whats up with my gas gauge

[70ptta]

When I fill up my tank the gauge show 3/4 of a tank how can I adjust this. This is on my 72 Trans am. thanks

[qwik1320]

Did you just put a new sending unit in? I had a similar problem and had to drop the tank and replace the new sending unit with another one. It would get hung up at about 1/2 tank. As far as I know there's no way to adjust the gauge (I'm assuming you have the factory gauges).

[70ptta]

I do have factory gauges. The last guy who had the car said he put a new tank. How hard is it to change the sending unit.

[dhardison]

I had the same problem after purchasing a "foreign" made sending unit. I finally ponied-up for the GM sending unit, re-dropped the tank and all is well now.

In order to change out the sending unit you have to drop the tank, then R&R the unit. Hardest part is dropping the tank, especially if it has any fuel inside.

[70ptta]

i got a full tank i guess ill wait till i burn it all

[qwik1320]

Yeah, dropping the tank is the hardest part. Enlist the help of a friend or two. Although unhooking the rubber fuel line was kind of a pain on my 73, I did it with the rear tires on so that limited my work room. There are two wires, one should have a plug that you just need to unplug, the other is the ground that's usually screwed into the frame rail. Don't forget to take that off or you'll get the tank half way down and...

If you jack the rear up high enough you can take the tank out without removing the front of the tank straps, just bend them out of the way toward the differential. Drop the front of the tank down and slide forward so the filler neck clears the rear body panel.

The sending unit's held in with a locking ring, carefully turn it with a screw driver and hammer until the ears line up with the slots. Make sure the top of the tank is super clean so no crap falls in to the open hole. There's a rubber o-ring seal under the lock ring. A new sending unit should come with a new seal and lock ring but don't throw the old ones away until you check.

[73novaguy]

Sounds to me like your sending unit is bad! If the potentiometer (little thingy actually attached to the sending unit with the float hanging out of it) is bad then your gauge would read off. My factory gauge in the car would only go down to 1/4 tank no matter how empty the tank was

I pulled out the sending unit and measured the ohms across the two terminals that were described earlier. and it read from 20 ohms to 120 ohms. Early GM sending units are 90 ohms, so when the tank is empty Float all the way down) the sending unit will actually read 0 ohms and when the tank is full (float all the way up) it will read 90 ohms! That is why my gauge would only go to 1/4 tank because it would only go down to 20 ohms!

So what I'm trying to say is drop the tank (very easy, usually just two 9/16 bolts holding the straps) and when half lowered unplug both wires (one for ground and one wire that attaches directly to the top of the sending unit (power wire). Loosen the lock ring with hammer and screwdriver and pull out the sending unit. With the sending unit on a clean bench, measure the resistance (ohms) across the power wire plug and the ground wire running from the top of the sending unit. My geuss is that your sending unit is only gonna read to around 75 ohms at full float height. But if your sending is reading the correct 0-90 ohms then check to make sure you don't have pinched power or ground wires and that the frame is clean where the ground plugs into!