends

Light Blue/orange stripe wire turns relay on/off, pin 22 at computer, same color at both ends

 

The fuse no longer blows. However, there is a draw on the battery that kills it and the truck won't start. I don't hear the fuel pump with the key on.
Hooking the fuel pump direct to the 20A fuse turns it on, but not off.

 

The fuel pump only runs for 2 seconds when key is first turned on, so you should not hear it after that, even with key on
Computer turns fuel pump on full time when engine RPMs are above 400

Which wire had the short?

 

The light blue /orange one at the PCM.
I hooked it back up directly to the pump relay but that made no difference, the truck won't start. It's as if there isn't any power to turn the belts.

 

The light Blue/orange stripe wire could not cause a short that blew that fuse,
first, it is a Ground wire, so short to ground would just cause fuel pump to come on(IF KEY WAS ON)
second, it doesn't get power from that fuse, its 12v comes from 30amp EEC/PCM fuse

Are you sure you are working on the FP relay and not the EEC/PCM relay

 

I'm working on the 2nd large relay in the center.
However....I hooked up a light to the negative battery terminal and cable and there's a short somewhere. I've unhooked all the fuses, the wiring harnesses and even the positive battery cables individually and am still seeing the light on. At least the 20A fuse isn't blowing any more lol

 

I started randomly checking pins in the wiring harnesses and I found 0 ohms at th the black/white wire pin and the red/blue wire pin. They lead back to one of the large fuses (not relays). I'm going to see I can find it on the diagrams you sent.