Reverse Polarity
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From: Tennessee
Reverse Polarity
86 F-150 4x4 swb 302 fi dual tanks. I recently had new fuel pumps-sending units and a tank selector switch put in my truck because the guage wouldnt read on either tank. Now when i fill the rear tank the guage reads empty and as it gets empty it starts climbing when @ 3/4 i have @ 4gal left. Dropped the rear to see if they did replace the unit and they did. while i had it out i put an omhsmeter on the float and it read 2 all the way down and 16 all the way up so i am guessing it works btw the truck will run on the rear and you can hear the sending unit when i turn on the key. This is an on-going problem. I have swapped the guage in the dash and cleaned the circuit board it didnt help? what am i missing btw the harness that goes to the rear tank will only plug in one way since it is encased in a plastic shaped piece
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Somewhere along the way they changed the way the guages worked, but I believe yours should be 90 ohms empty, and 0-10 ohms full. Your ohmmeter should have read 90 when the float was down, and then gradually fall at a nice smooth rate till it read around 10 ohms.
If you want to double check this, go back to the tanks and unplug the sender unit. This wiring diagram says it's yellow/white. http://www.autozone.com/images/cds/g...3d800ce996.gif
With the key in run, but the engine off, take this wire and ground it. If I am thinking right, the guage should rise up and read full. Then take the wire off ground and let it hang in the air. The guage should fall to empty.
If you want to double check this, go back to the tanks and unplug the sender unit. This wiring diagram says it's yellow/white. http://www.autozone.com/images/cds/g...3d800ce996.gif
With the key in run, but the engine off, take this wire and ground it. If I am thinking right, the guage should rise up and read full. Then take the wire off ground and let it hang in the air. The guage should fall to empty.
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From: Tennessee
The color of the wires in the diagram look like mine as best as i can remember. There are 4 wires at the connector but the unit in the tank only has 3 wires 1 black 1 red to the pump and an orangeish colored wire that goes to the float. this is the wire i tested. i put one lead to one end of it and one lead to the other when the float was down indicating empty the ohms were 2 and when i moved the float up to full it read 16. was this the correct way to test it. how would i know it is defective or not. i believe it is still under warranty. i will try the test you gave me when it quits raining and i can drop the tank again. You always give me good advice, thanks.
Last edited by 4rdrules; Apr 24, 2007 at 01:46 PM.
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I believe the dash gauges were reversed when the dash style changed in 87. The 85.5-86 302 EFI's used the old (and reverse of the new) polarity. The two sending units will fit in the same hole, but the resistor on the sending unit is different. The 85.5-86 ones are hard to find because they were used only for 1 1/2 years. I thought the plugs were different, but I could be wrong. All I know for sure is the newer sending units for the 87 and up are reverse of the older ones.
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