Age old fuel sending unit question...

[jhaag]

what about the gauge?

[msbaugh]

Thought of a couple of things to check
Check the resistance from the installed sending unit to any good ground on the frame/body...there should be no resistance, if so the ground is poor.

The sending unit wire I suppose could have up to 10-ohms resistance and still register a full tank when grounded...find a 10-ohm resistor and substitute for the sending unit, ground one end and connect the other to the wire & see what you register, if 1/2 tank you know the wire has an issue.

Thanks for the help!!!

Grounded confirms a full gauge reading
1 10 ohm resistor does exactly as you said and put 1/2 gauge reading (what it is while full hooked up to the sender)
2 10 ohms in series for 20 ohms puts it at 1/4 gauge
2 10 ohms in parallel for 5 ohms puts it at 3/4 gauge

This tells me I have too much resistance in my sender wire, correct?

Another thing to note my temp gauge also reads very low! My oil and ammeter gauges are perfect. There's no circuit board on this just a little limiter mounted on the back (rally dash)

Any advice? Thanks for walking me through this guys!

[msbaugh]

Just did my final test, I've got 40 ohms of reisstance between the sending unit wire and ground when bypassing the sender directly to the - battery.

Likewise, my temperature circuit is putting out 30 ohms when bypassing the sender directly to ground.

Now what in the hell is causing this? These should be close to 0 correct?  I need to find what could be causing this hmmm

[Bullitt-]

isn't that going through the gauge & volt limiter ?

[msbaugh]

Here's what I've drawn out

[msbaugh]

isn't that going through the gauge & volt limiter ?

Yes. I re-routed a new wire running directly from the back of the fuel gauge to the sender eliminating the old wire. Unfortunately it still reads half on the gauge. The extra resistance must be in the limiter or the gauge itself then, correct? That would mean new gauges are in order I guess

[Bullitt-]

I can't say their bad .... that long wire you have, use it to ground the sending unit at the tank to your battery to see if anything changes. 

[msbaugh]

I can't say their bad .... that long wire you have, use it to ground the sending unit at the tank to your battery to see if anything changes.

When I touch a wire from the - battery to the sender body there is no change.. It still stays at 1/2 a tank. I'm 100% positive that the sender has a great ground. 0 resistance to the battery.

There is no way to peg the gauge out full unless i bypass the sender completely and go straight to ground.
When the sender is Hooked up it automatically goes to 1/2 on the gauge. And replacing the sender wire makes no difference.

The resistance in the float travel is good, but when it's added to the circuit it's too low. There's something under the dash or in the gauge doing this to both the fuel and temp gauges!

[Racer57]

Another thing to note my temp gauge also reads very low! My oil and ammeter gauges are perfect. There's no circuit board on this just a little limiter mounted on the back (rally dash)

Hope this helps...
http://www.cuda-challenger.com/cc/index.php?topic=97102.0

[Bullitt-]

The resistance in the float travel is good, but when it's added to the circuit it's too low. There's something under the dash or in the gauge doing this to both the fuel and temp gauges!

Seems you've about eliminated everything but the fuel gauge... Oil pressure gauge would not be working properly if things were amiss on the power supply side.   

[msbaugh]

Thanks for the post! Unfortunately there's gotta be something wrong with the gauges themselves... At 200 degree coolant temp the temp gauge isn't even at the minimum. Likewise, the fuel gauge won't read above 1/2 way.

I've narrowed down a lot of y'all's great suggestions and everything I can think of. What would be the symptoms of a voltage regulator failing/putting out 2.5 instead of 5 volts?

Also when these gauges get old is it typical for them to start to read low?

[soundcontrol]

If both the gas and the temprature gauges are low, it seems to me like it could be the 5V voltage regulator. I believe the only gauges that uses the regulator is temp and gas and if the regulators voltage gets lower both those gauges will read low.

[JoeGrapes]

I've been screwing with the problem of my gauge not reading right for years. I have my old Chiltons book from back in 1972 and I tested the gauge the way they show and it tests fine. I've checked all the wiring, replaced the voltage limiter and even ran a ground wire in place of the ground strap. I do have and after market sending unit and I never really get the ohm readings everyone on here said I should. So out of frustration I just put exactly 1/2 tank of gas and bent the float are until the gauge reads 1/2. At least then I know how much gas I have once,lol

[msbaugh]

I just got a solid state limiter in I'm going to instal it and cross my fingers. If that doesn't work then I'll just plan on sending all the gauges into a dash specialty shop for some restoration/modernization. 

[msbaugh]

I fixed my problem!! Finally got the 5v limiter switched out once I got back in town and wa-la!! Everything has doubled on my gauges. If any of y'all have gauges that read low it would definitely be worth looking into replacing it with an rt solid state limiter like I did.

If you think about it, it makes perfect since. Basic ohms law says v=IR right? These under dash limiters must give out lower and lower voltage readings once they start getting old. I measured the output on my old one and although the voltage varies (normal), it never went over 2.5 volts. With half the voltage, the measured resistance was way off! Explains why my fuel gauge only went to 1/2 a tank and why my temp gauge barely moved. I also found out my oil pressure isn't quite as low as I thought it was lol! No need for heavy 10-40 oil

Very happy I dont have to tear into any of my gauges and have them rebuilt for $900