Fuel Gauge Issues (pulling my hair out)

[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??

[Dead Parrot]

Is it possible that one or both of the new to you gauges were meant for or pulled from a CUCV vehicle with a 24V system?

The other possibility is the resistor on the back of the gauge is either bad or not making connection. The resistor is the green on the white thing on the back of the gauge. I looked at two gauges I have. I think one is the original from my 75 that developed the 'bouncy' problem. It has a blue color and measures about 82 ohms on its own and when coupled with the gauge, I get about 42 ohms across the A-B terminals. A second newer gauge( don't know what year) has a pink resistor that might be bad since I couldn't get a reading across just the resistor. The combo yields about 82 ohms.

The resistor is easy to remove. Just gently remove the two nuts. The resistor will lift off.

Guessing that for a 12V system, you want about 42 ohms for the combo.

[msjulian]

When I pulled the resistor I got 87 ohms across it and then in the 40's across the gauge with it installed. On the second gauge I have, I got nothing on the resistor (guessing it's bad).

The gauge seems to be for 12v as it sweeps from E to F with 12v connected.

[Willowrun]

CUCV gage panels (and gages) are 12v. Just the starter is 24v.

[badchevy]

I know you replaced the fuel pump but how about the sending unit? A faulty sending unit can make your gauge read half when the tank is actually full. Have you tried pulling out the sending unit/fuel pump assembly and moving the arm manually. This way you can see if the gauge reads all the way empty or all the way full. Sorry if I missed something on your posts saying you already did this.

[hatzie]

You're hooking up the power and ground backwards.
Using your gauge picture

A = Ignition +12v
B = Sender
C = Ground

Grab an adjustable DC power supply or 12v battery.
Test leads.
Ohmage on the sender at Full is 88-92Ω and 1/2 tank is 43-47Ω and 0-4Ω at Empty. SO... Grab some radio shack 1/2W or 1/4W resistors... 100Ω & 50Ω should be all you need.

Hookup A to +14.5v or Battery + and then C to ground. The gauge should read past "F" Approximately 3:00... Simulates an unplugged "open" sender circuit.

Hook a jumper from B to C ( 0Ω sender ) and the gauge will read "E"

Hook a 100Ω resistor from B to C and the gauge should read some past F

Hook a 50Ω resistor from B to C and the gauge should read some past 1/2

[Axle]

Some additional reading for you;
http://67-72chevytrucks.com/vboard/s...d.php?t=640615

Alex.

[Dead Parrot]

With Hatzie's info, I performed some testing with the two gauges I have. When I tested the gauge with the known good resistor(blue), a 70 ohm across the sender terminal and ground yielded a reading of about 7/8. On the gauge with the open gauge resistor(pink), the same 70 ohm across the sender terminal to ground gave a reading around 1/8. Moving the good(blue) resistor to the other gauge caused it to start reading properly.

Did a further experiment and added a 2nd resistor across the A-B terminals. Total resistance there is now about 28 ohms. With the same 70 ohm sender test resistor, the gauge read almost to the 3 o'clock position.

So if the gauge resistor is too high or open, the gauge will read low. If the gauge resistor is too low or short, the gauge will read high.

[msjulian]

Hattie, thanks for the correction. That was the way I hooked up the 12v power supply. When I did this the needle went to the E mark and no further. When I jumpered B and C with 12v on A the needle went to the F mark and no further.

What I can't figure out is why I seem to be seeing the ohms drop at the gauge panel as I drive the fuel out of the tank.

I will try and pull the pump again and see if I can get the gauge when moving the float arm.

[msjulian]

After reading the suggested post from Alex it looks like I have one gauge with a bad resister(pink). With it off the gauge I get no ohm reading and no continuity. With it installed on the gauge I get double the ohm reading that was shown in the post from Alex. This explains why that gauge would only read 1/4 when the tank was full. The other gauge has a green resister and the ohm readings on it match was was explained in his suggested post +/- 2 ohms. Checking the terminals in the gauge panel I have 55 ohms across the ground and sender lead. This should be about 5/8 of a tank and that is what the gauge with the good resister reads and close to what I figure should be in there according to my mileage math. I will drive the truck this weekend and monitor the gauge and the ohms from the sender at the gauge panel and see if I can get the gauge to read past 1/2.

I am assuming that if it doesn't go past 1/2 on the way down my next step is to drop the tank, pull the pump and see if the gauge moves with the float arm.

[hatzie]

Originally Posted by msjulian

After reading the suggested post from Alex it looks like I have one gauge with a bad resister(pink). With it off the gauge I get no ohm reading and no continuity. With it installed on the gauge I get double the ohm reading that was shown in the post from Alex. This explains why that gauge would only read 1/4 when the tank was full. The other gauge has a green resister and the ohm readings on it match was was explained in his suggested post +/- 2 ohms. Checking the terminals in the gauge panel I have 55 ohms across the ground and sender lead. This should be about 5/8 of a tank and that is what the gauge with the good resister reads and close to what I figure should be in there according to my mileage math. I will drive the truck this weekend and monitor the gauge and the ohms from the sender at the gauge panel and see if I can get the gauge to read past 1/2.

I am assuming that if it doesn't go past 1/2 on the way down my next step is to drop the tank, pull the pump and see if the gauge moves with the float arm.

You are correctly reading the sender resistance. 55Ω should set the needle just proud of 5/8. The fact that the ohmage is dropping with the fuel level sounds like the sender and wiring is working properly up to this point.
I'd keep a fuel can in the bed and drive her 100-140 more miles. At 10mpg that should put you under 1/4 tank and at 15mpg it should still drive the needle down well under 1/2.

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 Gauges 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.

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-

[Jeepwm69]

http://67-72chevytrucks.com/vboard/s...6&postcount=21