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Old 09-02-2013, 10:50 AM   #1
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Fuel pump issue

Trying to help a buddy figure out his fuel issue. When the key is on or off the fuel gauge is pegged at full, the pump wont prime when the key is turned on, I am only getting 8 volts back to the pump, relays are good, there isnt a ground at the ebrake, tore apart the pdc to clean everything up, checked every inch of wire from tank to engine, replaced the connector ends, tested pump and it work, we actually have it wired to a toggle switch right now so it can be driven. Changed out entire under dash harness due to a security system that way its fresh and good to go. I think thats all ive tried, im sure im forgetting something. But I am at a wall, I am out of options besides swapping out the under hood harness. Oh yeah even tried another computer I had and still nothing. I know when I tested pins on the ecu I concluded the auto shutdown was being activated and have no clue why. Any help would be beyond appreciated as this has become a nightmare. Thanks

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Old 09-02-2013, 11:41 AM   #2
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the fuel gauge sending unit is just a variable resister which in effect is the same as the volume control on a radio..the wiper arm on this resister is connected to the fuel float.when the tank is empty with the float sitting at the bottom of the tank then the reading across the 2 wires on the variable resister is zero ohms which puts the gauge on empty..when the tank is full with the float at the top the resister will read about 90 ohms which will put the gauge at full.in between empty and full it will read a different resistence just as the radio volume control does.the wire on one end of the resister goes to ground..the wire on the other end goes to the gauge..so,all the gauge is looking for is the resistence between ground and the gauge which is controlled by the fuel level sending unit in the tank..should you lose the connection anywhere in either of those lines or lose the ground all together then you will have infinite ohms.since this is greater than 90 ohms the gauge will be pegged past full..it could be a broken wire or even a bad sending unit in the tank but 9 out of 10 times I have found it to either not have a ground or have a dirty connection on the back of the gauge..................on the fuel pump I guess the best thing to do is back track the circuit until you find where the voltage is dropping..when something like that gets to be too big of a pain to me,I am suject to put a fused line through a relay to the pump and then take a wire that gets hot when I turn the key on and have it control the relay

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Old 09-02-2013, 12:52 PM   #3
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I did check ohm resistance which I believe was in the 50's, it was about half a tank or so, I checked voltage at the relay which was good and also at the grey connector next to the pdc which also was good. In my research on the ecm pin 51 should have no voltage and pin 57 should have 12v but his pin 51 has 12 volts and pin 57 has 0 which is the auto shutdown. I also checked where it makes another connection down by the transmission and everything is good there also. For some reason something is causing the auto shutdown which is why at that point I only get 8ish volts back at the tank. Its gotta be something so simple its stupid.
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Old 09-02-2013, 02:26 PM   #4
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okay..if you have 50 ohms on the wire at the back of the gauge but the gauge is reading full then I would think the gauge itself would be bad..I can't advise you on anything involving the computer
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Old 09-02-2013, 03:03 PM   #5
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Subscribed, ill be PM'n you later on if you don't mind. I'm having a similar issue, only getting 3.2 Volts DC at the pump, can't figure out what's going on. Keep us posted on what you find.

Are you suggesting that the maxed-out fuel gauge causes the ECU to go into auto-shutdown? This would make sense for me b/c my Jeep stalled out going 40mph after filling up at the gas station, it was running fine all day before...
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Old 09-02-2013, 03:33 PM   #6
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leading up to it nothing out of the ordinary, just going to work and it randomly died. No warning nothing, gas gauge worked great and everything. When it died it spit and sputtered like it was out of gas and the fuel gauge went straight to full and whatever caused it apparently is triggering the auto shutdown. All grounds have been located and fixed, continuity between front and back are as good as they can be. The only thing I can think of is a sending unit failure or something but all is well when I have it wired to a toggle switch from the battery.
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Old 09-02-2013, 03:46 PM   #7
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Yeah. If it was me I'd wire in another relay and new ign wire to activate it when key is on. Bypass everything altogether.
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Old 09-02-2013, 04:21 PM   #8
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Thats basically where its heading, but its actually became a mission to figure it out now
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Old 09-02-2013, 04:50 PM   #9
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I could be wrong but as far as I know the fuel gauge has no input to anything..again,as far as I know the gauge only has power to operate the gauge and then the wire from the sending unit...plus the light.only way that I could see the two problems being related would be if the same low voltage on the pump is also the power wire to the gauge but I wouldn't think they would be the same............I like the relay deal..when I wired badboy I wired the fuel pump,electric fan,lights,wipers,basically everything through relays.the relays I used have small indicator lights on them which makes any troubleshooting very easy

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