Well that sucks Wtarmon. Sorry about that!
Yes, it's supposed to read accurately, and as far as I know for the most part they do. I know fuel level gauges have been the bane of Bronco owners existence for years. And this is why we went to some extra effort to try to get things more consistent than was the case with the old swing-arm style.
But check a few things first, just to make sure you don't go to extra trouble and try to change out the sender only to find out the new one reads the same. That would suck even more!
Presumably because you've just worked things out at the rear with the new tank, your ground is clean and voltage is good coming to the sender. But if you didn't mess with the wires at all, other than to convert them to work with the new sending unit connections, you might want to check that.
But first...
Check the input and output voltages at the instrument cluster IVR (instrument voltage regulator) to see what your gauge is dealing with. Should be full battery voltage input, and roughly 5-7 volts output.
If it's an original type, you may need an old analog volt-meter to check, because they are a thermal-breaker type that pulses the output voltage something like 4v to 8v so your gauge reads an "average" of that value. Or at least that's how I think I understand things.
If it's a newer unit it might just give you a steady output that's readable with a modern digital volt-meter. Either way, check it to see what is coming out of it and report back.
Either type is usually adjustable as well, so if you find that it's reading low on the output side it can be corrected if needed.
The good/bad thing about that is that your temperature and pressure gauges read off of this as well, and might go up accordingly. But if the output voltage was off, then all three gauges are reading falsely low. So changing it would be a net good thing in the end.
Hopefully it's just an odd voltage problem, or maybe even a wiring problem. But if it's indeed a sender problem we'll make sure to get you another one.
For sure you want to know what voltage is reaching the sending unit and how much is getting to ground of course.
I suppose a smart "first thing" to do would be to simply disconnect the sending unit harness and jumper the positive and negative wires together momentarily. As you do that your gauge should go up to beyond full. If it does, then it would seem to be the sender.
Ultimately you'd want to read the ohms on the sending unit, but without knowing exactly how much fuel, it would be hard to know precisely what your reading should be without consulting one of the charts here that members have put together for stock senders, or contacting the manufacturer to see what it should be.
Good luck.
But when in doubt, call it in to WH and see if anyone knows more about it, and might have other suggestions.
Paul
