alexandervdr

I got tired of the wild movements of the fuel gauge caused by fuel slosh. I read some threads about this but none that I found actually showed a tested set-up. I just did a quick prototype that seems to work nicely, the needle hardly moves in a sharp turn or when crossing a speed bump. What I use is a 100 Ohm resistor and an 10.000 uF (ten thousand microfarad) electrolytic capacitor. The resistor is there to limit the current when the tank sender resistor is at a low value. The electrolytic capacitor (minus side is grounded) buffers the voltage long enough to get rid of the fast fluctuations. I built it in 30 minutes with some scrap parts. I hooked up the unit temporarily at the fuel tank side cause that is easier acces than behind the dash. A nicer build version will go behind the fuel gauge. Have only been driving it for 10 minutes with the tank 1/2 full. Maybe some fine tuning of the resistor value will be required to accommodate for full and near empty tank. I'll keep you posted on this. The only 'quirk' I found is that the gauge needle goes full empty for a second or so when turning off power. That is cause the capacitor empties over the brown wire into the gauge. The green wire gets no longer voltage from car battery, so polarity provided by capacitor as seen by the gauge is reversed. Hence the needle going full empty. Don't think this is a problem.
The wire codes on a C2 differ per year, I believe the ones on the drawings below is for a 66. in my 64 'brown' was 'Tan' and 'green' was 'Black/pink'. When in doubt, refer to the S and I inscriptions on the sender/tank side

Frankie the Fink

I'll await a final version.

My 63 gauge bounces around like Dolly Parton in a zoomba class...

alexandervdr

Well, no Dolly Parton over here except at full freeze frame
I may actually just keep it in the back, digging behind the dash is not my favourite sport. It's only a condensor and a resistor with 3 wires coming out. Easy to weather proof in a rubber or epoxy casting.
Here is how it's temporarily done now for testing, condenser/resistor under the duct tape. I used the wires and connectors from an old harness to integrate.

Vitaminmopar

This is an idea I will store for future use. I think if you place a small diode in series into the modified circuit before the resistor that will keep the gauge from reversing. When power is removed the cap will discharge through the normal path back to ground. The diode will have a small voltage drop across it so you may have to adjust the value of the resistor to account for that drop. Full empty is an oxymoron but I know what you mean. Dolly Parton in a zoomba class, now that would be udderly worth watching.

426 Hemi

I see what you did there.

Bubba63

Alexander,

In what condition is your sending unit (original, rebuilt, replacement)? Mine would go through the gyrations and never show more than half a tank. After the rebuild, it works as expected without the wild swings. It still does move, but it is at an acceptable level.

GJM

alexandervdr

I had the diode in mind, but need to analyse where to put it so I don't inject any unwanted side effects. If I put it in line with resistor as you suggest, the capacitor may not discharge in some positions of the sender...I think a diode in one of the gauge wires is the better solution, cause there the current is always supposed to go in the same direction , so the wrong direction can be blocked by the diode. Something for next year, 2 days from now

Here is my first shot for putting the diode , but if someone sees a problem with this, let me know

alexandervdr

It's a brand new unit, which I agree sometimes does not mean anything cause doubtful quality of some newly made items....
Mine seems to work, when it's full, it shows full, and when it's empty it shows empty.

alexandervdr

I think I am getting closer to final. Version as in this diagram seems to work perfectly. Both at full, empty and in between gauge is way more stable than before. No more 'needle reversing' when turning off like I described some posts earlier.

Discovered that my gauge is a bit off centre. Full is indicating "4/5th" and shows "empty" when there is still about 1/5th in. Never noticed before, cause of needle bouncing. Nothing to do with the added circuit (I tried without the circuit, same problem) , non matching (new...) sender resistance is at the heart of this. I guess this can be corrected by bending the float arm. One day, when I have the courage to take sender out I may try that. In the meantime I added the red 50 Ohm resistor that brings the low position somewhat higher. Top still at 4/5th though. If the sender floater position is correct, there should not be any need for the 'red' resistor.

William Buckley

While you are talking fuel gauges, I have a '66 coupe with a fuel tank and sender that have just been replaced. This did not help my gauge accuracy as the tank can be filled to the top, and the gauge will only read 4/5 full no matter what I do. I really don't feel like pulling the dash to replace the gauge, but do not know of another method.

AZDoug

Are you sure this isn't matter of the float hitting the top of the tank and not letting the variable resister go to 100%?

Fuel sender units often need to be dorked with so the float goes full range top to bottom..

Doug

William Buckley

That is a scenario that could explain my problem. However, I'm not going to pull the tank or at least drain it to see if that is the problem. Right now I just write the mileage down when I fill up and refuel when the miles accumulate. Lazy, but that's the truth.

claf

When I built my Factory Five Cobr* replica I used a 1989 Mustang LS donor for a lot of the mechanical parts. I found the Mustang had a "factory" slosh module built into the original wiring. When I incorporated it into the Cobr* it worked faultlessly. The module was about 2" square and fit right next to the gauge behind the dash.

alexandervdr

I have one of these too from a 90-93 mustang, I just wanted to try with simple components first. Felt more 'sixties' than the electronics the nineties. And no need to go behind the dash cluster, not my favourite gymnastics

Frankie the Fink

How was this wired in ?

Vettegeezer

Let me tell you my fuel sender experience: bought my '67 in 2006 and found that sender was literally "burned out" (resistance wires burned open). Bought a repro. unit & installed, unit would not provide a "full" reading due to low sender resistance (about 80 ohms). Factory units are marked "90 OHMS", so decided to have my original GM sender rebuilt. The rebuild was done by a highly respected shop with excellent quality done on my sender - resistance measured 95 ohms, so I thought that should work. Wrong! Fuel gage registered about 4/5 at full tank, so I assumed that something might be amiss with the gage itself. I lived with this situation for about 4 years, but at the same time I searched for a high resistance sender always having my DVM with me. Bingo! Three years ago found an excellent sender that measured 110 ohms. The sender appears to be an original GM replacement part and found that the fuel gage now works perfectly. So, bottom line is that the dash gages, like everything else, are built to tolerances that may not work well with nominal tolerance mating parts.
Phil M.

William Buckley

Interesting reply, and it may be the source of my problem, but I had this problem before inserting the present new sending unit and new tank. Damn if I don't hate to think of pulling my tank again!

1snake

Are you sure it's "fuel slosh" and not the lack of dampening fluid in the gauge? Out of all the C-2's I've owned, I only had one that had a fuel gauge that did what you describe and that was solved by having the gauge rebuilt and the dampening fluid replaced. They never had this issue when new.

alexandervdr

you are probably correct that the damping fluid in the gauge is gone after 54 years, but going behind the cluster to take it out is less attractive than playing around with a capacitor and a resistor.
Still, one day when there is a need to take the cluster out I may consider. Any insights of where to put this fluid and where to buy it?

Avispa

Yes, this is it. The gauge itself should be mechanically damped. A correctly working gauge shouldn't need any other damping - not that the OP's solution isn't a good one for a gauge with failed damping. My own fuel gauge lost its damping before I had my cluster restored in 1999 and did pretty much as the OP described. I'm not aware of any tank sending unit having electrical or other type of damping.