Fuel Gauge Issues

[msjulian]

I know this has been covered to death but I can't seem to pin down what the issue is.

I have an '87 R10 with a single 16 gallon tank. The original fuel gauge had the standard Chevy truck needle bounce so I figured I would swap the gauge out. While I was at it I replaced the fuel pump as it was original as well. This is where things get weird. I purchased a NOS gauge still in the box off eBay and now the gauge doesn't bounce but won't drop past half tank. I thought it must be a bad gauge so I bought another gauge off eBay. This on the seller claims it was pulled from a running truck and works fine. Stuck it in and I get the same thing. The original gauge was swamped out at a buddies shop and was pitched so I don't have access to it anymore to try it. Testing the voltage and ohm reading at the gauge panel things seem to look right.

Across A and C I have 14.4v while truck is running
Across B and C I have a varying ohm reading. It is going down as I drive the truck.



From the best of my calculations when the truck had a 1/2 tank is was in the 40 ohm range and then has been dropping. At the time of this post is is in the 20's and doing the milage math I should be in the 1/4 tank range.

I have pulled the gauge and hooked 12v + to B and ground to C and the needle goes to E. When I short across A and C the needle goes to F.



So it appears that the gauge it operating through its full motion.

I am pulling what hair I left out trying to figure out whats going on...

Any suggestions??

[Irish_Alley]

did you read this
FUNCTIONAL TESTS OF FACTORY GM ELECTRIC GAUGES

[bd]

For everyone's edification, "A" is 12v ignition, "B" is sender, and "C" is ground.

The fuel gauge is a bridged voltage divider.  Two internal gauge coils physically oriented at 90° to one another are connected in series between 'A' and 'C'.  The fixed ceramic resistor on the back of the gauge connects in series with the variable resistance fuel tank sender between 'A' and ground.  The fuel gauge 'internal coil pair' is connected in parallel with the 'external resistor pair'.  The common connection between the coil pair is bridged to the common connection between the resistor pair at 'B'.  The two gauge coils, the ceramic resistor and the tank sender are approximately 90 ohms apiece.

Powering 'A' while grounding 'C' with 'B' disconnected should cause the gauge needle to swing well past Full to the 3:00 o'clock position.  Subsequently shorting 'B' to 'C' should drop the gauge needle to Empty.

[hatzie]

Beating a dead horse.  If anyone cares...  I posted this in the thread the OP posted on the 1947-present truck boards 73-87 subforum.
I did a bench test of three known good gauges using a DC bench power supply set at 14.5vdc and inserted a General Radio 1432M Calibrated Decade Resistance box in place of the fuel sender.
The Gauge Ohmages were very close to one another.  Plotting them in Open Office Calc or Excel shows the expected sender resistance is not a straight slope from Full to Empty.  The gauge sweep is actually weighted to slow the drop in reading from just below 5/8 to just below 1/4 tank to give the appearance of better fuel mileage.  It also explains, as you'll discover, that you'd better find a gas station ASAP if the gauge reads below 1/4 tank.  1/4 to E is a darn short trip because it's not really 1/4 tank. :lol:

This table is the ohmage that drove all three gauges.
-Ω-   Gauge Reads   
90Ω   -F-
76Ω   7/8
64Ω   3/4
53Ω   5/8
45Ω   1/2
37Ω   3/8
29Ω   1/4
18Ω   1/8
00Ω   -E-

[bd]

Useful data!  Thanks.

[Irish_Alley]

i always wondered why my last 1/4 seamed to go slowly. thought it was my driving habit changes when i realize i need to get fuel.

[roundhouse]

Saw a story somewhere about how Cadillac reprogrammed the gauge in their cars to give the appearance of better mpg


I can attest that you better treat 1/4 tank as empty

There's about a gallon left when the Gauge shows 1/4 tank
Mine drops from 1/4 to E and engine quitting   Very quickly

[Irish_Alley]

Yeah when mines on e when I stop its dry and will stall out.

[IdahoMan]

How do we fix our gauge's dishonesty then?

Intolerable. I expect my gauges to be accurate.

[Irish_Alley]

Maybe a digital gauge? Like Dakota

[bd]

msjulian - Did you get this resolved?