SOLVED: Fuel system electrical diag road block


ecgreen

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Problem -> pump not coming on when key is in on position

Where I am at:
- I have tested the pump out of the truck by connecting the ground and hot pins to a battery and it runs
- Fuse is OK
- 12.3 volts going into fuel relay
- red wire on relay when key is in 'On' position reads 11.5. volts
- red wire coming out of EEC relay going to fuel relay also reads 11.5ish volts
- voltage at intertia switch (which is bypassed) is 11.5 volts
- voltage at power wire on connector to pump is 11.5 volts
- voltage at black ground wire on connector to pump is 0 volts

The pump is still not working, so I am misunderstanding something.
 


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rusty ol ranger

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Sounds like a possible ECM failure.
 

farmer

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ECM grounds the relay when it calls for fuel, try grounding the relay and see if that works. if it does, ECU failure, or a broken wire.

Worst case Ontario, the ECU in my 91 doesn't do anything anymore, I think, could try swapping it in to be sure.
 

alwaysFlOoReD

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I cant see what year it is so this may not apply.
On frame rail pumps almost every one I've had has had loose connections right at the pump under the rubber protectors. I've replaced 3 pumps that I found out after were just fine. Crimped on new connectors and no more problems.
 

ericbphoto

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Could be bad ground after the pump, also. 0 volts on that wire doesn't equate to good ground connection.
 

ecgreen

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ECM grounds the relay when it calls for fuel, try grounding the relay and see if that works. if it does, ECU failure, or a broken wire.

Worst case Ontario, the ECU in my 91 doesn't do anything anymore, I think, could try swapping it in to be sure.
I might take you up on that if I can't track the issue down. Thanks!

How would I ground the relay?

BTW, is this the same farmer thats on ENH?

I cant see what year it is so this may not apply.
On frame rail pumps almost every one I've had has had loose connections right at the pump under the rubber protectors. I've replaced 3 pumps that I found out after were just fine. Crimped on new connectors and no more problems.
'89

Could be bad ground after the pump, also. 0 volts on that wire doesn't equate to good ground connection.
What is the proper way to test and see if the ground is OK?
 

farmer

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I might take you up on that if I can't track the issue down. Thanks!

How would I ground the relay?

BTW, is this the same farmer thats on ENH?
Pop the relay out of the holder, flip it over to see the backside of the connector, and use a jumper wire of any sort.

Yup the same farmer!
 

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What is the proper way to test and see if the ground is OK?
Measure resistance (ohms). With key OFF. Put one meter lead on the black wire where it connects to the pump and put the other lead on a good clean ground point on the frame. Ideal reading would be 0 ohms.
 

ecgreen

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Pop the relay out of the holder, flip it over to see the backside of the connector, and use a jumper wire of any sort.

Yup the same farmer!
Wow, your like a bad penny lol!

Let me get this straight: pop the relay and connector out of the holder, keep the relay plug in the relay and run a jumper wire from where the ECU connects to a known ground (the LB/O wire)?
Or are you talking about jumping with the connector unplug from the relay?

I notice the DG/Y also goes to the ECU. There are 2 wire going into the connector, and one is showing a bit of wire, like its not in all the way. What does this wire do?


Diagrams_ElectronciEngControls2_9_1of3.JPG



Measure resistance (ohms). With key OFF. Put one meter lead on the black wire where it connects to the pump and put the other lead on a good clean ground point on the frame. Ideal reading would be 0 ohms.
It measured 6 ohms. That's not enough to cause an issue, right?


Can't thank you guys enough, getting such a great education on this forum
 

ericbphoto

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6 ohms is more than I would like to see there. Should still let the pump run, but probably not at full speed and pressure. If that's an accurate reading, it could be reducing the voltage that the pump gets, possibly by 2 or 3 volts. Go on with the other checks you're pursuing with the relay and just keep this in mind as something to clean up later.
 

ericbphoto

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That dg/y wire going to the computer probably just tells it that the fuel pump relay contact is closed. That signal could be used internally for any number of things including setting a fault code if the relay fails.
 

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Disconnect the fuel pump relay from it's socket and make a good jumper to go between the BK/Y and DG/Y of the relay socket. If the pump comes on, the relay should be replaced (0.8 volts of voltage drop at the relay is excessive). If it doesn't start, check the ground that feeds the pump.

The other DG/Y wire that goes to the ECM tells the ECM that the fuel pump circuit is energized, and it wasn't used on Rangers before 1988.
 

ecgreen

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Disconnect the fuel pump relay from it's socket and make a good jumper to go between the BK/Y and DG/Y of the relay socket. If the pump comes on, the relay should be replaced (0.8 volts of voltage drop at the relay is excessive). If it doesn't start, check the ground that feeds the pump.

The other DG/Y wire that goes to the ECM tells the ECM that the fuel pump circuit is energized, and it wasn't used on Rangers before 1988.
This was what farmer was getting at I think. Thanks!
 

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1 ohm will drop 1 volt. 6 ohms will drop 6 volts, not 2 or 3.

The battery voltage KOEO appears to be about 11.5

The fuel pump requires a minimum of 5V to run.

With the fuel pump relay jumpered closed (ground it, jumper across its plug, W/E, just put power on the wire) check voltage between the pump's power wire and its ground wire to see what amount of power is available to the pump itself. With VBAT at 11.5 and 6 ohms of resistance on the ground I'm guessing you are getting only 5 votls. That might get it moving, it probably won't give you enough to start the engine. Starting a cold engine uses extra fuel.
 

ericbphoto

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1 ohm will drop 1 volt. 6 ohms will drop 6 volts, not 2 or 3.
I don't know how much current a stock fuel pump draws. So I made a conservative guess so I could use ohms law. Thanks for jumping in.
 


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