Ford truck fuel gage..empty?

Friends 1998 Ford diesel powerstroke 250 with dual fuel tanks has a problem with one tank's fuel gage. It reads empty all the time, even when the tank is full.
Question is, should I be looking for a short or an open? Is the fuel transmitter a float activated rheostat like in the olden days, or??????? Any tips are appreciated before I bundle up and brave the wind and snow to check it out. Thanks in advance, Lynn
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So... let me get this straight. a dual tank machine, and presumably the owner would have some clue as to how many miles to expect per tank.
Where the normal practice would be to drive/run 2/3 of the expected capacity from a full tank.. then switch to the one with a gage.
That must be a helluva friend. I mean, even if he's plowing surely it could wait for a break in the weather. Just switch to working gauge when he drops the blade.
Use a voltmeter, not ohmmeter, two reasons... it's NOT 12v so not confusing.. mistake can blow your ohmmeter, start at the switch with key on
looking for zero to five volts (which is what gage supplies) and the sender grounds it
full tank always was near or no resistance (a short) so youre looking at straight five volts until you come to the break in the wire or bad connection
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wrote:

Thanks Krusty, (Friend thanks too) I think I've got it. The gage sources 5 volts, and the tank sender grounds this through a rheostat (or equivalent), with a near zero resistance to ground indicating a full tank? Sounds like a possible short to ground somewhere along the path from the switch to the sender (or maybe the switch or sender themselves) Friend says Krusty can't be that bad. Lynn
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Now I am confused.. remember, the sender grounds.
In the legacy case, taking the wire off an oil, temp, or gas tank sender causes gauge needle to drop to 'zero'.. and grounding it sends the gauge to full or high or hot.. ie, the opposite of no needle movement.
thus you are looking for the source point of a disconnect.. not a 'short'. On the gauge side of the failure point, the wire should read 5 volts. ignore fluctations, if any.
Be aware that some think a 'short' means a break.. in reality, it's connecting something that shouldnt be connected.
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Well, Gee Whiz, I was wondering how you could be so bone-headed....... Now I see that in my original trouble description, I said (but did not mean it) that the tank gage always read empty. BUT I SHOULD HAVE said that it always reads "FULL".
Very sorry!
Maybe I'm getting too old to think and type at the same time!
Thanks again, Krusty! Lynn (and friend)
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Aw, crap! That changes things JUST A LITTLE!!!!.
Once you've determined which wires carry 12 volts (remember fuel pump and/or relay) test wire with FP's running, switches in both positions.
You can eliminate them.
THEN try to determine which wires head toward fuel tank and use your ohmmeter with ign switch off.
Hopefully the working sender shows 'not a full tank"... to make them easier to distinguish.
Another thought... with switches in both positions, each time, do a 'tug test' along the path of the wiring. Be advised, you may well have a bad sender, and that's in-tank.
But - unlike a gasoline model - that tank could be, with sensible precautions, removed inside a garage.
Like no open flames or things that spark, like a drill or compressor and leave gar door cracked at bottom.
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I think that's wrong/opposite, if you yank the wire off of the fuel sender I *think* the fuel gauge will go full. (maybe chevys are that way and ford isn't?) Also, the first place to look is the switch(probably built into the fuel line switch), but since your crawling under it anyway visually inspect everything you can under there first. If you get a book with a wiring diagram it will make the diagnoses oh-so much easier. Make your friend buy one, you will probably need it again anyway :)
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chevy`s are darsenal movements and i think fords are heated bimetals
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PCK wrote:

Just about every automaker uses this type of movement now: auhttp://students.concord.edu/murphypv/NEETS/Chapter%2052%20Summary.htm The bimetallic strip is really old stuff. The difference between a Ford (and Chrysler) vs. GM fuel SENDER is that the first start with high ohms resistance for full, and low resistance for empty; GM does it the other way. Grounding a Ford sender will send the gauge high.
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Lynn Coffelt wrote:

disconnect the wire at the tank and check for power by shorting it to ground .... does gauge go to full? (if yes check to see if tank is grounded (plastic tank?) if ground is ok replace fuel sending unit(note fuel units in ford became shitty. they are ceramic with carbon coating for a rheostat and the carbon wears off... no connection)) step 2 : if thats not it then find the dual tank switch and disconnect the wiring to the gauges... check wire for continuity from switch to sending unit. (obviously if it works on one tank the problem is either with the switch or the wiring to the other tank .)
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You're bass ackwards here. If the wire is grounded it will go to empty and an open circuit will go past full. Bob
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Bob wrote:

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Well, I never said they were "wired like a Chevy" but yea, an open circuit will cause the fuel gauge to go over full. Been that way for a long time too. Bob
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Bob wrote:

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