Hi all,
I just swapped out the gas tank on my '85 GSL-SE. My tank was super rusty on the outside, with odors of gas, and the sender gauge would drop to zero when half full. But the "outta gas" (not "oiuta" - can't edit the subject title) light worked.
The replacement tank came with its own sender. Now this sender does sense the gas level all the way down, but the "outta gas" light is always on. For this reason, it can't be something in the dashboard, it must be in the sensor sender unit. Before I go under the car once more, I'd like to know -
Does any electrical wizard out there know how that little cylindrical sensor works? There is one wire coiled around the rod that holds the sensor, so I'm thinking the rod is the ground. That wire could be broken, or it could this be a matter of a bad electrical connection to plug to the sensor (does an open circuit light up the dash light?)

Also, when I get around to it (hopefully not right now) the "rheostat" bar in the old sensor gauge that was dropping to zero had several broken windings, so when the float lever went below them, the gauge went to zero. I'm going to try to swap a new one into it from a more recent vintage unit, if that's possible. So there may be a follow up on that...

 

If you use a new sender from something else. the ohms have to match.

Fuel level sending unit- full=6 ohms / 1/2=31.5 ohms / empty=80 ohms.

 

Thanks KC,
Regarding replaceing the level sensor, I'm thinking that the resistance should match.
The left photo is the damaged rheostat (brken windings). The little finger tabs that hold it in place are tough to pry but doing it carefully, they didn't break.
The center photo is the replacement gauge that I mis-ordered and waited too long to return it (so I own it) The rheostat bar is the exact same size, and seems to have the same number of windings (I'm winging it here).It's a "FC02-60-960 RX-7 FC3S 1989/01-1992 Fuel Tank Gauge Level Sensor" currently selling for $95 to $110 on eBay. I had to keep reminding myself that it's a $100 part, as I pried it loose and unsoldered the connection. I used a dremel grinding wheel to cut the little finger tabs that held it in place.
The right photo is the replacement (salvaged from the mis-ordered gauge) installed and soldered into my RX7 sensor. There is a slight difference in the way the rheostat connects to the outside world, but the common location allowed enough overlap to solder the link.
I also "harvested" the "empty tank" sensor (cylinder) and I'll use that to replace the one currently in the tank, should it not be an electrical connection problem.

 

How hard is it to replace the part? My ‘83 reads at 1/4 on a full tank of gas. The rest of the time it reads empty -lol. I got used to it and simply go off the mileage and hitting the reset button when I fill up.

 

Replacing the sender is not difficult. The tank should be removed to replace it. This also gives you access to the soft lines on top of the tank. If they are weak, it would be a good time to replace them. The hard part will be tiny Phillips screws that hold the sender to the tank. Often times the heads are rusted and they must be extracted with vise grips. I'm not sure if they are still available from Mazda, but when I was building my 85 in 2008, I replaced the sender and the screws, and I undercoated the screws to preserve them. I dug around and found a couple pictures from when I replaced the sender after the car was already up and on the road:

 

I refurbished my tank about a month ago. Had to replace the sender with one from my parts car. I used M4-0.7 x 8mm stainless screws to replace mine. Make sure the screws are not too long. I also used a lock washer and blue lock tight.

 

"How hard is it to replace the part? My ‘83 reads at 1/4 on a full tank of gas. The rest of the time it reads empty -lol. I got used to it and simply go off the mileage and hitting the reset button when I fill up."

Same here, but that seemed sketchy, so I kept 2 gallons of gas in the back as reserve (I'm not comfortable with that!) And it relies on that "outta gas" light to function properly. So when I prepped to replace the sensor by proactively removing and reinstalling (with never seize) the metal shroud deflector guard and tank straps last spring, I ended up with the smell of gas. When I finally got around to cleaning out the spare tank, I was ready to drop the tank to replace it, I could see the rust all around the clamshell welded seam. The tank was toast, and the sensor screws were almost impossible to remove even using vice grips (2 broke). There was rust stuck to the rubber gasket. Rochester NY climate is a friend of steel.
I installed the "new" used tank with the sensor that came with it, and now, the level gauge seems to work fine, but the "outta gas" light is on all the time, so ...
when the tank gets down to near empty (250 miles±) I'll have to drop the tank again, and either swap in my old sensor with it's brand-new rheostat, or replace the "outta gas" sensor on the one that's in it. Either way, it's the same amount of work.
I also put never-seize on the screw threads, and topped the heads with fluid film... in case I ever had to remove it again, which is now way too soon...
-- "I do it right, because I do it twice!"