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Old 07-19-2004, 12:11 PM
"SLOWMOE"'s Avatar
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feul sending unit failure??

i orderd a new fuel sending unit from LMCTRUCK.COM for my 67 chevy p/u wich has it's original feuling system. no return lines. anyway, after having the tank cleaned and coated, i installed the new unit and it has worked fine for about 3 months. the problem is ..........now i am not getting any reading from this feul unit to my gas gauge in the dash! i have checked the wiring(it only has one wire from the unit to the fuse box) and when i unplug the wire from the fuse box with the switch on, the needle for the feul gauge goes from empty to full and then some! thus when i plug it back in to the fuse box it goes from full to empty! there is 3/4 tank of gas in it so i know that i have gas, by why is this thing reading backwards?? i am assumeing that the tank it self is grounded by it's bolts to the body wich i have checked also. cleand of paint and making good contact! could this fuel sending unit be deffective after only 3 months???
thanks in advance for any help!!

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Old 07-19-2004, 12:19 PM
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I thought the wire from the fuel sending unit should go to the gas gauge, not the fuse panel.
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Old 07-19-2004, 12:22 PM
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In my experience bad grounding is the problem the majority of the time on these things. Not much to go wrong with them but it does happen occasionally. I would check the electrical circuit in depth, including taking the sender out of the tank and stroking it with temp wiring B4 buying a new one.
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Old 07-19-2004, 12:24 PM
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I checked my '67 Chevy shop manual and the wire does go to the gauge. Right now you don't have anything connecting the gauge and the sender. Right?
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Old 07-19-2004, 12:32 PM
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yes i do have the wire going directly from the unit to the fuse box wich has a place marked " fuel gauge " this is the way it has been working.
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Old 07-19-2004, 12:52 PM
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Maybe trucks are different, but in my '67 car the circuit goes from the fuse panel to the gauge. Then, from the gauge to the sending unit which is grounded.

Do you have a truck manual that has a wiring diagram you can reference?
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Old 07-19-2004, 01:03 PM
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unfortunatley i don't! how ever this is quite confusing. it seems that from the fuel unit....the wire goes back to the fues box and from the fuse box to the gauge! it has one of those flexible circuit trace panels that attatches to the back of the instrument panel. it also has one harnest with 9 wires that plugs into it. i geuss i really need to find a book that has this wiring huh? thanks for the help
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Old 07-19-2004, 02:12 PM
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I'm making an assumption that Chevy used the same color codes for both cars and trucks in '67.

There should be a 20 ga tan-colored wire in that harness clip on the back of the instrument cluster. That tan wire should go back to the gas sending unit.

In a car, there is an interim harness consisting of about 6 wires that runs from the dash, under the carpet and seats, and into the trunk. All of the wires in this interim harness are black, for some unknown reason. Anyway, in the trunk the tan wire from the sending unit should connect with the interim harness.

As I understand it, you now have the gauge and the sending unit both plugged into the fuse panel, with no connection directly between them.

I think the curcuit should go:
Fuse panel to gauge
Gauge to interim harness
Interim harness to sending unit.

And, as Willys said, watch that ground on the sending unit. I fiddled with my circuit for a couple of years before I finally swallowed my pride and redid the ground.

Hope this helps.
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Old 07-19-2004, 03:05 PM
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Tan has been the standard for decades. Don't know why they would ever change it.
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Old 07-20-2004, 01:27 AM
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Could be a problem in the tan wire itself. An open circuit (disconnecting it or a break inside the insulation) causes a full+ reading on the gauge. A short circuit (grounding it somewhere between the dash & the tank) causes an empty reading.
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