Curious Fuel Gauge Behavior

[darth_linux]

So I just today got my fuel gauge working again in my '66 Newport by changing out the float. While things were apart, I took note of the fuel gauge with the float set to "full" and "empty," noting that the gauge read just over a quarter for "full," and just about an eighth for "half." I ran the tank to empty in order to do the job, so I added a gallon from a gas can, and then went to the corner gas station and added 12 more gallons, for 13 total, or just about "half." Much to my surprise, when I started the engine, the fuel gauge rapidly rose to the "halfway" point, and then slowly started falling, not to the 1/8 point as expected, but closer to the 1/4 point.

It would appear my fuel gauge reads completely correct for the first 5 seconds and then falls to about 50% of the expected value.

Has anyone observed this before? The sending unit is a MoPar 2580624, which came up as "correct" for 65-66 C Body. Not sure if it's original, or just a factory replacement, but it's not from China . . . . as far as I know.

 

[CBODY67]

No matter where it was made, the noted behavior should not be accurate. Wondering if there is some resistor in the circuit which might be warming up and changing values?

What about the voltage limiter in the instrument panel?

CBODY67

 

[darth_linux]

No matter where it was made, the noted behavior should not be accurate. Wondering if there is some resistor in the circuit which might be warming up and changing values?

What about the voltage limiter in the instrument panel?

CBODY67

Voltage limiter is a new unit made by “standard” products. Verified to work correctly with volt meter, pulsing ~5v about every 3 seconds.

 

[halifaxhops]

Other guages OK?

 

[Big_John]

I suggest one of these. TechnoVersions - MeterMatch for Analog Gauge Correction

I have one in my 300 vert.

 

[darth_linux]

Other guages OK?

Yes!

 

[cbarge]

All 3 C's gas guage is not accurate.
I bought 1 meter match and will be installing it in Grace.
Full is pegged but 1/4 is half .
Here is a link to a member's thread pertaning to meter match.
Fuel Sender calibration using Meter Match

 

[darth_linux]

Thanks for all the tips guys. I’m gonna leave things as they are for now since the gauge reads accurately upon initial start up. I’ll drive for a couple of weeks and see how it reacts to the gas level getting lower and lower.

 

[CBODY67]

I'[m wondering what happened to the accuracy the sending unit/inst pnl gauges had when the cars were "used cars" in the 1970s? Certainly, it seemed that they all took a while for the needle to move lower than "F" with a full tank, which was normal for all vehicles back then . . . even some current production cars, too. By the time they got to "1/2" they were usually pretty accurate. What happened near the "E" mark was variable and needed to be watched, especially on the A-body cars A friend had a '70 Dart Swinger 340 and claimed that when the needleh it about "1/8", you'd better be driving into a gas station right then . . . on the three or so A-body cars he'd owned, all were the same.

Is it the wiper arm/sending unit rheostat coil, the aged wiring terminal contacts, the dash gauge resistances, or all of this together which is making the gauges inaccurate? Or the connections at the bulkhead connector?

Just curious,
CBODY67

 

[Dmbrassy]

How bad are the meter matches to wire in? Do I have to take the dash apart

 

[CBODY67]

How bad are the meter matches to wire in? Do I have to take the dash apart

Most seem to install them near the tank and leave them in the trunk. A few threads in here on them.

 

[Big_John]

How bad are the meter matches to wire in? Do I have to take the dash apart

Real easy.

The size is less than a cigarette pack and you can put it about anywhere. It does need power, so I get that at the fuse block. It is electronics and not a sealed unit, so under the car is a bad idea.

A good place is under the kick panel on the drivers side, but being able to see the gauge while tweaking the Meter Match is necessary. The wire from the tank runs up through there and it's not far to run a power wire. Mine is just under the dash with a tie wrap.

Here's my adventures with one: Gas Gauge Fix