1994 ranger, low voltage at fuel pump

cardana24 · Mar 25, 2018

Hey guys, my truck was running fine yesterday. It’s a 1994, 4.0 Manual trans. I wanted to change my fuel filler neck since it was leaking and thought I should change the pump while I was in there. I finished the job and now my truck will not start. I am only getting justover 7 volts at the pink wire at the pump. Also checked the inertia switch and I am only getting the same 7 volts. At the fuel pump relay I get 12v. What in the world did I do? I tried swapping in the working pump that I just took out and that one does not turn on either. Any ideas?

RonD · Mar 25, 2018

You are getting correct voltage, after the 2 second FP relay opening and closing.

Computer is connected to inertia switch to monitor Fuel Pump relay closing and if inertia switch has been "tripped".
This monitor wire has 5 to 8 volts when computer/key is on.
And thats what you are seeing most likely, it has no AMPs just volts, so can't run fuel pump.

When you cycle key on the computer will boot up and close FP relay for 2 seconds, and 2 seconds only.
You can cycle key on and off as much as needed to check for full Battery Voltage on the Pink wire, but it will only show for 2 seconds, after that, if key is left on, it will drop to 5-8volts

Fuel pump relay's socket will have 4 connected slots, one slot will have 12v all the time, from fuel pump fuse, always hot
One slot will have 12v but only with key on, from EEC(PCM) relay, key on
One slot "should" have that 7volts from computer monitor wire at inertia switch, key on, if not then inertia switch could be bad/tripped
4th slot is the Computer's Ground control to turn FP relay on and off, open and closed

There is a trick you can use to test FP voltage easier.
OBD1 connector in the engine bay has a FP test slot, in 1994 it will be between fuse box and brake master, may have a cover over it
Look here: http://www.therangerstation.com/how-to/ignition-charging-computers/testing-eec-iv-equipped-engines/

In the drawing the Fuel Pump slot is labelled, that slot is connected to Computers Ground control wire on the FP Relay
So if you put a jumper wire in that slot and ground it, FP Relay will be grounded full time, so when you turn on the key FP relay will close and you should get full battery voltage all the way back to Fuel Pump, fuel pump on, it will go off when you turn off the key

FYI, the 2 second thing is a safety feature, if there is an accident an electric fuel pump could just keep pumping out gasoline.
Pre-computer electric fuel pumps were hooked up to oil pressure switch, so no oil pressure = no power to fuel pump, but those were Carburetor days, so engine could be started from gas in the float bowl, then electric pump could come on
Fuel injection doesn't have a reserve/float bowl so 2 second thing was used
Inertia switch "should" trip and cut power to fuel pump in an accident, but as a backup there is the 2 second thing.
Computer will only close FP Relay full time if engine RPMs are above 400.
If a gas line should break in an accident then engine would stall, so under 400RPMs and then computer shuts off FP relay
For startup it Primes the system for 2 seconds, and will Prime it each time key is turned from off to on.

cardana24 · Mar 26, 2018

RonD said:
You are getting correct voltage, after the 2 second FP relay opening and closing.

Computer is connected to inertia switch to monitor Fuel Pump relay closing and if inertia switch has been "tripped".
This monitor wire has 5 to 8 volts when computer/key is on.
And thats what you are seeing most likely, it has no AMPs just volts, so can't run fuel pump.

When you cycle key on the computer will boot up and close FP relay for 2 seconds, and 2 seconds only.
You can cycle key on and off as much as needed to check for full Battery Voltage on the Pink wire, but it will only show for 2 seconds, after that, if key is left on, it will drop to 5-8volts

Fuel pump relay's socket will have 4 connected slots, one slot will have 12v all the time, from fuel pump fuse, always hot
One slot will have 12v but only with key on, from EEC(PCM) relay, key on
One slot "should" have that 7volts from computer monitor wire at inertia switch, key on, if not then inertia switch could be bad/tripped
4th slot is the Computer's Ground control to turn FP relay on and off, open and closed

There is a trick you can use to test FP voltage easier.
OBD1 connector in the engine bay has a FP test slot, in 1994 it will be between fuse box and brake master, may have a cover over it
Look here: http://www.therangerstation.com/how-to/ignition-charging-computers/testing-eec-iv-equipped-engines/

In the drawing the Fuel Pump slot is labelled, that slot is connected to Computers Ground control wire on the FP Relay
So if you put a jumper wire in that slot and ground it, FP Relay will be grounded full time, so when you turn on the key FP relay will close and you should get full battery voltage all the way back to Fuel Pump, fuel pump on, it will go off when you turn off the key

FYI, the 2 second thing is a safety feature, if there is an accident an electric fuel pump could just keep pumping out gasoline.
Pre-computer electric fuel pumps were hooked up to oil pressure switch, so no oil pressure = no power to fuel pump, but those were Carburetor days, so engine could be started from gas in the float bowl, then electric pump could come on
Fuel injection doesn't have a reserve/float bowl so 2 second thing was used
Inertia switch "should" trip and cut power to fuel pump in an accident, but as a backup there is the 2 second thing.
Computer will only close FP Relay full time if engine RPMs are above 400.
If a gas line should break in an accident then engine would stall, so under 400RPMs and then computer shuts off FP relay
For startup it Primes the system for 2 seconds, and will Prime it each time key is turned from off to on.

Great information! I'll ground the fuel pump via the test port and see what readings I am getting. I am doing everything my self so running back an forth from the key to the tank is not working very well.

Also I will check the relay better, I know I am getting constant 12v on one pin but beyond that I am not sure. Should another pin show 12v when the key is switched on? I assume this signal would come from the ignition switch?

Spott · Mar 26, 2018

cardana24 said:
Great information! I'll ground the fuel pump via the test port and see what readings I am getting. I am doing everything my self so running back an forth from the key to the tank is not working very well.

Also I will check the relay better, I know I am getting constant 12v on one pin but beyond that I am not sure. Should another pin show 12v when the key is switched on? I assume this signal would come from the ignition switch?

The second pin you describe will only have 12V for the first 2 seconds after the key is turned on (or while the engine is running or the fuel pump test connector is grounded).

If you turn the key to "run" while it's sitting in the driveway, and then get out and walk around to the relay and stick your meter probes on it, then your 2 seconds has already expired and the output pin will show no voltage.

If you ground the fuel pump test connector, then it will energize the relay continuously and you can then perform all your tests without worrying about that 2 second window of time.

RonD · Mar 27, 2018

One FP Relay socket slot will have 12v full time from FP Fuse
2nd slot will have 12v continuously with key on, not 2 seconds, this 12v comes from EEC(PCM) relay, which is closed when key is turned on, EEC Relay also powers spark and fuel injectors, along with the EEC(computer)

Computer grounds the FP relay to close it, this ground wire also goes to OBD1 connector, which allows you to Ground it manually with jumper wire to ground

cardana24 · Mar 27, 2018

RonD said:
One FP Relay socket slot will have 12v full time from FP Fuse
2nd slot will have 12v continuously with key on, not 2 seconds, this 12v comes from EEC(PCM) relay, which is closed when key is turned on, EEC Relay also powers spark and fuel injectors, along with the EEC(computer)

*I have not had a chance to test anything yet* So if I don't get a second pin to get 12v on the FP relay then I need to look as the EEC (PCM) Relay? Assuming the rain holds off I should be able to get some readings this evening.

cardana24 · Mar 27, 2018

So I did some testing. I forced the fuel pump to “run” by grounding the pin on the test connector. I was getting battery voltage at the fuel pump connector but when I plugged the pump in it still did not run. So I started wiggling wires at the connector and it would run and cut off. So something is not making good contact at the fuel pump connector. Luckily I figured that may have been a problem and I had alr day ordered a new plug and it’s is supposed to be here Thursday. Hopefully I can put this one to rest.

alwaysFlOoReD · Mar 27, 2018

The older Rangers and F150's had a high pressure frame rail pump as well as an in-tank low pressure pump. I've found the frame pump connections at the pump itself, not the connector, to be loose and act like a failed pump. Replace the pump and same problem. I ended up changing the crimp connectors as a matter of course any time I have to remove the frame pump.
This may be what you're experiencing...or not.

cardana24 · Mar 29, 2018

so I changed out the fuel pump connector yesterday evening. Plugged it in to my pump out side of the tank (working pump removed from truck), and it turned right on when I turned the key on. So I plugged it in to the pump installed in the truck...hit the key....nothing. So now I have a warranty claim in to rockauto, it seems that I received a pump that was DOA.

akoz · May 25, 2018

I'll apologize right up front for jumping on this thread, but it's current and I have the same truck (94, 4.0), and have the same issue.

I'm already on my 2nd fuel pump and swapped fuel pump relays. The 20A fuse is good. Looking at the FP relay socket, the top slot is 12V (hot as long as the 20A fuse is installed/good). I started checking continuity (going clockwise) and get: OPEN, 1360 Ohms, 510 Ohms. The center contact is N/C (I don't see a contact installed).

I checked continuity between the FP socket contact that measured 1360 ohms and that is connected directly to the Green w/yellow stripe wire at the inertia switch. I re-installed the relay and try 2 tests (setting the meter to capture the max voltage):

1) turn the key on - measure 1.0V max at the Green/Yellow wire at inertia swtich

2) GND the FP test slot pin - turn the key on - (same as above, only 1V)

It looks like I have a problem with not getting 12V at the intertia switch when I turn the key on. I'd be saying bad relay but swapped it with another.

I've been trying to trace that Green/Yellow wire and haven't had much luck (I just took off the inner fender on passenger side and don't see it coming through the firewall, but I do see the Pink/Black wire there).

Anything I might have missed?

Thanks,
Al

akoz · May 25, 2018

Look like I have solved 1 problem by replacing the FP relay with another spare (instead of swapping around what seems to be a bad one).

Now I have 6.8V on the Green/yellow wire at the inertia switch.

I take out the FP relay, I GND the FP test slot, and put the key to ON and measure (clockwise) 12V, 0V, 6.8V, 12V.

Remove the GND at the FP test slot, put the FP relay back in and still no start (no fuel at the fuel rail). It looks like I have a bad inertia switch if I understand how it works meaning that it will short the monitor wire (Green/yellow) so the FP relay doesn't get the monitor voltage. If that's right, then I'd expect to be able to start with the inertia switch connector disconnected...still, no start, no fuel.

akoz · May 25, 2018

Problem solved...2 BAD RELAYS!!! What are the odds?...apparently better than zero

RonD · May 25, 2018

The Pink/black wire goes back to fuel pump.

Green/yellow wire is your problem, as you surmised, from the testing
So the 12v from the FP fuse is not making it to the inertia switch when FP relay closes

But you should have 5-8v at inertia switch green wire with key on.
There should be 2 green wires at the inertia switch, and one pink wire.
One green wire comes from FP relay, the other green wire goes to pin 8 on computer, it monitors FP power.
The 5-8v has no amps its just a monitor circuit
This computer wire can also be blue/orange.

These 2 green/yellow wires can also be together at the FP relay socket

Underside of relay socket can get corroded, which would effect both green wires if they terminate there.
I can't see wire getting cut without effecting other systems in the harness.

You can run a new wire, but would have to also run one for pin 8 at computer, or CEL would come on

akoz · May 27, 2018

Hey, thanks for the reply.

I definitely only have 1 Green wire at the inertia switch. There is a 3rd slot on that connector but it's empty (no contact in it).

I also have the shop manual (for '92) which diagnosed it to a bad relay, but I am still not 100% convinced that I might have a EEC issue.

Considering that I only have a 2-wire setup, what does it mean with respect to the monitor voltage?

This problem is strange in the sense that there have been days where it will run and the others where it won't start and all it's been doing is sitting (not my daily driver).

Al

RonD · May 27, 2018

Well grounding the OBD1 connector's fuel pump wire will cause FP relay to close(key on) so that should tell you if its a computer issue.
i.e. if relay closes using jumper but doesn't when computer only is controlling it then computer, or that wire to computer pin 22 is bad

(you can leave that OBD jumper connected and drive the truck that way, FP shuts off with the key, bit of a safety concern in that FP could stay running in an accident, thats the main reason for this type of FP control circuit, computer opens FP relay if engine RPMs are under 300 or so, so stalled engine cuts FP power)

On my 1994 wiring diagram it shows light blue/orange wire from FP Relay(ground for coil) to OBD connector then same color from there to Pin 22 on computer, so there should be 2 wires on that OBD slot.

You may want to pull computer and test the wires, and also pop the top off the computer and have a look at the circuit board, there are 3 Blue Capacitors that will leak over time and that will cause some systems to be intermittent or stop working altogether, cost under $5 to replace them.

And you could also see if pin 8 on computer has a wire connected, thats for the FP voltage monitoring, it may not have that, I see it on the 1994 diagram but that doesn't mean it is there for sure.
It was a test for inertia switch passing voltage in later years, so wiring was different, it connected to the Pink wire side of inertia switch

Interior of EEC-IV computer seen here: http://www.auto-diagnostics.info/ford_eec_iv

3 blue caps are seen
Also pin numbering

1994 ranger, low voltage at fuel pump

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