The Sending Unit is the First Suspect:
R._. :   I have two 1970 Chevelles, and both have given me trouble with the gas gauges. In the first car the gauge would bounce all the time. I took that gas tank out and looked everything over and nothing was out of order. If Ihit the brakes the gauge would go down, and if I took them off it would go up. My second car would do the same. Then it started going to empty when I turned the key off, but when I turn the key on it goes up. Sometimes itsticks and will not move when I add gas. I will get down to almost empty on the gauge, and then add three maybe four gallons. Suddenly, the gauge shows almost half a tank. What do all of these problems indicate?
B.M. :   Replace the sender.
P._. :   I am not a 1970 expert, but I think that it is safe to say that gasgauge operational design did not change much during the 60s and early 70s. From your symptoms, I would guess that you have a bad sending unit in both of your vehicles. If your symptoms are not consistent, you may have a bad ground in the area of the sending unit or gas tank. A sending unit is typically nothing more than a variable resistor (rheostat/potentiometer). The float is attached to the wiper or tap of the "pot". As the fuel level in the tank changes, the resistance of the "pot" changes. This alters the current flow in the gauge circuit and changes the reading on the gauge itself. Sometimes the windings of the rheostat ("pot") within the sending unit can get bad spots on them (i.e., dirt or shorts). This can cause the gauge to read incorrectly or erratically, but the symptoms for this should be consistent (i.e., when the tank level drops to 1/2, the gauge bounces between 1/2 and 1/4, and then after a while it settles at 1/4). If you remove the sending unit, you may be able to test with an ohm meter. I do not recall what the normal max/min resistance is, but I think it is something pretty low like in the "hundreds" of ohms. This should be able to be measured between the sending output terminal and the case of the sending unit. As the float is manually moved up and down very slowly, the resistance should change gradually without seeing big jumps in the resistance reading. This may be tough to see if the bad spot is small.
B.R. :   I think it is common for this to occur. My 1972 Chevelle was reallyempty at a 1/4 tank, but that was because it had a dent in it. My 1970 does the same thing your cars do.
Archivist: Tom Wilson