Not sure if this is the correct area to ask the question but here goes.

My fuel gauge on 71 Cutlass showed very far past the full area after fueling. The gauge would go past the hot area on the gauge. Also the needle would bounce all over the place.

I replace the sending unit and now the needle goes to the half mark not full mark after fueling but does not move all over the place. I have not yet driven the car enough to see what happens to the needle after it starts using fuel.

Sending unit was purchase from OPGI and it looked exactly like the one I replaced.

Any suggestion would be appreciate. Bad sending unit gas gauge?

Thanks in advance for your help.

Don

 

As I recall, disconnecting the sender and turning power on will move the needle one way, and grounding it will move it the other way. This will help ID gauge vs sender issues.

 

This is the classic symptom of an open circuit, often a bad connection at the point where the ground wire coming off the sending unit attaches to the underside of the body. Very common in these old cars. But the problem could also be a bad sending unit or an open in the circuit somewhere else. Another test is to actually disconnect the sending unit from the wire coming from the front of the car (the connection is usually in the area behind the license plate or behind the trunk latch) and ground the side coming from the front of the car. Turn the key on and the gauge should move to E. Remove the wire from ground, and the gauge should go way past F as you've seen.

This could indicate that the bad ground is intermittent. When the ground is good, the needle moves to show the fuel level. A moment later, the ground goes bad, and the gauge moves to past F. Etc.

Could be a bad sending unit. Or an incorrect unit that has the wrong total resistance as these sending units are just a variable resistor with 0 Ohm being Empty and 90 Ohm being Full. If your sending unit has a resistance of only 45 Ohm when in the "full" position, the gauge would read 1/2 when the tank is actually full. "Looking just like" the one that was in the car isn't a for-sure way to know if the replacement actually is correct.

 

I'm thinking that the gauge itself has a dampening mechanism; the needle movement shouldn't be erratic. But verify the wire harness from gauge to tank, and from tank to ground first.

 

Gentlemen here is an update on my fuel gauge issue.

As stated before after installing new sending unit from OPGI for a 1971 Cutlass and fully refueling the fuel gauge registers half full, needle not bouncing all over the place.

I checked and assured that the tank unit was grounded. Than disconnected wire coming from the front of the car going to the fuel tank, turned on the key and the gauge needle went way pass the F. Than grounded the wire and the needle went to E. Does this rule out the gauge as the issue?

Reconnected wire coming from front of car and turn the key on and back to the half full mark. Bad sending unit?

What is the next course of action? I think I know but want advice before removing the tank again.

Thanks again.

Don

 

Yes.

It would seem. Perhaps something is preventing the float arm from rising more than half way.

The only thing more annoying than having to remove a gas tank once is having to do it twice. I had to do it twice on the '67 Delta 88 I owned by have since sold. I tried to resurrect the original sending unit, but it wouldn't work consistently, so I ended up having to lower the tank a second time and putting in a replacement sending unit.

 

Disconnect gauge wire at/near tank. Measure ohms from gauge wire to ground.

If the tank is full, you should see ~90 ohms on the ohmmeter. If you're seeing 45, I'd be jacking up OPGI about which sending unit you got. Did they send you a 0--90 sending unit, or a 0--45 sending unit? At any rate, if you're not seeing ~90 ohms across the sending unit with a full tank, you'll be dropping the tank. Good luck with OPGI, I'll NEVER do business with them again.

I have heard about aftermarket sending units that have a crappy float--they float in water but not very well in gasoline.

If you have to drop the tank, you'll be able to do a more-exact measurement of the sending unit resistance, AND you can connect the sending unit electrically, then move the float arm manually while someone watches the gauge.

 

OK I guest I'll be dropping the tank. Measured ohms form the tank and it reads about 45 ohms. Not sure if the sending units is 45 or 90 ohms. If the float is being restricted from reaching the high level will it still read 90 ohms or the ohms level where is is restricted? Once I use over a half tank of fuel we'll see if the gauge starts to move down. Either way the tank needs to be dropped again. It will take some time for me to use enough fuel to try again.

I'll update after I complete the rework.

Thanks for all the help.

Don

 

Hello,

I have the same issue with my fuel gauge. I installed ttt and this entailed replacing the dummy lights with gauges. My fuel tank is always reading past fuel.
1. I grounded the wire in the trunk going to the gauges, and the Fuel needle went past E
2. I put an Ohm meter on the trunk connector to the sending unit - I go ~90 Ohms

Any next steps here?

 

WHAT VEHICLE?

Does the sending unit ever read something other than 90 ohms? Is the float stuck? Does the sender ever go to "0" ohms?

 

I just metered the sending unit again and this time I would get all types of numbers from 0 to 90 Ohms. I believe this is to a bad ground, meaning as I moved the probe in the trunk to a better ground the gauge moved.

The car is parked in the garage so I do not feel the float is moving.

I know the tank is full (I filled it because I had no way of knowing how much gas I have left.

On the dummy gauge I replaced, the fuel gauge worked fine.

 

And the car is a 1970 Cutlass S.

 

ok before I drop the tank I have another question.

I have driven the car about 90 miles and the gauge still reads half tank. I disconnected the gauge wire at rear and measured the ohms from gauge wire to ground and still see 45 ohms. Should the ohms be going down and needle start moving toward E? Or will it not start going down until it goes below 45 ohms on the sending unit?

This is driving me nuts (lol)

Thanks for the help.

 

The resistance should change continuously as the float height changes as the amount of fuel changes. If your gauge is stuck at 1/2 and the sending unit resistance stays constant at 45 Ohms, it appears that the dash gauge is working fine, which you established earlier, and that the float is stuck. Time to bite the bullet and drop the tank.