Fuel Gauge Inaccurate - SOLVED

[oldchev]

Greetings. I have a 2004 Yukon Denali @260K+ miles.
My issue today is fuel gauge malfunction. According to mileage and previous economy(?), its off 1/4 to 1/3 depending on actual. Up until last week it was or seemed accurate, but Saturday was reading under 1/4 tank and I filled it with 13.2 gallons and that was squeezing it in. The reason I kept trying to fill more was I knew (previously) it would take much more at that reading.
So today It changed from just over 3/4 to under 1/2 only driving 25 miles. Once arriving I shut it off and turned the key to on to see the reading, I watched it move a little then slowly creep up above 1/2 about midway to 3/4. Later I did the same thing and it was under 1/2 and stayed there. After driving 25 miles home it was now at 1/4. From full on dead full to 1/4 in 106 miles total.
I understand it could be one or many things. I did replace fuel pump/sending unit with factory (was it bosch? don't remember) whatever the original was... at about 180K (4 years ago) and never had an issue with fuel level ever until now.
I am thinking stepper motor for fuel gauge?
Anyone had similar issue with stepper motors for these gauges? Testing process?
No stranger to fixing stuff on this baby just want to hear from others on this. No TECH2 available but most everything else. Don't want to drop the tank (again) unless that's it. Not a fun job. All the best, thanks.

 

[MassHoe04]

You might be able to check what the fuel level sending unit is telling the gauge with a Volt-Ohm multi-meter to verify nothing wonky is happening in the tank.

Here is a link to what values are.

Fuel Gauge And Tank Capacity

I have a 2005 Tahoe. A quick google search says it has a 26 gallon tank. Haven’t paid much attention to it before but noticed today that my gauge is right at empty and the truck says I’ve used 18 gallons. Is it actually a 26 gallon tank and my gauge is just off or could it be a smaller tank?

www.tahoeyukonforum.com

 

[oldchev]

It looks like the access to the ohm wire is at the instrument cluster 24 pin connector. Using the Service Manual it says that wire would be B7-yellow wire. I'll try my Ohm reader on that wire and see what the values are. If I had a signal generator I could make the gauge move and see but so far I do not.
Anybody else please chime in. Whatever I do, I'll report back so we know how this was resolved.

 

[OR VietVet]

If was a Bosch pump, it is not OE. The two things it could be have already been said, the sender or the stepper motor. Post test results please.

 

[MassHoe04]

Bosch is certainly not junk (usually). If AC Delco is not available, for whatever reason, Bosch would be my next choice.

You are looking to confirm either the sending unit is wrong and the gauge is OK; or the sending unit is OK and the gauge/stepper motor in your cluster is bad.

I think you can measure voltage at the cluster end of the wire, but for measuring Ohms resistance... I think that has to happen right on the socket at the sending unit itself.
If your tank is full, you should expect to see either 250 ohms resistance at the sending unit terminals or a full 2.5V on the wire at the cluster.

Let us know what you find.

 

[oldchev]

Similar to what MassHoe04 posted (in the link) the values for everybody's enjoyment (attached fuel level specs).
I have all the factory manuals to follow and refer to but sometimes this forum has experienced shortcuts.
Would a Kent Moore J33431C signal generator be any help?
Looks like I'll do the easier test on the cluster Thursday.
Regarding the fuel pump assembly ... I really don't remember what it was but it was the same one going in as I took out. It was an assembly with integral sending unit.

 

[rockola1971]

Bosch is certainly not junk (usually). If AC Delco is not available, for whatever reason, Bosch would be my next choice.

You are looking to confirm either the sending unit is wrong and the gauge is OK; or the sending unit is OK and the gauge/stepper motor in your cluster is bad.

I think you can measure voltage at the cluster end of the wire, but for measuring Ohms resistance... I think that has to happen right on the socket at the sending unit itself.
If your tank is full, you should expect to see either 250 ohms resistance at the sending unit terminals or a full 2.5V on the wire at the cluster.

Let us know what you find.

Using the 2.5v at full level I would just take a potentiometer and 9v battery and turn the pot up until 2.5v is reached and mark that as max on the pot and a value not to be gone higher than. Disconnect the fuel pump and stick a negative wire off the pot and the positive on the SIGNAL wire to the cluster. See if the fuel gauge is showing the correct level. Lower pot to roughly 1.875v which should be 3/4 tank level and see if the gauge follows. Keep stepping down .625 for 1/2 tank, 1/4 tank and empty at 0.

*Make sure you are on the signal wire and not the 12v supply (source) power wire.* *Adjust voltages below the 2.5v with a voltmeter reading and pot DISCONNECTED from truck wiring harness in case you go the wrong way so you dont send 3-9v to the cluster*

DO NOT Hook up a signal generator to the fuel pump circuit. A signal generator produces AC voltage and an variable frequency sound wave. Fuel tank was never designed to have an AC signal/voltage introduced into it. It could cause a spark and an explosion.

 

[MassHoe04]

I'll be the first to admit, I am less experienced when it comes to electrical diagnostic stuff.

@rockola1971 gave some good documentation that even I might have a chance of understanding! - Thanks for that write-up!

 

[Fless]

Note that the signal wire sender voltage is inversely proportional to the indicated fuel level. Less voltage = more fuel level.

 

[oldchev]

I have a conflict. A chart copied from this forum (first one) shows just the exact opposite volts and ohms readings for the entire range most notably Full and Empty. The second chart is from my OEM shop service manuals for 2004 (Yes, I have all three). Is it possible the first one is from an earlier manual? Can anyone explain the variation?