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Discussion Starter #1
I did do a search of the forum and did not find my issue. My gage was working fine then after filling up it went down to about 1/2 and has stayed the same. I replaced the sending unit about a year ago then stored the car with a FULL tank. Got it out a couple weeks ago the gage was working fine. drove the car for awhile the gage was down to about 3/4 or so... filled it up and like I said it went to about 1/2 and never moves regardless of the amount of fuel in the tank or if the ignition key is on or off.. so... anybody have any ideas? Is there a separate fuse for the gage or is the fuel pump fuse the same one? If I understand it right if the circuit goes open it goes to full if it's shorted it goes to empty right? so what the heck does stuck 1/2 way mean? Thanks
 

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First Easy Test

This gauge stops where it wants when the Ignition goes off. So do not worry what it reads when ignition is Off.

On a 94 the I/P indc fuse #11 powers temp, oil, and fuel gage. Also the "low and hot" lights in the cluster. Trunk release, defogger and air bag.
If everything else works you should start at the tank end of the circuit.

The easy first test would be to unplug the trunk electrical harness plug. Driver's side top of fender between trunk hinge and grommet by driver's tail light. Wire colors are Lt green, Dk green, brown, yellow, gray, purple, and black.

On the tank side purple(sender) and black(ground) are the gauge wires. Near 0 should be empty and near 90 Ohms should be full. Half full may not be 45 Ohms so you are looking for a change in numbers between full and empty. With low Ohms values from the sender your meter may be 0-10 Ohms off the theoretical number.

If the Ohms are changing between full and empty then we know the problem is forward of the trunk plug. I do not know of a plug between the trunk and the back of the cluster.

If you are getting a steady (same#) Ohm number full to empty you can try the same test at the tank plug by the fuel fill.

If the Ohm reading are not changing from the tank sender you may be pulling the tank. The sender may be damaged or the float is stuck some how.

When I say empty I mean low. I never let my car fall below 1/8.
Please tell us the year and model of your car.
 

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I did do a search of the forum and did not find my issue. My gage was working fine then after filling up it went down to about 1/2 and has stayed the same. I replaced the sending unit about a year ago then stored the car with a FULL tank. Got it out a couple weeks ago the gage was working fine. drove the car for awhile the gage was down to about 3/4 or so... filled it up and like I said it went to about 1/2 and never moves regardless of the amount of fuel in the tank or if the ignition key is on or off.. so... anybody have any ideas? Is there a separate fuse for the gage or is the fuel pump fuse the same one? If I understand it right if the circuit goes open it goes to full if it's shorted it goes to empty right? so what the heck does stuck 1/2 way mean? Thanks
The circuit is a Resistive circuit, meaning the sender works by increasing or decreasing the resistence as the float arm moves. Just check the "Refurbish your sending unit" thread for the specs on the resistence value and get an idea of how this works. The gauge just responds to the changes it see's in resistence. IIRC - It's 90 ohms when full and down to near 0 ohms when empty, so it varies in that range. Check the Instrument Cluster Link here for the schematic of the circuit to the Fuel gauge.

http://goldsswagon.com/diagrams/wiringindex.htm


It's probably a issue with the sending unit but be sure to check things first before you drop the tank and tear into it. Check the Schematic and see where the ground is for the gauge, I believe it is in the trunk and make sure it is connected to clean metal and solid. If you know you have a 1/2 a tank of gas, you should see approx. 45 ohms when you meter from ground to the Purple wire at the sending unit connection. At least there are some things you can check first before diving into the tank again.
 

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Discussion Starter #4 (Edited)
This gauge stops where it wants when the Ignition goes off. So do not worry what it reads when ignition is Off.

On a 94 the I/P indc fuse #11 powers temp, oil, and fuel gage. Also the "low and hot" lights in the cluster. Trunk release, defogger and air bag.
If everything else works you should start at the tank end of the circuit.

The easy first test would be to unplug the trunk electrical harness plug. Driver's side top of fender between trunk hinge and grommet by driver's tail light. Wire colors are Lt green, Dk green, brown, yellow, gray, purple, and black.

On the tank side purple(sender) and black(ground) are the gauge wires. Near 0 should be empty and near 90 Ohms should be full. Half full may not be 45 Ohms so you are looking for a change in numbers between full and empty. With low Ohms values from the sender your meter may be 0-10 Ohms off the theoretical number.

If the Ohms are changing between full and empty then we know the problem is forward of the trunk plug. I do not know of a plug between the trunk and the back of the cluster.

If you are getting a steady (same#) Ohm number full to empty you can try the same test at the tank plug by the fuel fill.

If the Ohm reading are not changing from the tank sender you may be pulling the tank. The sender may be damaged or the float is stuck some how.

When I say empty I mean low. I never let my car fall below 1/8.
Please tell us the year and model of your car.
It's a 96 SS

So I pulled the harness apart in the trunk and I have 30 ohms. Guess I'll drive it until I figure I've used at least a quarter tank and see if that number changes. We know it's getting power otherwise it wouldn't run correct? What pisser... just changed the damn thing.
 

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Discussion Starter #5 (Edited)
The circuit is a Resistive circuit, meaning the sender works by increasing or decreasing the resistence as the float arm moves. Just check the "Refurbish your sending unit" thread for the specs on the resistence value and get an idea of how this works. The gauge just responds to the changes it see's in resistence. IIRC - It's 90 ohms when full and down to near 0 ohms when empty, so it varies in that range. Check the Instrument Cluster Link here for the schematic of the circuit to the Fuel gauge.

http://goldsswagon.com/diagrams/wiringindex.htm


It's probably a issue with the sending unit but be sure to check things first before you drop the tank and tear into it. Check the Schematic and see where the ground is for the gauge, I believe it is in the trunk and make sure it is connected to clean metal and solid. If you know you have a 1/2 a tank of gas, you should see approx. 45 ohms when you meter from ground to the Purple wire at the sending unit connection. At least there are some things you can check first before diving into the tank again.

A bad ground for the gage seems very plausible. On the other hand I am getting a 30 ohm reading at the trunk harness [ tank side ] so the black wire must be grounded correct?
 

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Is it just my browser or are all the links of the schematics from your link above broken? There's no actual data.
A bad ground for the gage seems very plausible. On the other hand I am getting a 30 ohm reading at the trunk harness [ tank side ] so the black wire must be grounded correct?
The link I posted should take you directly to the "Wiring diagrams" on the www.goldsswagon.com site. you then select "Instrument Cluster" for the diagram of the Fuel gauge. The Fuel Pump circuit is listed in 1 of the 3 Engine wiring diagrams. If the link doesn't work just goto goldsswagon.com and check his site for the links. Lots of other good info there as well.

As for your issue, if you unplug the 3 wire connector at the rear bumper and then meter across the black and Purple wire of the connector coming from the tank, you are pretty much reading directly across the Float resistor. See what reading you have now and then go and add 5-6 gallons which should be about a 1/4 of a tank. Now check to see what the resistence is. If it increased by about 20-25 ohms then the float circuit in the tank is most likely fine.

Now you need to troubleshoot the otherside of the circuit, make sure the ground wire in the trunk is good. I would suspect it is or the Fuel Pump would not run. You may need to then meter the cuircuit back to the gauge and make sure all is good there and also check the grounds for the cluster which I think are at the Lower Driver Kick Panel. If all the wiring and all checks out, it could just be a flacky gauge or connection to the gauge at the cluster.

EDIT _ You mentioned you replaced the sending unit about a year ago. Was it a Brand New unit? GM or Other? Cause the Resistor for the float could be acting squirrely even though its new and the reason the refurb thread has you clean the contacts.
 

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Discussion Starter #7
The link I posted should take you directly to the "Wiring diagrams" on the www.goldsswagon.com site. you then select "Instrument Cluster" for the diagram of the Fuel gauge. The Fuel Pump circuit is listed in 1 of the 3 Engine wiring diagrams. If the link doesn't work just goto goldsswagon.com and check his site for the links. Lots of other good info there as well.

As for your issue, if you unplug the 3 wire connector at the rear bumper and then meter across the black and Purple wire of the connector coming from the tank, you are pretty much reading directly across the Float resistor. See what reading you have now and then go and add 5-6 gallons which should be about a 1/4 of a tank. Now check to see what the resistence is. If it increased by about 20-25 ohms then the float circuit in the tank is most likely fine.

Now you need to troubleshoot the otherside of the circuit, make sure the ground wire in the trunk is good. I would suspect it is or the Fuel Pump would not run. You may need to then meter the cuircuit back to the gauge and make sure all is good there and also check the grounds for the cluster which I think are at the Lower Driver Kick Panel. If all the wiring and all checks out, it could just be a flacky gauge or connection to the gauge at the cluster.
I did see the schematic by browser wasn't happy for some reason works now.

The tank is now full I filled it up this morning so I'll have to drive it around awhile to see if the ohm reading changes... if it does guess I'll check the ground by the driver kick panel. That would be sweet if it was just a bad ground and something easy to get at!
Guess I'll have to buy some beer and drive up north to the family cottage,,, stay a day or so then come home and re-test the ohm reading after using 1/2 tank of fuel. ... its really the only way.
 

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Drive the tank down.

"So I pulled the harness apart in the truck and I have 30 ohms. Guess I'll drive it until I figure I've used at least a quarter tank and see if that number changes. We know it's getting power otherwise it wouldn't run correct? What pisser... just changed the damn thing."

I am assuming that the ground is good. It grounds in the trunk on the driver's side hinge support. It also grounds the tail lights, brake lights, and the fuel pump. If your pump is working you should have a good ground into the tank. It will have to be checked if you drive test shows the sender works.

I think this ground is a weak point and have added a ground wire between the tank plug and the ground point. This allows me to run high power sequential turn signals with out burning out the trunk plug.

I have not tested a Impala sending unit but most do not have a direct Ohm to level measure. It has something to do with the way a fuel gauge works.

My friend has a ford focus and the dealer had him back 6-8 times changed the sender and sent him out with a gas can to check it. It ended up to be the gauge but it was a gong show of dealer incompetence. I hope the forum can help you with this.
 

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Re: plug test.

"The tank is now full"
The trunk plug-Ohm test bypasses the grounds. If you have 30 Ohms when full I think you have a problem in the tank or the wires to the tank.cwm8

It could be worse, to fix the sender in my dodge truck the safest way is to take the box off.
 

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Discussion Starter #10
"The tank is now full"
The trunk plug-Ohm test bypasses the grounds. If you have 30 Ohms when full I think you have a problem in the tank or the wires to the tank.cwm8

It could be worse, to fix the sender in my dodge truck the safest way is to take the box off.
I just unplugged the truck harness and turned the key on and sure enough the gauge went to full. So... seems like it's the frickin sender. Maybe the float is stuck? Either way looks like I'm dropping the tank...why am I surprised it's only brand new... :mad:
 

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I have not tested a Impala sending unit but most do not have a direct Ohm to level measure. It has something to do with the way a fuel gauge works.

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Not sure what you're saying there, but it is just a resistive wire in the tank. I have worked on both the original style and the newer replacement offered by GM and Spectra. I have taken apart the stock replacement and improved the ground design to work with a ractronix kit.

It is just a resistor wire that varies by the point where the float arm makes contact with the wire. The original SU just has a thin wire that wraps multiple times around a board about 2" long. Check the refurb thread and you will see the wire wraps that are cleaned with the eraser. The more wire length, the greater the resistance. The newer style sending unit is the same concept it just uses a different style of resistive board.

The fuel gauge adjusts the needle to the amount of resistance it see's. At least that how I understand the gauge itself, however, the resistor wire which controls the gauge just acts similar to a rheostat.

I just unplugged the truck harness and turned the key on and sure enough the gauge went to full. So... seems like it's the frickin sender. Maybe the float is stuck? Either way looks like I'm dropping the tank...why am I surprised it's only brand new... :mad:
"Hold on there bubba looey". :D. You don't want to drop the tank unless you're sure that is the problem. I'm not sure that unplugging the sending unit wouldn't just peg the gauge needle anyway. Just stick with the simple test of metering the resistance across the purple and black wire and see if the ohm value is changing with the fuel level. If it does, odds are things are fine in the tank and you need to look to the other side of the gauge circuit.
 

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Discussion Starter #12
"Hold on there bubba looey". :D. You don't want to drop the tank unless you're sure that is the problem. I'm not sure that unplugging the sending unit wouldn't just peg the gauge needle anyway. Just stick with the simple test of metering the resistance across the purple and black wire and see if the ohm value is changing with the fuel level. If it does said:
My thoughts on it was if I unplug the trunk wiring harness then the reading the gauge see's is infinite ohms so if the gauge still stayed at 1/2 then I'd be worried the problem is the gauge itself or some wiring in the harness up front. But it did in fact peg past full showing that the gauge itself is working... no??
 

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Gauge full unpluged good.

Quote the chevy diagnostic chart: (8A-82-10 1994 service manual)

"Disconnect C400 Does gauge read full?"
"Yes"
"Replace fuel sender"

I would still check the tank plug by the fuel filler tube. If it is not seated or it is damaged it might be the problem. Same purple-black test should be 90-100 Ohms full tank.
 

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My thoughts on it was if I unplug the trunk wiring harness then the reading the gauge see's is infinite ohms so if the gauge still stayed at 1/2 then I'd be worried the problem is the gauge itself or some wiring in the harness up front. But it did in fact peg past full showing that the gauge itself is working... no??
I don't know for certain how that would act and what that indicates. But there may be different ways to determine the functionality of the gauge and or sender.

Since a tank drop is a hassle and the SU fairly new, I would want to first confirm that the float is at least working. It seems pretty easy to check this with a meter after burning through a few gallons of Gas, meter it, top off the tank and meter it again.
 

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Discussion Starter #15
Quote the chevy diagnostic chart: (8A-82-10 1994 service manual)

"Disconnect C400 Does gauge read full?"
"Yes"
"Replace fuel sender"

I would still check the tank plug by the fuel filler tube. If it is not seated or it is damaged it might be the problem. Same purple-black test should be 90-100 Ohms full tank.
It's 30 ohms there too.... guess the writings on the wall. :(
 

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Discussion Starter #16
I don't know for certain how that would act and what that indicates. But there may be different ways to determine the functionality of the gauge and or sender.

Since a tank drop is a hassle and the SU fairly new, I would want to first confirm that the float is at least working. It seems pretty easy to check this with a meter after burning through a few gallons of Gas, meter it, top off the tank and meter it again.
I think I might put a 5 gallon can of gas in the trunk and just try and get real low. Who knows maybe the float is stuck like you mentioned... maybe going low will free it up. Gotta get it low to drop the tank anyway. I'll check the ohm reading again after I burn through some fuel. But I am a firm believer in Murphy's law... so I know what it is... It's whatever is the biggest sob to get at. always is it seems.
 

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We all agree on most things.

Unplugged gauge should read full.
30 ish Ohms is halfish
Shorting purple to ground would be empty.(I would not bother)

4DoorSS said" if you unplug the 3 wire connector at the rear bumper and then meter across the black and Purple wire of the connector coming from the tank, you are pretty much reading directly across the Float resistor."

So we both suggest you check the tank connector before R&R ing the tank.
 

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You posted while I was on the phone

"It's 30 ohms there too.... guess the writings on the wall. :("

I hope we have covered it all, but I think you are correct:

"I am a firm believer in Murphy's law... so I know what it is... It's whatever is the biggest sob to get at. always is it seems. "
 

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If it is not changing, the float arm my have slipped out of the clip for the potentiometer arm. I tied mine to the arm with a tiewrap. Sometimes the float catches while assembling the pump module in the tank, and pops out of the nylon clip. It could have been partially out when you assembled it, and completely popped out while driving. The only way to find out is to drop the tank.
 
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