gas gauge reading low/empty after emptying tank - Chevelle Tech
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post #1 of 15 (permalink) Old Oct 15th, 19, 11:14 AM Thread Starter
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gas gauge reading low/empty after emptying tank

Used a Harbor Freight hand pump to get rid of some old gas
Sender was new...now reads E or barely above if I got say 8 gal

Guessing I damaged it or is it stuck? Keep driving it or? Dont wanna take it out less i have to I get a hell of a headache from fumes. plus Ipaid to braze 1/2" pickup rather not do it again

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post #2 of 15 (permalink) Old Oct 15th, 19, 3:15 PM
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Re: gas gauge reading low/empty after emptying tank

I wonder if the float is not floating ?. If it's heavier than it was before then it will sit lower in the gas in the tank to where now the float may be at the bottom of the tank but the tank still has fuel in it.

Maybe you did damage something so I would maybe try filling the tank up and then either driving it to where you can get a feel if it's reading right or wrong and then go from there. If after driving it for so long and the fuel gauge then gets down to empty and you still have this 8 gallons left in the tank then it may be time to drain the tank and see what's going on with the sender.

Also do the simple stuff too by checking your wiring connections and doing some tests at the sender connection in the trunk.

I know I have stated many a time if you ground the sender wire in the trunk and turn the ignition to run that the gauge "should" go to empty or below but this can give one a false positive result as it is not taking into account how the circuit would react with an issue with the gauges shunt resistor.

This is how a 33 ohm resistance on the sender wire affect the gauge without the gauges shunt resistor in place:



And this is how a 33 ohm resistance on the sender wire should have the gauge reading properly with the gauges shunt resistor in place:



If you can,maybe disconnect the sender wire in the trunk and do some tests with resistors just to verify from that point forward all is good:


The above video I made a few years back does not give any info in regards to how the gauge would react with an issue with the gauges shunt resistor and eventually I'll doa more in depth troubleshooting video.

Jim

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post #3 of 15 (permalink) Old Oct 15th, 19, 4:03 PM
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Re: gas gauge reading low/empty after emptying tank

Absolutely check sender wiring around the tank. I used to work in the factory that built the originals and have calibrated hundreds.
If the float is black they can absorb fuel and will not read correct. Sometimes the winding's on the unit need cleaning. Maybe a can of seafoam. It is standard practice to bend the float arm rod that connects the float to the unit. If you take it out you can connect wiring and work the float and bend as needed. Tiny bends at a time!!


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post #4 of 15 (permalink) Old Oct 15th, 19, 9:09 PM Thread Starter
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Re: gas gauge reading low/empty after emptying tank

Checked the wiring outside seems to be fine..reads about the same as that first plc. Worked perfect til I stuck that hose down in there I musta damaged something?

Hope its not the gauge I hear shiftworks stopped making them? wouldnt know what a shunt was anyways

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post #5 of 15 (permalink) Old Oct 16th, 19, 11:54 PM
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Re: gas gauge reading low/empty after emptying tank

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Originally Posted by cuisinartvette View Post
Checked the wiring outside seems to be fine..reads about the same as that first plc. Worked perfect til I stuck that hose down in there I musta damaged something?

Hope its not the gauge I hear shiftworks stopped making them? wouldnt know what a shunt was anyways
Yep you might have damaged something but who knows sometimes but with some good troubleshooting and investigating, you should be able to find the issue{s}.

It would be nice if someone could reply to your issue and say it's this or that and who knows the gauge you have might have gone south but right now I'm thinking that is not the issue.

As far as knowing what a "shunt" is a good question. I too never knew about shunts but they are used a lot in gauges and if they fail or are not working properly there can be issues with how accurate a gauge can read. This is why I posted pictures of a stock gauge with and without a shunt in place and how it affects things. Like I said, I used to think that just grounding and ungrounding the sender wire on a gauge was enough for two tests but it's not in all situations.

If you can, keep this thread alive and let us know what you find out. Too many times there are issues that people ask about but then later we never hear back except for the cricket noises.

It's also nice to know now that you are dealing with an aftermarket gauge. The more info from the get go, the better.

Jim

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post #6 of 15 (permalink) Old Oct 17th, 19, 10:16 AM
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Rick
 
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Re: gas gauge reading low/empty after emptying tank

The shunt is on the unit and is a small wire wound electric part that gives you the readings when the float arm moves.
If the float arm got bumped after in the tank it could very easily have knocked it off calibration. If you take it out and check it and it is off, use 2 pair of pliers and gently bend the longest part of the float arm.


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post #7 of 15 (permalink) Old Oct 17th, 19, 11:01 AM
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Re: gas gauge reading low/empty after emptying tank

Can you pull the sending unit and inspect it for damage? You could also then wire it and move the float manually to see if it activates the gauge appropriately. Testing the gauge itself is really easy to do. If fumes are a problem, get a good mask, like those used in a paint booth.

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post #8 of 15 (permalink) Old Oct 18th, 19, 11:08 AM Thread Starter
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Re: gas gauge reading low/empty after emptying tank

Thanks..Ill have to see if I can pull it next week, entering first show tomorrow
Sure I damaged it putting the hose in. Ive drained more than Ive burned but at least its getting driven

SS gauge pack from shiftworks..like it but my vision is so bad debating putting the big gauge and clock back in. Nah..
Ill keep the thread going...guess Ill have to learn how to use one of these 5 multimeters I have..that I cant find.
Living with a hoarder this is the norm.

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post #9 of 15 (permalink) Old Oct 26th, 19, 10:28 AM Thread Starter
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Re: gas gauge reading low/empty after emptying tank

Added 5 gal last night it finally moved. Shows 1/4 tank. .Ill keep an eye on it not sure if its accurate or not I suspect its holding more.
Maybe it was stuck??? Thanks for the tips if somethings wrong Ill post what i found.

Finally getting some miles on this thing, love it.

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post #10 of 15 (permalink) Old Oct 26th, 19, 11:59 AM
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Re: gas gauge reading low/empty after emptying tank

Before you go ripping the tank apart check the gauge. My car is doing that right now, all factory wiring in the rear, gauge responds as expected when grounded or disconnected. Hook it up to the new sender and it reads a 1/4 tank when its half full. Tried a different sender that came with the car and it reads 1/2 tank with the float full up. I believe mine is related to the resistor on the back of the gauge. If in fact it is even there at this point.
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post #11 of 15 (permalink) Old Oct 27th, 19, 12:25 PM
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Re: gas gauge reading low/empty after emptying tank

The resistor could cause it, but I believe it's there to keep the gauge from being erratic. The motion has a sort of delay so that when the fuel is sloshing due to bumpy roads or cornering, the gauge isn't looking stupid ranging a half tank for awhile. The delay evens out the signal allowing the fuel to settle down. An aftermarket gauge may require a different value.
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post #12 of 15 (permalink) Old Oct 27th, 19, 1:09 PM
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In electronics, a shunt is a device which creates a low-resistance path for electric current, to allow it to pass around another point in the circuit. The shunt protects the small fine wires of the coil in the gauge from to much current. The shunt is mounted in parallel to the gauge.

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post #13 of 15 (permalink) Old Oct 27th, 19, 1:30 PM Thread Starter
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Re: gas gauge reading low/empty after emptying tank

Dread having to pull the dash we will see. The repop switches are garbage and need replacing anyways. Going to put my orignals back in.
If its the gauge Im pulling the ss stuff out putting the stock clock/gas gauge in and will figure something else out on temp stuff. Disappointing I love the look of the ss stuff.

Worked perfect before I stuffed the hose in had to be something i did. As usual lol

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post #14 of 15 (permalink) Old Oct 27th, 19, 4:34 PM
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Re: gas gauge reading low/empty after emptying tank

I have some limited info but on a gas gauge from one of my Nova's is they have this external shunt resistor across the back two terminals of the gauge and if this shunt resistor is not in place or of the right value, then doing the two tests of grounding the sender wire to get the gauge to read empty and then having it connected to nothing to get the gauge to read full can give you the impression the gauge is fine but you have to do further tests with resistors on the sender terminal of the gauge to really confirm things.

This is a gauge without the shunt in place on the back of the gauge and then has a 33 ohm resistance attached to the sender wire of the gauge.



It does not read correctly as it should read like below with the correct shunt on the back of the gauge.



Now here is the same gauge without the shunt in place on the back of the gauge and then has a 75 ohm resistance attached to the sender wire of the gauge.



With this 75 ohm resistor the gauge does not read right but should read like below with this 75 ohm resistance AND the gauges shunt in place.



The more I talk about and research things about the fuel gauge, the more I feel one needs to do more than the two tests with grounding and then not grounding the sender wire. One needs to buy some resistors such as one for testing a 0-90 ohm system with getting a 22.5 ohm resistor, a 45 ohm resistor and a 90 ohm resistor. By then putting in a 22.5 ohm resistor between the gauges sender terminal and ground the gauge should read 1/4 full. If then the 22.5 is changed to a 45 ohm resistor then the gauge should read 1/2 full. If you then series the 22.5 ohm resistor to the 45 ohm resistor and hook it up to the gauges sender terminal and to ground then the gauge sees 67.5 ohms of resistance and then should have the gauge reading 3/4 full. Then as a further test, putting in the 90 ohm resistor by itself should make the gauge read Full. The same kind of testing can be done with the older 0-30 ohm system and checking things with a 7.5 ohm resistor, a 15 ohm resistor and then a 30 ohm resistor.

Keep in mind some replacement gauges have the gauges shunt resistor now built inside the gauges housing while an original had it externally bolted in place and if one did not know what was going on, bolting on an external shunt to a gauge that already has this shunt built inside would throw off how well the gauge would actually read.

Jim

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post #15 of 15 (permalink) Old Oct 27th, 19, 5:20 PM Thread Starter
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Re: gas gauge reading low/empty after emptying tank

How many gallons does a 67 tank hold anyways?

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