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Fuel Gage Not Reading Right?

PurpleBeeper

PurpleBeeper

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OK, I've read about a half dozen posts on this and I'm stumped. 1970 Roadrunner with a factory fuel tank (99% sure it's original). The fuel tank has an aftermarket "dual outlet sump" on it, so fuel goes out those two outlets & the original fuel line at the sender is capped off, but the sender is still hooked up to operate the gas gauge. The car sets just about level...maybe 1-degree nose up. The gas gauge reads a little less than 1/2-tank when the tank is completely full. Here is what I've done, in order:
* grounded the sending unit wire & gage pegged FULL
* ran an extra ground wire to the sending unit "ground strap" to the body of the car (since it has aluminum fuel line)
* installed a new aftermarket sending unit
* replaced the aftermarket sending unit with a working used original sending unit
* replaced the gage itself with a new reproduction gage

After all that, the gage still reads about 1/2-tank when the tank is completely full. There has been a tiny bit of variation, but the gage reading is off by just about the same as what I started with. The only piece that is original now is the wire between the sending unit and the gage.....what the heck am I missing here? How critical is the resistance on that sending unit feed wire? If I've got some type of patch job on that wire (likely) would that throw off my gage a lot?
 
Someone posted that there is a 5ohm resistor modification you can do to fix this. I dont remember who posted it but it fixed this issue.
 
Take sender out of the tank and attach it to ground and your sender wire. Move the arm through the range from empty to full, see what the gauge does. If it goes from empty to full then you know the sender works and the gauge works. What I’ve found is the sender arm needs tweaked to have the float reside in the right part of the tank at the right time.
 
I operated mine out of the tank and it went from empty to just over 3/4 full.
Put it in and it is the same as yours - tank dead full and it reads just over 1/2.
I live with it.
 
You solved your problem in you 3rd. sentence. aftermarket. None of them are right. Or accurate
 
Last edited:
There have been cases where the old original brass floats have developed small "pinholes" over a long period of time that can allow some gas to seep in and make them sink a bit. I think I read somewhere that Ford pickup truck floats from the seventies will interchange with the Mopar floats if needed, but am not sure about it...
 
PurpleBeeper said:
* replaced the aftermarket sending unit with a working used original sending unit
* replaced the gage itself with a new reproduction gage
Click to expand...
Using a working used original (tank) sending unit. In which ways did you determine it was 'working'?
On my 64, using original sending unit, ohm tested the full/empty range. Actually had to combine parts from two units, to make one good one. Float should be empty, brass or other. The other can get gas soaked, heavier, and won't float right.

Repo gauge? Calibrated right? Checking the gauge itself, it should at least make the full sweep.

Last, but not least, sending unit wire. I was able to use my original. It ohm checked at 0.2 ohms. Barely any resistance. No resistance would be 0.

Matter of checking all involved parts. (Mine reads right!)
 
Thanks for all the input guys:
@toolmanmike --> I have an original sending unit in there right now.
@69L48Z27 --> Good idea. I can at least check the wiring/gage easily with the aftermarket sending unit I took out. The only test I've done like that so far is to ground the sending unit wire which did peg the original gage. I ran that test before I installed the aftermarket sending unit (which I later removed). It did at least cross my mind that the tank isn't original and I never checked to see if the float is half sunk and/or the float arm moves freely.
@BeepBeepRR --> good idea, I will have to look for that post.
@Rockland71GTX --> cool little gadget. I may give that a try + you can add a "low fuel" warning light which would be nice.
 
OK, I had my gages, reverse lights, turn signals & wipers all stop working at once. I traced it to the black wire (2nd one in) on the ignition switch plug down at the bottom of the steering column. All I've done so far is push the two connectors inwards by pushing on the back of each one with a tiny screwdriver.....anyway...that connection ain't so good (the power supply). Could THAT make my fuel gauge read really low vs. the actual amount of fuel in the tank? (a borderline positive power supply, not the typical ground/resistance issue near the sender, or the sender itself, that causes gauge to read wrong)
 
Yes a reduction of power in to the gauge from the required 5 volts would affect the gauge.
It may not be your problem but you would need to check the voltage coming out of the gauge supply.
Due to it being a circuit anything that affects the power supply side or earth side will make the gauge read funny.
 
i used following tube-type from summit. need to tack weld steel ring on inner side of tank to allow threading of screws.

https://www.summitracing.com/parts/cin-sn38t9

not as linear as i wanted. havent tested running it to empty. im running to an autometer fuel gauge 0-90ohm. this particular sender is 0-90ohm GM style


watermelon
 
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