Question for professional restorers.

[Joegrapes]

Ok, I've been screwing with this fuel sending unit problem for years. I have not found an aftermarket sending unit that works. I mean works right. They all seem to make the gauge needle move but not at all accurate. When you guys restore a car for a client and you need a new sending unit what do you do? Look for an NOS unit or have you found someone that makes an exact repro of an original unit that works right?

[70 Challenger Lover]

I'm not a professional restorer but I've read up on this stuff for other cars with similar problems. One thing some guys have done in the Corvette world is to measure the resistance at various float levels and then add a resistor of the correct size behind the gauge to get it closer to factory specs. It's never as perfect as a modern car but it gets it fairly good.

The only part I care about is where the needle is pointing as the car runs out of gas. I've had cars that show a full needle width below the empty line as they are going dry. Others that were a full needle width above the empty line. That was good to know when I was young and cruising around with little change in my pocket. Now I just top it off whenever it's below half.

[Cuda Cody]

At best, they are not accurate IMO.  But they are consistent.   You know how big your tank is, right?  Drive it until it's at half and then fill up.  Some simple math will give you a good idea what you have left in the tank.  Do the same for 3/4 and 1/4 full and you'll always have a great idea of the fuel left.

[70 Challenger Lover]

On Mopars, it's been my experience that they are not far off when displaying empty but they always seem far off when the tank is full. This is fine with me as I'm more interested in accuracy when I'm running out.

[Joegrapes]

Oh yeah. I do exactly what you guys suggest. I've lived with it that way for years. I just can't believe that somebody like Graveyard Cars tell their customers "everything on the car is perfect but the fuel gauge will never read right, you'll just have to guess at" LOL

[gzig5]

The new one I got with my tank reads empty with half a tank left.  I shouldn't ever run out.

I saw a thread on another forum or maybe here recently that said the original sending units used a non-linear resistor and the new ones use a linear one.  Its a wire wound resistor and new ones are done on a rectangle and the original is tapered from one end to the other.  There was a pic that made visualizing it easier.  Supposed to be an aftermarket box you can wire in to compensate for the different resistor.  my links are on the other computer.

[Joegrapes]

You're right. I've had them both apart and I can see that the aftermarket ones can never work properly.

[70 Challenger Lover]

There are specs for adjusting the fuel gauge for so many ohms of resistance full, so many empty, and so many at half. It's been a while so I can't recall the resistance numbers but I used to redo the gauges way back when using a 5 volt power source and the correct resistors purchased at a Radio Shack. With the gauge out, you can actually tweak it inside until you get a more precise reading using the specified resistors. You couldn't make it perfect everywhere though. You had a choice of trying for balance where all three points were consistently close but none were perfect or you could adjust one area of the gauge for perfection but the other two points would be off by a little. I always tweaked it so empty was as perfect as I could get it and half and full were wherever they landed (usually very close).

One variable was if your power source was slightly higher or lower than five volts, it affected the reading on the gauge. They make modern voltage regulators for our gauge sets that produce a more precise power output so I always used them to eliminate that variable, ditching the old one.

The catch with all that is the sending unit in the tank. If it did not produce the exact same resistance numbers as your radio shack ones during bench testing, then the gauge would read differently. So to do it right, you would need to measure your new sending unit and check how much resistance it has at empty, then calibrate the gauge so that it was in sync at empty. Half full and full just falls where they fall and you would have to live with that but they would likely be fairly close.

For the fuel gauge to be precise throughout its entire sweep, you would need to redesign it inside so that it produced a precise resistance reading at empty, quarter, half, three quarters, and full. Obviously not realistic. I think modern sending units have floats that go straight up and down on a tube so they don't get the distortion effect of one on an arc like ours.

[Joegrapes]

Thanks for all the help everyone. I know now this problem is pretty common. I'll just make a note on where the gauge reads at empty and half full and call it a day.

[gzig5]

Here's one iteration of the calibration boxes.  I've read they work OK but no experience.  I'll probably play with resistors to get mine to be more accurate on the low end of the tank.  I don't really care what it is above half a tank.

http://www.tanksinc.com/index.cfm/page/ptype=product/product_id=748/category_id=184/mode=prod/prd748.htm

[70 Challenger Lover]

That's pretty cool. On my Corvette, I plan to Add EFI and a non original tank more compatible for the conversion. Something like this might pair up the original gauge with the aftermarket sender.

[Joegrapes]

Hmmmm, that looks interesting. I'll check into it, thanks.

[bcudachris]

No pro here, but these guys did me right after fighting with the aftermarket junk.

http://tristarrradiator.com/

Sent them my 5/16" returnless, got it back a 3/8" with a return.  Used my rheostat assembly and mount plate.  Just as accurate as it was before.

[gzig5]

Good to know.  Unfortunately I recycled my old sending unit with the old tank.  Maybe find a used one...