W-27

I just replaced gas tank, sending unit, sending unit wire and all gas lines but the gas reading drops way below what's in the tank as I drive it. Example it read 1/4 of a tank but I could only put 1/2 a tank of gas back into it. Could it be a short? could it be a bad ground? Is it my gauge? It's a 68 442 conv. Your help is appreciated.

MDchanic

This is a recurring question. One to which there are recurring answers, some of which can be found here, here, and here, as well as in your Chassis Service Manual.

- Eric

oldcutlass

With the key in the "on" position, if you remove the wire from the sender it should go past the full mark. If you ground that wire it will go to Empty. If both actions get these results then your sending unit is not adjusted properly. If not, then your problem is with the gauge or the power supplied to it.

W-27

I didn't know that the sending unit was adjustable but I sure hope that it doesn't entail removing the gas tank.

Professur

Have you ever adjusted the float in a toilet tank? Same thing.

jaunty75

It's not a short. Your gauge would be reading empty all the time if your sending unit wiring were shorted, and it would be reading F or well past it if the ground were bad.

We need a little more info here. What happens when you fill the tank? Does the gauge go to F? Past it? Not quite up to it? The fact that your gauge moves as the fuel level changes, even if it isn't accurate, suggests that the wiring is not a problem. You could do the tests that oldcutlass indicates to verify that your dash gauge is moving freely between E and F and not getting hung up somewhere.


I doubt very much that the float is the problem or that adjusting it, assuming it can be adjusted, would do much good. Adjusting it would be difficult because you have no real way to test its operation without reinstalling everything, and having to drain the tank, remove it, make the adjustment, reinstall the tank, and refill with gas over and over again as you make each adjustment is just not practical.

Keep in mind, too, that fuel gauges are and were rarely perfect. I don't think I've ever had car where the gauge stopped right on top of the F when the tank was filled. It'll usually go past the F. On my '73 Olds wagon, the gauge is "offset" by one hashmark. That means that, when full, the gauge is pointing about one hash-mark's worth past the F. When the tank is half full, the gauge is pointing at 3/4. When the tank is nearly empty, the gauge points at about 1/4. I know this, and I live with it just fine. I just need an indication of the amount of gas in the tank, and I have it. It certainly not worth the trouble of removing the tank and trying to make any adjustments.



I just recently bought a 2014 Mazda 6. The fuel gauge on this car isn't even a needle. It's a semi-circular row of LED's that lights up, with the entire series lit when full and one at a time going out as the fuel is used up. The times they are a changin'!

oldcutlass

Unfortunately you will need to take the tank back down.


Yes, the sending unit can be crudely adjusted, as the Professur stated similarly to the arm on a toilet float. To test I just extend the feed and ground wire and test with the tank on the ground under the car.


As Jaunty stated, the gauges can be fairly inaccurate as for determining the exact level of fuel remaining due to the shape and angle of fuel tanks. Mine will tell me when it's full and when it's empty, anything in between is judged by the needle moving back and forth while I'm driving, then I know there is fuel in it. I generally fill it when it drops below a 1/2.

Professur

Hey ... I represent that remark

MDchanic

My wife says I'm a little crudely adjusted. Gets nervous when we go out to dinner.

- Eric

W-27

With the "accessories" ON I grounded the sending unit wire and it made the needle go slightly past the HOT on my "idiot lights" but once I start the car the needle goes down to slightly below the "Full" line, when I disconnected the wires going to the sending unit nothing happened. When testing for current on the incoming wire on my test light it doesn't show any current coming in (is it supposed to be that way?)

oldcutlass

The key needs to be in the "on or run" position. The gauge has 12v on it at all times, the sending unit is a rheostat that basically is giving resistance variations of the ground side of the gauge. It will register from 0-90 ohms depending on fuel level.

76olds

LMAO!!! I just can't comment !

W-27

OK, with the key in the "ON" position I grounded the wire coming from the sending unit and the gauge went slightly over the "HOT" but when grounding the tan wire coming in from the wiring harness the needle dropped just slightly under the half tank mark. What does this indicate?

oldcutlass

How does a fuel gauge read hot? You need to test the gauge from the wire at the tank to ensure the problem is at the sender. There is a 3rd post on the back of the gauge that is tied to ground.

MDchanic

Yes, I'm not completely understanding either.

Did you read the links I posted?

- Eric

W-27

I read everything, it's an idiot light gauge therefore the needle can go past the "F" mark (which it does) and it stops at about 2MM past the "HOT" on the idiot light gauge, when shorting the wire coming from the gauge the needle drops slightly below the "half tank" mark. I can take pictures of it.

MDchanic

Okay, you should not be shorting anything.

The sender, as stated in the links above, has a 0-90Ω rheostat, which is connected to ground.
It puts a resistance between zero (direct connection) and 90Ω in the sender line between the gauge sender terminal and ground.

If you disconnect the sender wire, the gauge needle should go past F as far as it can get.
If you ground the sender wire, the gauge should go to E.

If you are grounding the sender wire and the needle is only going to to the half-tank level, the next thing I would do would be to ground the sender terminal at the back of the gauge, and also check that the gauge is getting a full 12V, and that it is well grounded.

If you do all of this, and the gauge still reads half full when the sender lead is grounded, then the problem is in the gauge.

- Eric

W-27

W-27

Do you recall what color is the wire of sender terminal at the back of the gauge? I hate the uncomfortable position that we need to be in order to check the back of the gauges! Thanks for all your help.

MDchanic

The sender wire is light brown.

- Eric

oldcutlass

I believe there is a tan wire for the sender and a pink wire for 12v. If you ground the tan wire side of the gauge it should go to E.

Fun71

This sounds like typical operation. Mine does the same thing when I disconnect the wire where it passes through the trunk floor behind the bumper.

This is not typical operation. Mine goes all the way to EMPTY when I ground the wire at the bumper.

It sounds as if you have an additional resistance in the wiring between wherever you are grounding the wire and the gauge (or within the gauge itself?)

freds cutty

the tan wire is for fuel pink for 12volt that is correct. You need a good meter to check your ohm reading prefer a digital unit you can check all this without removing tank but if you need to adjust well you do

jaunty75

C'mon, he didn't say this, and it's not that hard to figure out what he means. The gauge cluster is ROUND. The gas gauge part occupies the top portion, between about 10:30 and 1:30 on the clock face, with the F being at 1:30. The HOT light is at about 2:30, and the other lights, for GEN, OIL, and BRAKE are scattered the rest of the way around.

See the image below:





With the wire from the sending unit to the gauge separated at the connector (open circuit), the needle moves well past the F on the gas gauge portion of the dial all the way over to where the HOT indicator light is because it's reading infinite resistance, which is way more than 90 ohms, which is the resistance corresponding to F on the gauge.

This, boys and girls, is how fuel gauge can read HOT!


P. S. I've seen gas gauges over the years with a little post sticking out just past the F to prevent the needle from going too far past it. That's actually unfortunate as it prevents the gauge from acting as a diagnostic tool to troubleshoot a fuel gauge problem.

MDchanic

All well and good, Jaunty, but not everybody has a Cutlass from this year range, with this gauge and light layout.

I figured out what he meant, but, in general, it is less than helpful when posters assume that all of us can see exactly what they're looking at, and therefore fail to properly describe what they are seeing.

- Eric

oldcutlass

The fuel gauge on my 67 is a little rectangle and is in no way near the hot light , and yes initially I got confused as couldn't figure out what he said. I got it after reading all the other links Eric posted.


GM has used this same style gauge operation from 1965-1990, it's not difficult to trouble shoot.


Hopefully the OP has it figured out.

W-27

I used the volt meter to see the current coming in and the tan wire going to the sending unit as 9 to 9.5 volts on it and the back lights have 13 volts coming in. Is this any indication?

W-27

W-27

Jaunty, I couldn't have explained it any better. Thanks!

W-27

W-27

It reads full when I fill her up but goes down at about twice as much as it should as the gas is consumed. Also it jumps a bit as its going down and while driving the car.

oldcutlass

I've never used a meter to troubleshoot a fuel gauge circuit. If your on the terminal that the sender wire is attached to, then your reading voltage that's coming through the gauge on the negative side. So yes, there will be a voltage drop due to what ever resistance is in the gauge.


Did you ground the tan wire side of the gauge, and did it go to empty?