Fuel Gauge Inaccurate - SOLVED

[oldchev]

I see when I posted the charts, they flipped. So the one I stated as first is the dirty looking one (GMC Fuel level#2), whereas mine is the clear white one that says Fuel Level Specs.

 

[Fless]

Ok, so I'm very curious about this. Seems like less resistance (nearer 0 Ohms) would allow a higher sender voltage on the circuit. However, what I don't know is if the BCM (if that's what controls the gauge) says higher voltage = lower level on the gauge display.

 

[oldchev]

Here is what I have discovered so far. Today 2/18, I checked the signal circuit at connector 152 (under the fuse box, eng comp).
I pinned into the signal circuit (purple) and low reference circuit (black). Using my DMM, read Ohms. I have two meters so checked both on each test to verify. To clarify, The fuel level meter is the gas gauge.

Test #1: Fuel level meter read one line under 1/4 @ 198 Ohms.
I added 6 gallons gas.
Test #2: Fuel level meter read one line above 1/4 @ 170 Ohms,
I added 5.84 gallons.
Test #3: Fuel level meter read one line below 1/2 @ 120 Ohms,
I added 6.05 gallons, apparently topping it off.
Test #4: Fuel level meter read FULL @ 46 Ohms.

I know what some have said about the specifications however my 2004 GM service manual has the chart (attached below) with 40-66 Ohms = 88-100% or basically FULL.
My Fuel Level Meter seems to drop from Full to about 1/2 when using about 6 gallons and from 1/2 to about 1/4 using 6 more gallons the rest is anybody's guess.
So with all that said, not having a signal generator to test the actual stepper motor Fuel Level, I am leaning towards the stepper motor being defective since the ohms read outs better define the actual fuel level.
Can anyone think of any further tests I can do without purchasing a TECH2 scanner? I have an Innova 3130c scanner that does a lot but no signal generator properties.

 

[rockola1971]

Check power at fuel tank plug. Is it there and within spec? If it is then sounds like your stepper motor is bad OR your needle is loose on the stepper motor output shaft (Like the inertia belt slipping on the outer ring of a harmonic balancer)....you'll never get a good reading.

 

[MassHoe04]

Check power at fuel tank plug. Is it there and within spec? If it is then sounds like your stepper motor is bad OR your needle is loose on the stepper motor output shaft (Like the inertia belt slipping on the outer ring of a harmonic balancer)....you'll never get a good reading.

Mine reads about 1/8 more than actual. It reads 1/8 on gas and 6 MPH on the speedo, before I even turn the ignition on. But, that is because PO put some cluster decals on the faces of the gauges and did not align the needle when they put it back together.

With two gauges being off and the PRNDL display contacts needing re-soldering... I'll probably just send out for a rebuilt cluster. At some point. No rush there. I got things like control arms and tires to get first.

In the meantime, I subtract 6 MPH from what I see on the speedo and I use the digital information center to tell me how many estimated miles are left in the tank.

 

[oldchev]

Update:
After two full fill ups, the gas gauge reading is close if not at the standard reading. I have nothing to attribute this to. I have monitored the distance and gallons used, then on fill up (all the way) it corresponds with what actually was used and the gauge needle seems to be back to normal and is relative to actual fuel level. Ain't that a peach!
So, watching it, but so far so good.

 

[OR VietVet]

When you say: fill up (all the way), do you mean fill till pump nozzle kicks off or put as much gas as you can in there? You will damage your evap system if you put as much as will hold in there.

 

[BadOlHoe]

Did a quick search and this post is relevant to almost the same issues I have had in the last few months. A lot of the info was over my head seeing as I have next to zero electrical knowledge... other than don't stick a fork in a toaster. With that said I am having similar issues. Is there a simple fix to a fuel gauge all of the sudden reading 3/4 at full fill up and then once it hits 3/8 it's like you can watch it go down to 1/8?

Apologies for hijacking this post

 

[oldchev]

Further update 4/07/23:
From the original post, Fuel Guage was displaying low reading faster than use. It "corrected" for a couple months then back to, or worse. My gallons consumed on the trip meter was accurate as each fill up corrosponded to the gallons used. So I went with that so as to not get caught short. Per the previous posts, I did a Digital Multimeter check of ohms at connector 152 (under the fuse box, eng comp). This led me to believe the sending unit was ok as the ohms vs gallons used was very close.
The correction process I used was to send my instrument panel to Dr. Speedometer for all new stepper motors, plus he also replaces all light bulbs. Now the guage displays accurate and corrosponds to fuel used.
With a little homework, I was able to narrow down the problem, which is now solved.

 

[Fless]

Thanks for coming back and explaining the solution. Gauge stepper motor issues are common in the 2003-06 clusters. Easy enough to replace if you have basic mechanical and soldering skills.