Bad fuel pump relay?
 

Hey guys i am trying to diagnose why fuel pump won't turn on. If I apply 12v directly to pump out of tank it turns on. But only getting 6v when I put the meter on the plug that connects at top of the unit. I get 6v to and from the inertia switch. Could it be the relay is only sending half the needed voltage or is something else wrong. Please help.

What year?

The 5 to 7volts has no amps, it is from the computer.

The Computer turns on the fuel pump relay and then monitors if it did come on, that circuit will bleed 5 to 7 volts when key is on but fuel pump power is off.
But it has no amps so can't run fuel pump.
And yes that computer wire, with the 5-7volts, is usually connected to one terminal of the inertia switch


When you turn on the key the computer Boots up and GROUNDS the fuel pump relay for 2 seconds, and 2 seconds ONLY.
Computer will not turn on fuel pump relay again until Engine RPMs are above 400, engine started.

But you can cycle the key off and on as much as you want when testing, it won't hurt anything.

(If wondering why it only runs for 2 seconds, it is a safety issue, it prevents an electric fuel pump from continuing to pump out fuel from a broken fuel line, i.e. line breaks engine stalls, RPMs under 400 power is cut to fuel pump)


Power for fuel pump goes like this

Battery----30amp fuse-------Fuel pump Relay----------inertia switch------------fuel pump

Fuel pump relay is not expensive, $5-$8

Inertia switch can be reset with button on the top............but once inertia switch OPENS, it must be replaced, reset button is there so vehicle can be moved after an accident, it will never fully "reset".

Relays have 2 parts
Load, in this case the two Load contacts touch together to send 30amp 12volt power out to fuel pump
Coil, this is the low amp side, when coil has 12volts and a Ground it becomes and electro-magnet and pulls the Load contacts together and holds them together until coils 12volts or Ground is cut

Fuel pump relays "coil" gets power from EEC relay, when it closes(key on) EEC relay sends power(12volts) to EEC(computer), fuel injectors, spark system and fuel pump relays coil.
So Computer can GROUND the other side of the coil and relay will close and send power to inertia switch, and then fuel pump


If you want to test relays socket pull out the relay
Key off
test each slot for 12volts, one will have 12volts, thats power from the 30amp fuse, connected 24/7(all the time)
Now turn on the key
test slots again, another slot should now have 12volts, this is power from the EEC relay, the coil power

Thank you for that detailed answer it solidified a lot of different info I have acquired but wasn't sure about. I will try to test the relays and report back. The truck is a 1990 Ranger 2.9L xlt regular cab 2 wheel drive.

ok this is what I have gotten.
B/Y wire 12v constant
Key off all other wires red 0.
Key on
Red reads 12v
LB/O reads 5.59 which is also the wire that goes to inertia switch and to pump also what I get on multi meter at both locations.
I bought a new relay and swapped it with the fuel relay and still nothing. I then swapped the new on into the EEC relay thinking maybe it was bad and still same readings.
Not sure what to do please any help would be appreciated.
http://asheepdogproject.info/wp-cont...281089888.jpeg
http://asheepdogproject.info/wp-cont...ls2_9_1of3.jpg

Voltages all look good

Does the Fuel Pump relay Click on when key is turned on?
It should click on for 2 seconds then click OFF

If not the you need to find the VIP(OBD1) connector.
It is in the engine bay and "usually" near firewall in corner on drivers side but also passenger side, follow main engine wiring harness.

In your above diagram you see VIP test connector, center right, spliced to the Light Blue/orange stripe wire
In your picture it looks like it might be right there next to relay base

Image here shows you what to look for: Ford Ranger - Testing EEC-IV Equipped Engines
Second drawing down

It may have a cover that has EEC on it

Fuel Pump slot on the VIP connector is labelled in the drawing.
That Slot is the GROUND for the fuel pump relay, computer grounds(pin 22) this wire to turn ON the fuel pump, close the relay.

So with key on use a jumper wire in that Slot to a Ground, engine, fender bolt, battery "-" any ground will do.

You should hear the "click" of fuel pump relay closing and fuel pump should start running.

You can leave this connected and start the engine, won't hurt anything, you are doing the same thing computer should be doing.

If you don't get the "click" then the Ground wire at the relay is bad, assuming your test voltages were accurate.

If you do get the "click" then computer is not Grounding pin22 wire or wire from pin 22 to fuel pump relay is bad.
computers rarely fail, so 99.99% a wire issue


You can drive the truck using the jumper wire in place, fuel pump will go off when key is off.
But it is a safety issue, computer cuts the Ground to relay if engine stalls(RPMs below 400), i.e. broken fuel line in an accident, which prevents a fire, inertia switch should as well, so just a heads up on that.

OK I found the VIP connector and and grounded the wire and the fuel pump now turns on when key is turned on but truck still won't start.
I have replaced spark plugs, others were badly fouled black.
I am getting fuel pressure at the rail, and in cylinder because new spark plugs smell of gas after start attempt.
I have replaced cap and rotor.
I have replaced spark plug wires.
I am getting red/orange spark at plugs and from coil. I thought it was supposed to be blue/white. If so I think it is the coil.
What am I missing

Well you could try spraying Ether(Quick start) into the intake, it has a much lower ignition temp, so even a weak spark will ignite it.

But yes, blue/white is the color of a good spark.


Before replacing the coil see what voltage you have at the coil when cranking the starter motor.
Select DC volts, Ground meter's black probe then put red probe on coil +
Turn on the key, you should see battery Voltage, 12.5 volts

When cranking starter motor, it will drop down 2 volts, so 10.5volts which is OK
If it gets down to 9.5 volts then spark will get weak.
So a battery or wiring issue not a coil issue

ok so I went to go test the coil and battery was dead something is draining the battery. I charged up the battery, replaced the starter solenoid because it had stuck engaged twice on me recently. I also checked compression between 140-150 on all cylinders. I then went to go check the voltage at the coil and now when I turn the key on I have no power to gauges or anything in cab I have to jump the starter solenoid to get engine to turn over.
This truck is driving me crazy

Also does anyone know what this plug goes to it sits right behind the battery. I am wondering because the 2 large black wires coming out of the plug both are going to the battery negative and that doesn't seem right.
http://asheepdogproject.info/wp-cont...8853818993.jpg

can anyone help identify this plug

Look like the Ground connection for the Relays

still trying to diagnose what is going on with the truck. Fuel pump only comes on when the obd connector is grounded.
Truck runs if starting fluid is sprayed.
I have 40 psi at the fuel rail.
Injectors removed and cleaned and spray if I connect directly to battery but not when plugged into wiring harness.
I have a good ground on engine block and on frame.
Could it be the computer?

Fuel injectors are controlled directly by computer so yes, at this point computer should be looked at

Open it up and look at circuit board

Should look nice and clean like this: Ford EEC-IV

Also on the above page are the GROUND wires for computer, test at the connector that all are still good grounds, one bad ground will stop fuel pump and injectors from working

I have a very similar problem with my Ranger. Did you find the solution to this problem?