Fuel pump issue 94 B4000

[rt520]

My fuel pump is not turning on. Truck is a 94 B4000 4x4 extended cab

Here is what I've done so far.

I tripped and reset my inertia switch.

I checked the 20amp fuse.

I took a voltage reading at the wire harness at the pump. 9.4 volts

I took a power probe and hooked it up to the pump, it turned on.

I pulled out the fuel pump relay and put the power probe in #30 that goes to the fuel pump and was able to run the pump.

#85 on the relay gave me a volt reading of 0.4

#86 gave me 11.9

#87&#87a didn't read anything.

Any suggestions on where to go from here? I was thinking about going out and buying a new power relay and fuel pump relay and seeing it that does anything.

 

[RonD]

Relay removed:
Fuel pump relay slot 30 should have Battery Voltage(12.3v-12.8v), all the time, 24/7 from that 20 amp fuse(#22 I think)
If it only has 9volts then wiring is bad or battery is "dead"

slot 87 will have 5 to 7volts when key is on, that comes from computers monitor circuit, no amps just volts, so can't run fuel pump, that may have been what you saw at the fuel pump.

Slot 87a isn't hooked up to anything

Slot 85 should show Battery Voltage when Key is ON, nothing when key is OFF
Slot 86 is Ground for the relay coil, it is operated by the computer, it will only test as a good ground for 2 seconds just after key is turned ON, computer will not Ground relay again until engine is running, RPMs above 400, or unless you turn the key on and off again, for another 2 seconds.

Issue reads like wire from fuse to slot 30 is corroded or loose

Slot 30 has power all the time, full battery voltage
Battery "+"------------fuse--------slot 30

Slot 87 is what slot 30 is connected to when relay closes, computer monitors if 12volts is on that wire
slot 87-------------------/-inertia switch-------------------------fuel pump
Computer Monitor------/

 

[rt520]

Relay removed:

Slot 87 is what slot 30 is connected to when relay closes, computer monitors if 12volts is on that wire
slot 87-------------------/-inertia switch-------------------------fuel pump
Computer Monitor------/

I took out the relay and used a jumper wire from 87 to 30. The fuel pump kicked on.

 

[RonD]

That would be correct.

That is what the relay does when activated by the computer, 30 and 87 are connected and fuel pump comes on

But test voltage at 30, it needs to be exactly the same as battery voltage, if lower then wiring is the issue, and pump can shut off when it starts to draw more amps.

1994 is the last year of the EEC-IV (OBD1) computer in Rangers, there will be a VIP(OBD1) connector in the engine bay, usually near firewall drivers side, it is just hanging loose.

Diagram of it here: http://www.therangerstation.com/tech_library/OBD_I.shtml

Second drawing down.

Fuel Pump slot is labelled in that drawing

That slot is connected to the Computer's wire that is used to Ground the fuel pump Relay to activate it.
With key ON that slot should show Battery Voltage, and if you use a jumper wire in that slot with other end to a Ground then Fuel Pump Relay should "click" closed and fuel pump should come on.

You can run the engine with this jumper in place, you are not "shorting" anything, the jumper is the same as if the computer was grounding this wire, which needs to be grounded as long as engine is running.

If using the jumper does not cause relay to "click" and fuel pump to run, then either the relay is bad or the Ground wire at the relay is bad.

From you reading pin 86 is the 12volt power for coil, 85 the Ground

Fuel pump relay coil 85---------------------------/-----Computer--------Ground
VIP connector(FP slot)--------------------------/


A relay's coil doesn't have a polarity, when pin 85 has 12v and pin 86 is a Ground then coil becomes an electro-magnet and pulls metal relay bar down connecting 30 and 87
So 86 can be the 12v and 85 the ground, doesn't matter to the coil, as long as BOTH 12v and ground are present.

Ford, like most car makers, uses the Ground side to control most things, it is safer to have Ground wires running around the vehicle, less chance to a 12volt short since vehicle metal is a Ground.

 

[rt520]

Bought a new relay and it fired right up. Thanks for the help!!!

 

[RonD]

Good work

Thanks for posting the fix