71Cutlass "S"

I have a 1971 Cutlass "S" Coupe and the fuel Gauge is pegged to the right, it happened once before and it corrected itself. does any one know where to start troubleshooting

hookem horns

Chassis manual has a troubleshooting checklist. Don't have mine handy but others have said it's on the wildaboutcars website. Probably the sender on the tank, either bad ground or shorted supply. Can't recall which pegs it to full.

OldsManNJ

If/when you figure it out let me know - same thing on my 71 S. Right now the gauge has moved to the right a bunch - it is empty at 1/4 tank mark and full is way over the full mark - but it still works - just moved to the right.

frankr442

Probably a broken wire/connection. Search " fuel gauge pegged F - what next" and you'll find a pretty detailed thread

jaunty75

A fuel gauge that pegs all the way to F or beyond at all times indicates an open circuit (infinite resistance) in the gauge circuit.

To test if the gauge itself is ok, disconnect the wire coming from the gauge from the wire coming from the sending unit on the tank and ground the wire coming from the gauge. With the ignition ON, the gauge should immediately peg to E. If it does this, the gauge itself is OK. You can find where these two wires connect to each other in the area around the rear license plate or trunk latch area.

The fact that the gauge settles above F when the tank is full and never goes below 1/4 when the thank is near empty suggests either a misadjusted gauge, which seems unlikely, or a partially malfunctioning sending unit, which seems more likely. But at least your gauge works at all. Many times, the sending unit fails completely causing the gauge to peg at E or F at all times, and it needs to be replaced.

hookem horns

Tried to take a picture of the chart in the manual but too blurry to post, pretty much what jaunty75 said. I'm betting sending unit but go through the tests before dropping the tank.

Run to Rund

If the gauge doesn't read consistently, bad ground, partially broken wire, or contaminated sending unit rheostat are good possibilities. You can "calibration check" your system by using resistors. If you ground the hot lead, the tank will be full; 80 ohms in the grounding lead will do the same, as should the sending unit with a full tank. A 10 ohm resistor in a pigtail that grounds the hot lead will mimic 1/8 tank, etc.

jaunty75

I'm not sure what you mean here.

The only hot lead comes from the gauge. It goes to the tank, through the sending unit, and to ground. If you ground the hot lead coming from the guage, that mimics an empty tank, and the gauge should read E. As soon as you disconnect the wire from the gauge to the tank at any point along the way, including pulling the wire off of the sending unit on the tank or disconnecting the ground from the sending unit, the circuit is open, which means infinite resistance (well above the 90-ohm maximum the sending unit provides), and most gauges peg well past the F mark.

Putting various resistors in series with the line the line from the sending unit to ground will mimic various stages of fullness of the tank, and you can test the gauge this way. Put in a 45-ohm resistor, for example, and the gauge should read about half full.

Run to Rund

Yes, a 40 ohm resistor will be about 1/2 tank, but a direct ground of the hot lead will read empty. 80-90 ohms reads full so infinite ohms (hot wire left hanging) is pegged past full. A direct ground will read empty; adding 10 ohms gives 1/8 tank. Sorry, I typed it wrong above.

Flying Elvi

Would this be the same for a 1973 Delta 88 Royale? My gauge is pegged all the way to the right.

jaunty75

That means you have an open circuit. It could be the ground side of the sending unit, which should be attached to the frame under the car, or a break in the line between the gauge and the sending unit.

Find where the wire coming from the sending unit attaches to the wire coming from the gauge. There should be a connector somewhere around the rear license plate area, possibly in the trunk near the latch area. Pull the connector apart and ground the side coming from the gauge. Then put the key ON. The gauge should immediately peg on E. If it does this, your gauge is fine and you need to find the open in the gauge-to-sending-unit wire, fix the ground, or if neither of those work, drop the tank and replace the sending unit.

Flying Elvi

Thanks jaunty! I'll check it out tonight

Federated52

I also am having a problem with my '69 Camaro. I've tried unhooking every wire that i was able to access without pulling the tank. Every wire I removed still pegged the gauge to full. So now I will be pulling the tank to see if there is a wire off the sending unit. Any other suggestions?

Fun71

You need to ground the sending unit wire to see if the gauge then moves to EMPTY.

oldcutlass

Unhook the sender wire that runs forward to the gauge, touch it to ground and see if the gauge goes to E with the key on. If it does, chances are you have a bad sender or ground to the sender assy.

Federated52

Do you mean unhook the wire off the sending unit? Or were it is grounded on the frame?

Federated52

The gauge worked when I replaced the tank & sending unit, but that was about 6 years ago. The gauge stopped working about a year ago.

oldcutlass

Off the sending unit or the plug in the trunk.

oldcutlass

Its an electrical component, it could work for 50 years or 5 minutes, sometimes not at all right out of the box.

Federated52

You got that right. HaHa

Federated52

Ok, pulled the tank and did not find any broken connections. There is approx. 10v With the key on & off going to where the sending unit is plugged into the wiring harness. (In the trunk).

oldcutlass

Did you ground the wire that has 10v on it? You use an ohm meter on sender it should read 0 ohms at E and 90 ohms at F.

Federated52

It reads 0.4 ohms with the tank having approx. 1 gallon of gas in it. That is with the sending unit not hooked up at all to the car.

Fun71

Ground the wire from the gauge to see if the gauge goes to EMPTY. If so, the gauge and wiring is good and the issue is elsewhere.

Federated52

Which wire there are 3 one ground, one wire on the empty side & another at the top of the gauge. This gauge is in the console not in the dash. Thanks

Fun71

I have a 1971 Cutlass, not a 1969 Camaro, so I don't have experience with which wire you need to check on your car. All I can say is the one that goes from the gauge to the sending unit, which on my Cutlass is the brown wire that goes through the trunk grommet above the license plate/gas filler neck.

Federated52

Ok, well thank you. I will try both wire & see. I might have a spare gauge lying around somewhere. Maybe I'll try that one if all else fails. Thanks again for you're help.

oldcutlass

Did you ground the wire in the trunk to see if the gauge goes to E?

Federated52

No, will have to try that. Thanks.

Federated52

Well, I give up on it for the rest of the season. Will work on it come fall. Everything I've tried so far has just led to the same thing happening. With the key off it is about at 3/4 tank, then when i turn the key on it goes way beyond full. I even hooked it up to the fuel gauge in the dash & does the same thing. All buttoned back up now.

jaunty75

For the record, this means nothing. It is only when the ignition is on and the fuel gauge system is getting power does the gauge position have any potential meaning.

This means something very simple. You have an OPEN CIRCUIT. There is break in the wiring somewhere between the ground at the fuel tank and the connection to the battery at the front of the car.

The most common causes of this are:

A) a bad ground at the sending unit. One wire from the sending unit is connected to chassis ground somewhere in the vicinity of the gas tank. A common problem here is corrosion at the connection point, and removing the screw and cleaning off the rust until you get to shiny metal will fix the problem.

B) a break in the wiring between the sending unit and the dash gauge. The wire from the dash gauge to the rear of the car connects to the other (non-ground) wire coming from the fuel gauge. The connection is usually somewhere behind the rear bumper or in the trunk compartment behind the trunk lock. Pull this connector apart, and ground the side coming from the front of the car. Then turn the key on. The gauge should immediately go to E. If it does this, and then goes back to F or beyond when the wire is removed from ground, then the gauge itself is working fine, and you can concentrate on the rear of the car. This is usually the FIRST step in diagnosing a fuel gauge problem because it is easy to do and because it can immediately eliminate the dash gauge itself and the front-of-car wiring if this test is passed. You should be able to perform this test whether the car has been "buttoned up" or not as all you need to do is find this connection and make the test. No disassembly required.

C) If A and B are not the problem, then the sending unit is the culprit. There is no other possibility. Replace the sending unit.

Federated52

I have done option A.,
I have grounded the wire in the trunk, coming from the front of the car. It did not go to E. I have done A & B on both the gauge in the dash as well as the gauge in the center console. Both results were the same. I also tested the sending unit itself & with about a gallon of gas in it it read 0.4 ohms, I guess my next step is a new sending unit. Thank You for the tips. I will try all steps again, before getting the unit, just to make sure.

jaunty75

You did this with the key on, right? This is an immediate indication that there is a front-of-car problem. I would find and fix that before doing any fiddling with the rear of the car.

You have TWO gauges? Did they ever both work properly? Do they both do the exact same thing whenever you're performing whatever test you're performing?

This sounds right. The sending unit is a variable resistor, usually 0 to 90 ohms with 0 being empty and 90 being full. If you're reading 0.4 ohms, that's very close to 0 compared to 90, so your dash gauge, if working properly, should be reading nearly empty. One gallon of gas in a 20 gallon gas tank is nearly empty, so your sending unit appears to be working correctly. This is another indication that your problem is not at the rear of the car at all but is in the front-of-car wiring. That's where you should be focused.

I would not spend the money on this or go through the bother of dropping the tank until you've fixed whatever electrical problem there is with the dash gauge(s). You've already confirmed that the sending unit does actually appear to be working correctly, so there is no need to replace it at least until you've corrected the front-of-car wiring problem.

Federated52

Yes both gauges are doing the same thing, but I can only hook one up at a time
The one in the dash is the original one that was hooked up. The one in the console was never hooked up until I had a problem with the dash.
They both did work very good at first.
I will have to try to trace the wires coming off the gauge to see if I can find the problem, but I think they all merge into one big group of wires & they are wrapped together.
My friend hooked up the gauge in the console off of the wiring harness for the dash gauge and it did work for a short time.
I was pretty sure the sending unit was fine after measuring the ohms.
Thanks again. We'll see what I can come up with.

Federated52

Well, no luck on searching for a bad connection. I took the lens off the fuel gauge & moved it by hand back to empty. When I turned the key it just went right back to over full. Checked the fuses, they were all fine. Don't know what to do next. My next step would be taking it to a local repair shop that specializes in electrical problems. Although it would probably be a lot cheaper for me to replace the sending unit & both gauges. HaHa

jaunty75

Fuses wouldn't have anything to do with the problem. If a fuse in the gauge circuit were blown, the gauge wouldn't move at all. The fact that it does, and goes way past F, says that there is an open circuit. You need to find the break in the wiring or the bad connection. It calls for tracing. It's as simple as that.

You say that the gauge wire disappears into a bundle, which is to be expected. You could splice into the wire at that point and just run a wire along the outside of the car to the sending unit wire in the rear of the car. If this fixes the problem, you know roughly where the break is. You could either take apart the bundle and try to find the problem.or you could just run a permanent second wire that's routed out of sight.

And it would be a waste of money. We've pretty much established that the gauges are fine and that the sending unit is fine. The problem is in the wiring. Replacing the sending unit and gauges won't fix that. You'll just have new gauges and a new sending unit that together still don't indicate the amount of fuel in the tank.

Federated52

Ok, will try splicing the wires & see what happens Thanks again.

Federated52

Ok, Finally got it. (Thank You Jaunty75) It ended up being the wire from the sending unit to the gauge that was bad. I ran a new wire & now it is going to empty. (Which it is). I am very glad the gauge is working correctly again. Thank You to everyone who had some advice. I sure am glad I can go on here and get help with the little things that creep up.
Thanks again, and stay safe & healthy everyone.

jaunty75

Glad it got solved!