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My gas guage reads beyond full. I have visually checked the ground wires. It went bad while driving and I think I have about 1/4 or less gas. What should I check and what is the fix? I am thinking of dropping the tank and checking the sending unit. I would'nt think the wiring would just go bad while driving!
 

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Re: Gas Guage Failure. . .How to fix

Common issue with first gens. Easy troubleshooting to isolate if it is the gauge or the sender or the harness.

Unplug the sender lead in the trunk - in a 69 it is the brown single wire connection.
Turn key to on position.
Gauge should read past full.

Now ground the end of the harness leading back to the gauge - NOT THE LOOSE LEAD GOING UNDER THE FLOOR TO THE SENDER.
Turn key to on position.
Gauge should read past empty.

If it does both of these the gauge is in working condition and harness back to instrument panel is in working order. If not you can test the ohms being sent to the gauge as described below for the sender test, or have a buddy loan you his gauge for a second and plug theirs in and test as above.

If both gauges fail - and your description of your situation sounds as if the gauge is fine to me - the harness may have cracked or broken wire. You can also alligator clip the brown lead of the back of the gauge to a ground to do the above trunk testing to isolate wiring.

But if the gauge moves to past full then past empty with the trunk connector test, then problem is most likely the sender unit in the tank, or its ground.

The sender uses a ground fixed to the back passengers side tank strap and that is where I would start if you can get under the rear you should be able to visually inspect if the wire lead is corroded or loose. If it looks ok and is clean of corrosion it still may not be ok, and you can hook a test lead alligator clip to the connector at the tank strap and then the other end to a new clean ground you have made on the chassis and see if your gauge now reads correct with the key in the run position.

If you have a good ground at the tank or with your new temp ground, you reconnected the lead in the trunk and you still have a past full gauge reading, then maybe the sender unit itself has lost the will to live.

To test the sender you need to test the ohms (resistance) with a simple multimeter.
You measure across the loose lead in the trunk to the connector to the gauge. You
should read anything from 0 to 90 (empty to full). If you are getting something close to what you think is in the tank, the sender maybe ok. You can manually manipulate the float valve with a bamboo rod or wooden dowel through the filler neck. DO NOT USE A METAL ROD. Last thing you want to do is set your tank of gas and your ride, garage and house on fire with a spark. You can also affect change by removing or adding fuel to the tank as you check the ohms.

If you get a change in resistance the sender is most likely ok and the connector in the trunk may have been loose, or the float may have been stuck. if it is flaky better to replace then rebuild in my humble opinion. Dropping the tank is not an easy project without help or a lift and storing gas may not be easy where you live (condo owners take notice your neighbors may rat you out). And if you rebuild, how long would it last before it found its own groove again and left you high and dry.

Hope this helps you find your issue.
Brian
 

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Re: Gas Guage Failure. . .How to fix

Very detailed and appreciated diagnosis. I will be following your instruction this week end. The good news, I live in the country on an acre so no problems. . .I have all my Camaro parts along side the shop in the back!
 

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Re: Gas Guage Failure. . .How to fix

Ok, I appreciate the fuel gauge test procedure, but I have one question. First of all, let me say that with two different sending units, with the float all the way UP, my gauge barely reaches half.

Now, when I disconnect the sending unit wire from the plug in the trunk, and turn the key on, the gauge goes WAY past full. The needle goes around so far that it is horizontal. Is it supposed to go that far, or would you say my gauge has had it?
I hope one of you can help me.
Thanks,
Chris :confused::confused:
 

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Re: Gas Guage Failure. . .How to fix

What year and style of gauge?
If 1969 - located in right dash pod - yes and if your test resolve that it goes the same way past empty you can reset the pointer if the ohms are correct for half a tank of fuel = 45-ish.

If not the resistor on the rear of the gauge maybe damaged.
 

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Re: Gas Guage Failure. . .How to fix

the guage on my 67 camaro goes from 8 o'clock (empty) to 12 o'clock (full) when unplugged the wire in boot guage sits on full mark perfectly tested the sender seems to work fine filled the tank and i got 91 ohms, so would this be a resistor problem? or is the guage niddle in the wrong spot?
how and where is this resistor?
thanks
 

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Re: Gas Guage Failure. . .How to fix

the guage on my 67 camaro goes from 8 o'clock (empty) to 12 o'clock (full) when unplugged the wire in boot guage sits on full mark perfectly tested the sender seems to work fine filled the tank and i got 91 ohms, so would this be a resistor problem? or is the guage niddle in the wrong spot?
how and where is this resistor?
thanks
Okay lets just make sure we both know what you found:
Needle sitting past full usually means a bad sender ground at the rear of the tank hanger strap.

so you unplugged the connector in the trunk or "Boot" you get past full.
Next step in the troubleshooter was to ground the gauge side of that lead in the trunk to see if you get past empty. Did you get past empty?

If "yes" then the gauge and wiring to it from the trunk is working.
If "no" then you need to test continuity through the wire to the gauge at the cluster end.

If that checked out then the gauge should be tested as it is suspected as being bad. If the continuity failed on the harness, then you need a new run to the trunk for the gauge or replace the Fisher Body harness.

When you tested the ohms from the sender was it before the tank was full? What was the reading? If not siphon half of the fuel back out to a Gerry Can and test again. You should have 45 ohm's with half a tank. If you still get 90 plus your sender is stuck in the up position.

Dash mounted fuel gauges in 1967 & 1968 used a printed circuit board with no external resistors on the back. But I think there is an adjustment screw or maybe that is for the Tach if equipped.

You can bench test the gauge with a 0 to 100 ohm "potentiometer" which will send a various amount of ohms at the unit to electronically reproduce the senders signal to the gauge. If it is working the gauge should move from empty to full and every where in between as you select a "volume" in the potentiometer.

But the long and the short of it is that if you can get 45 ohms from the sender and get half a tank-ish from the gauge, Then your 90 plus is maybe from getting more then 18 US Gallons in the tank and it really is past full. I would also if I had the cans pump the tank dry and see if I get past empty, then with the gauge on pour in one 5 gallon Gerry Can and see if I had about 1/4 on the gauge.

Hope that helps find the gremlin
 

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Re: Gas Guage Failure. . .How to fix

What year and style of gauge?
If 1969 - located in right dash pod - yes and if your test resolve that it goes the same way past empty you can reset the pointer if the ohms are correct for half a tank of fuel = 45-ish.

If not the resistor on the rear of the gauge maybe damaged.
Brian,
Sorry for not mentioning that I was working on a '69. With the plug in the trunk grounded, the gauge goes just under the empty mark, but if it is NOT grounded, and without the sending unit being hooked up, the gauge goes WAY past the full mark, and lays flat, horizontal. The only thing I have not done so far, is to check the ohms.

Is there an easy explanation for telling how to reset the pointer?

Thanks for ya'lls help!

Chris
 

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Re: Gas Guage Failure. . .How to fix

Okay lets just make sure we both know what you found:
Needle sitting past full usually means a bad sender ground at the rear of the tank hanger strap.

The needle only sits way past full if the connector in the trunk is not plugged in to anything.

so you unplugged the connector in the trunk or "Boot" you get past full.
Next step in the troubleshooter was to ground the gauge side of that lead in the trunk to see if you get past empty. Did you get past empty?

When the gauge side of the plug was grounded, the needle went to a "normal empty" position.








If "yes" then the gauge and wiring to it from the trunk is working.
If "no" then you need to test continuity through the wire to the gauge at the cluster end.

If that checked out then the gauge should be tested as it is suspected as being bad. If the continuity failed on the harness, then you need a new run to the trunk for the gauge or replace the Fisher Body harness.

When you tested the ohms from the sender was it before the tank was full? What was the reading? If not siphon half of the fuel back out to a Gerry Can and test again. You should have 45 ohm's with half a tank. If you still get 90 plus your sender is stuck in the up position.

I have not checked either of the two sending units I have with an ohm meter yet. One is in the full tank of gas, and the other one is just laying in the trunk right now. Both are giving me the half tank reading with the float all the way up.

Dash mounted fuel gauges in 1967 & 1968 used a printed circuit board with no external resistors on the back. But I think there is an adjustment screw or maybe that is for the Tach if equipped.

You can bench test the gauge with a 0 to 100 ohm "potentiometer" which will send a various amount of ohms at the unit to electronically reproduce the senders signal to the gauge. If it is working the gauge should move from empty to full and every where in between as you select a "volume" in the potentiometer.

But the long and the short of it is that if you can get 45 ohms from the sender and get half a tank-ish from the gauge, Then your 90 plus is maybe from getting more then 18 US Gallons in the tank and it really is past full. I would also if I had the cans pump the tank dry and see if I get past empty, then with the gauge on pour in one 5 gallon Gerry Can and see if I had about 1/4 on the gauge.

Doesn't the sending unit have stops? I mean that if I could squeeze 20 gallons in the 18 gallon tank, shouldn't the guage only go so far?

Hope that helps find the gremlin
Thanks again for all the help!
Chris
 

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Re: Gas Guage Failure. . .How to fix

Chris,
The sender float just "stops" at the full range, but the arm can stick if too much fuel pushes on it long enough.

Bench test the sender laying in the trunk and see if as you manipulate the float you get a change in ohm's from it. If you don't take a good look at the electronic portion of the float arm assembly for loose or missing parts. Once you can get it to report a full range on your meter hook it up to the gauge harness connection in the trunk and ground the sender to the decklid latch catch near the connection. Move the float and see if the gauge reacts.

If it does you can either drain the tank or drive it bone dry, and swap out the unit, then bench test the unit that was inside the tank and see if you can get a working spare out of it.
 

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Re: Gas Guage Failure. . .How to fix

Chris,
The sender float just "stops" at the full range, but the arm can stick if too much fuel pushes on it long enough.

Ok, gotcha!

Bench test the sender laying in the trunk and see if as you manipulate the float you get a change in ohm's from it. If you don't take a good look at the electronic portion of the float arm assembly for loose or missing parts. Once you can get it to report a full range on your meter hook it up to the gauge harness connection in the trunk and ground the sender to the decklid latch catch near the connection. Move the float and see if the gauge reacts.

Ok, on the sender I took out of the tank and is now on the table. It goes from about 4 ohms, to 80 in the middle, to 135 ohms when the float is moved all the way up.

If it does you can either drain the tank or drive it bone dry, and swap out the unit, then bench test the unit that was inside the tank and see if you can get a working spare out of it.
Now, using the same meter, but with the other sending unit still in the tank with real close to a full tank, I'm getting a reading of 86 ohms. Sounds good I'm thinking. So, the gauge at this point does not have a sending unit plugged into it in the trunk. I look at the gauge, it's horizontal, past full. I plug in the sending unit, and go check the gauge again, and now it's on empty!! I go back in the trunk, unplug the sending unit, and the gauge is still reading empty. I am about to arrive at the decision that the gauge needs replacing, what do you think? :mad:
 

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Re: Gas Guage Failure. . .How to fix

I just found this:
The wire that needs power on it is the other wire on the dash unit, and it should have power when the key is on. If it does, and the gauge doesn't read upscale, temporarily ground the wire that goes back to the sender, and briefly turn on the key. If the gauge responds by moving upscale in slow jerks until it gets up toward "full" on the gauge, the dash unit and the gauge regulator is OK, and your problem is associated with the sender in the tank. DO NOT LEAVE THAT WIRE GROUNDED for more than the time it takes to see if the dash unit responds - immediately turn OFF the key and remove the temporary ground before you turn the key on again. Failing to do that can burn out the dash unit.


Is this telling me that there is a voltage regulator that could be bad, rather than the gauge? If I bought a repro gauge, does it have all that is necessary to make stuff work, as long as the sending unit and wiring is all ok?
 

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Re: Gas Guage Failure. . .How to fix

Now, using the same meter, but with the other sending unit still in the tank with real close to a full tank, I'm getting a reading of 86 ohms. Sounds good I'm thinking. So, the gauge at this point does not have a sending unit plugged into it in the trunk. I look at the gauge, it's horizontal, past full. I plug in the sending unit, and go check the gauge again, and now it's on empty!! I go back in the trunk, unplug the sending unit, and the gauge is still reading empty. I am about to arrive at the decision that the gauge needs replacing, what do you think? :mad:
Sounds like the unit in the tank is for a Camaro and the unit on the bench is another application.

The power was on when you got the conflicting readings? It maybe the flaky ground the 67 an 68 gauge cluster used at the top like a paperclip if I remember right. Maybe run a secondary ground and see if the gauge output stabilizes.

Brian
 

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Re: Gas Guage Failure. . .How to fix

Is this telling me that there is a voltage regulator that could be bad, rather than the gauge? If I bought a repro gauge, does it have all that is necessary to make stuff work, as long as the sending unit and wiring is all ok?
Sorry missed this post when I responded to the one above. After checking the grounds, also check the connection to the rear of the gauge. If it is an original printed circuit board on the back it may have cracked and is damaged. Replacement boards are available and just bolt on.
 

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Re: Gas Guage Failure. . .How to fix

Just for clarification here, I am working on a '69. Sending unit in the tank is a new reproduction, and the sending unit laying in the trunk that I am comparing, is the unit that I just replaced, so they're both for '69's

Confusing huh!! Grrrrrrrrrr
 

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I am having the same problem on my 67 with console gauge. Went back to trunk and unplugged the sender. Grounded the wire to the gauge at the trunk plug got empty on the gauge. With the wire unplugged in the trunk I get full. Put a 40 ohm resister on the wire in the trunk got little under 1/2 on the gauge. Put a 10 ohm resister on the wire in the trunk got almost empty. I have to assume its the sender. Stuck a dowel rod down the fill port by my license plate but could not seam to find the sending unit with the dowel rod to see if it was stuck. Do you have to remove the fill neck in the trunk to use the dowel?
John
 

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John - The 69 it is a straight shot to see the float from the filler neck. 67-68 it is not easy if at all possible to manipulate the float from the filler neck. I think others have accomplished that but don't know if dis-assembly was required.

But I think at this point the next test would be to connect an alligator clip test lead, or create a long test lead and connect one end to your sender's ground wire connection at the strap and shunt the other end to a known good ground, or the trunk latch and see if the gauge comes alive. If not I would be feeling confident if I drained and dropped the tank it would not be a waste of time. The sender does seem to the the issue - but it might just be the ground. If not you can remove the sender and bench test it.
 

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Brian
Good advice!
I forgot to say I did run a wire from the tank ground wire to another ground location with no change. I also forgot to say that when I unplug the trunk plug to the sender and measure resistance to ground it is always 90 ohms or so. I used my ohm meter on the sender while trying to get at the sender with the dowel. Figured if I touched the sender with the dowel that I should see a change in ohms on my meter. No luck there!
John
 

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Guys, I have a similar problem with my 69 with console gauges. Using the troubleshooting guide noted here (along with some good advice) it looks like it is my sending unit because I get about 135 ohms at the trunk connection; the tan wire.

The question I have now is: what would cause the gauge to go past full when I turn off the ignition? Based on the troubleshooting, that would lead me to believe that the tan wire is grounding out somewhere between the lead in the trunk back to the sending unit.

Am I way off here? I am waiting on a new sending unit to arrive and to use up the gas so I can drop the tank, I suppose that when I do that, I'll be able to tell if the tan wire is in good shape.
 

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Bill,
Not sure I understand your problem but from my test 135 ohms would make your gauge go past full I think. Go to radio shack and buy a 10 ohm a 40 or 50 ohm and a 100 ohm and try them one at a time on the tan wire in the trunk. One end of the resistor needs to be grounded. If you get 1/8 tank on the 10 ohm 1/2 tank on the 50 ohm and full on the 100 ohm then its your sender.
I assume you tried to ground the tan lead and get empty.
I am thinking about making a box with a 3 position switch with the resistors in it. The box would just plug into the trunk plug and then this test would be easy.
John
 
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