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Troubleshooting fuel pump circuit


Old 04-05-2016
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Join Date: Apr 2016
Location: South FL
Posts: 1
Icon5 Troubleshooting fuel pump circuit


I am having a problem with my 94' Ranger 2.3 not getting fuel. The pump is new but not running.

I have done a good bit of troubleshooting already and at this point I suspect some kind of short in the wires from the fuel pump relay to the connector at the sending unit. I would like to consider other possibilities before I tear into the truck more.

I checked the fuel pump fuse in the engine bay and replaced the fuel pump relay. Battery voltage is 12.6V. There is 12.6V coming into the fuel pump relay on one pin. The other 3 outer pins vary by a few tenths of a volt but are all around 11.5V, which is the same voltage I am getting on one pin at the connector to the sending unit. When I try to power the pump on that pin it won't run and the voltage drops to zero on my tester. Thus I suspect a short in that line.

If I power the fuel pump with the pins to the fuel pump relay, the one with 12.6V powers it well and voltage stays consistent on my tester. 2 of the other 3 pins with around 11.5V run it but a noticeably slower and voltage drops to 6-8V while running the pump. The third pin with about 11.5V will not power the pump.

The inertia switch was not tripped and tested to function fine. I have 11.5V at the inertia switch.

Have I missed something, or is it time to grab the ohm meter and test the wires from the relay back to the sending unit connector until I find the short?

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Old 04-05-2016
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Location: Vancouver, BC
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Ford computer runs the fuel pump.

Fuel pump should only get 12volts for 2 seconds when key is first turned on, it's a safety feature.
Fuel pump won't get power again until engine RPMs are above 400, OR unless you turn the key OFF and then ON again, then it will come on for another 2 seconds.

Fuel pump relay
yes in its plug in there should be 1 slot with 12volts all the time, key on or off, that from the fuel pump fuse.
Then there will be another slot with 12volts but ONLY when key is on.

The computer controls the Ground for this relay, so even though it gets 12volts when key is on it can't "activate" until it is Grounded, computer does that, for 2 seconds.

To test if relay is working you will need to find the OBD1 connector, usually near firewall drivers side, by power brake booster.
It just hangs loose in main wiring harness.
Drawing of it here: Ford Ranger - Testing EEC-IV Equipped Engines

In the drawing you will see a slot in that connector labelled Fuel Pump.
That is the Ground for the fuel pump relay, it is spliced to the computers wire used to ground the relay.

So find it
Turn on the key
Put a jumper wire in that slot and ground the other end
Relay should "click" closed.
You can leave jumper connected, doesn't hurt anything, same as if engine was running.
That should give you full 12volts at inertia switch for testing and at the connection by the gas tank.

When you are testing voltages you have to keep in mind what is connected to the wire(s).
If powered you will see 12volts going into and coming out of a motor, like a fuel pump, that's because a motor is just a long wire, voltage in voltage out, what a motor or light bulb for that matter, needs is 12v AND a ground, then it will run.

For example if you where to test for voltage on that fuel pump slot in the OBD1 connector when key was on it would show 12volts, thats the power passing through the relays coil of wire, but relay doesn't close until it is Grounded.

Sender in gas tank runs on 5 volts, it comes from the dashboards pulsed 5v power supply

Here is a 2.3l wiring diagram for engine wiring:

Upper middle is fuel pump relay
VIP test connector is the OBD1 connector

yellow wire is from the Fuel gauge to the sender
Orange wire is the Ground for the sender

Pink wire is fuel pump power
Black wire is fuel pump ground

Also because Computer monitors if Fuel pump is getting power you will always read some voltage at inertia switch when key is on, that is from the "monitor" circuit, not the Fuel pump Relay, you will read full battery voltage when relay is close.

Last edited by RonD; 04-05-2016 at 07:40 PM.
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