So the '71 has been converted to under truck tank location (damn thing looks factory) but it doesn't have the sending unit hooked up and I don't want to have to guess as to when I'm about empty.

Now there's a wire right behind the seat that I know is the sending wire for the gauge and the tank looks to have the sending unit in place with a few wires (or hoses, didn't have any light source to check it).

Is it as simple as connecting wire to wire until the gauge moves???

I'd say yes...can't hurt to try. But make sure it's grounded properly so you don't get crazy readings. Do you know if that SU has ever been hooked up? If it has...it may not read out, and then send you looking for something else, when in reality the SU is just bad.

I don't know, I'm just trying to get it working as fast and as easy as possible... I'll go out tomorrow and snap a shot of the tank and maybe that'll help...

Main trick is hte resistance of the sending unit. I don't remember the range right now, but if the resistance is wrong, the gauge won't work.

If the resistance isn't right, what can I do to make it right???

You would need to find a resistor sender in the right range. J C Whitney has some universal senders that you could make work if you have to

Where can I figure out the resistance needed?

Mil1on should have the specs... as should BKW, otherwise, service manuals are a source, but whitney does ask the resistance, not sure if the Fords stayed the same back then. I do know at 92 it was different than the earlier versions...

Here, this should help... http://www.fordification.com/fuel-sending-units.htm

IIRC ...From 0- 90 Ohms

Re: Ford Gauges


They are calibrated to read empty or zero at 70 Ohms of resistance. The full or maximum reading will be at 10 Ohms. All of the senders are calibrated for this 70-10 Ohm range. The fuel sender at 1/2 tank should read 40 Ohms.

~Ohms >> Reading
150 >> No movement - same as if the wire were broken or disconnected
73 >> Off scale 1/2 needle width below E
68 >> E
50 >> 1/8
26 >> 1/2
15 >> 7/8
12 >> F
10 >> Off scale a needle's width above F

Since the gauges are all a little different these numbers are not exact, but close. Shorting the sender wire to ground is a way to test the gauge but since its Zero Ohms to ground it will be overheating the gauge if you keep the short there for very long.

Even with an empty tank the needle will move just a bit when the key is turned on so that it is at the 73 Ohm position. That tells you the wire is not broken. With the 16 gallon tank and an original Ford float properly adjusted the first 1.5 gallons does not change the gauge reading; Ford's way of getting you to fill up early so you won't run out of gas. Likewise the last gallon does not change the slightly more than F reading; makes you feel good that the needle didn't drop after the first 10 miles of driving.
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