My fuel gauge “bounces all over the place”

November 1, 2015 | By Staff

Question:

I just finished reading the August MOD item concerning Chuck McGee’s 1969 Grand Prix gas gauge problem and your sending-unit diagnosis. So I thought you might be able to offer your opinion on how to solve my wavy fuel gauge reading on my 1968 Oldsmobile Delta 88 4-door hardtop. We’ve owned this car since new and the gauge always worked well until this past year.

Basically, my original canister-style sending unit stopped working correctly. As the fuel level changed in the tank the resistance from the sending unit would not change according to the fuel level in the tank, causing my gauge to not read correctly. I suspected the fuel sending unit in the tank as being the problem. However, before dropping the tank I wanted to do some diagnosis first. I started by checking the tank sending unit ground wire and then I moved on to the fuel gauge wiring circuit and gauge.

The tank ground circuit was good and so was the fuel gauge and wiring. I verified the fuel gauge and circuit operation by disconnecting the lead in the trunk for the tank sending unit and connected a decade resistance box to the fuel gauge circuit and ground. With the resistance near zero ohms my fuel gauge read Empty, at 95 ohms the gauge read Full and at approximately 45 ohms the gauge read ½ a tank. When I finally dropped the tank and removed the sending unit it did not meet the shop manual specification of zero=Empty and 95 ohms=Full.

Holes
Holes in the sides of the canisters controlled slosh and helped the gas gauge to read correctly.

This was also when I discovered I had a GM canister-style fuel tank sending unit. The only replacement-style sending unit available for my vehicle is a swing-arm type. While the aftermarket replacement swing-arm style unit does fit and operate the fuel gauge to the correct level when the vehicle is not moving, the new sending unit does not offer any damping. As the fuel sloshes in the tank during acceleration, braking or cornering, my fuel gauge level bounces all over the place. It appears the old canister-type sending unit design prevented this by having small holes along the side of the canister preventing any large changes in the fuel level within the canister and reading to the gauge.

A search of the Internet has found a number of GM brands (Chevy, Buick, Pontiac & Olds) from the 1960s used this canister-style sending unit. And people who have had to swap their canisterstyle unit for a swing-arm sending unit now have the same problem, with no solution that I have been able to discover. Some Ford guys have wired in anti-slosh modules found on a 1980s Mustang and some European guys have wired a capacitor into their fuel level circuit to dampen the signal to the gauge.

My questions are: Is there a fix for my problem? Is there a replacement canister-style sending-unit available, or should I try the anti-slosh module or a capacitor approach?

Answer:

This is a common problem with swing-arm replacement sendingunits for certain GM products from the era of your Oldsmobile. However, another possible cause of erratic readings from your gas gauge is that someone worked on the car previously and removed the ceramic resistor from the back of the gauge in the dash. Some people erroneously assume it is not necessary or they lose it or possibly break it, but it is vital to keep your gauge from reading erratically because it controls how fast the needle sweeps across the gauge.

The one on your Oldsmobile will be white ceramic on a green rectangle and attaches directly to the back of the gauge.

If the resistor is sound, there are sources that can rebuild your canister-type gauge or furnish you with a rebuilt, but they are not cheap. GM Obsolete can rebuild your old one or provide a replacement for about $300, and Tri Starr can also rebuild them if necessary. Try these parts sources before you go to more creative solutions such as wiring in capacitors:

GM Obsolete

909 W. Magnolia St. Phoenix, AZ 85007

gmobsolete.com

Tri Starr LLC

1452 State Route 89, Seneca Falls, NY 13148

tristarrradiator.com