Fuel Gauge Stopped Working

[GC427]

The fuel gauge was working when I left home on my last drive. I noticed it stopped working on my way home. The gauge reads empty even with a full tank of gas. This weekend I intend to find out what the problem is. It could be the gauge, a loose wire, the sending unit, blown fuse, or? If someone has experienced this problem and repaired it, I'm curious what you found caused the problem in your situation.

Thanks.

[Blas]

If you lift up the carpeting on the trunk floor (it's velcroed on) you will find a hatch cover that gives you access to the area where the fuel sending unit enters the top of the fuel tank. Remove the bolts holding the hatch cover on and there will sit the top of the fuel sending unit (most likely with one wire disconnected or shorted to the hatch cover) Look for a problem in this area
first...Email me directly if you have specific questions....(my email address is below)
If that does not appear to be the problem, we'll explore other options, but it's the most common reason for a dead fuel gauge....
FYI...The fuel system is a 10V loop not a 12V loop...And as always, be careful working around gasoline. Working outside the garage is always a good safety tip when working with open fuel tanks. But you should have no reason to open the tank top at this time.

[GC427]

Thanks for letting me know about the hatch cover access to the fuel sending unit. I did not know it's there. I would of probably ended up dropping the fuel tank to get to the fuel sending unit. I'll disconnect the battery before working on the wiring to avoid an accidental spark around the fuel tank.

I'll send an e-mail regarding the wiring diagrams mentioned.

THANK YOU again for your help.

Gil

[Fred Hamilton]

Gil

I had the same problem with mine #884. I had an extra gage and sending unit and it was still T/U. What I found was that Smith gages need an Voltage Stabilizer for the fuel gage.
Its located behind the dash. You can order one from Gaugeguys.com. The part # is
BR1307-00 Neg Ground.

Fred
SPF158 & SPO 884

[Blas]

Correct...Actually it's a step-down to 10V. So the gauge does not react to voltage swings when fans and headlights go on and off...

[GC427]

Fred,

Thanks for letting me know about the voltage stabilizer for the fuel gauge and the part number.

Blas,

I sent and e-mail directly to you regarding the wiring diagram. Did you receive it?

Gil

[Blas]

Yes, I sent information about the wiring diagram to you via email. I resent you the information again today. Apparently the first email didn't make it through properly. Hopefully this email will go thru. Thanks also for the inquiry about my Superformance Wiring Diagrams. I’m always happy to help fellow owners with questions too. FYI, there is an information brochure in the “For Sale” section here on Club Cobra and there are screen shots of the diagrams in my gallery of all three versions of the Wiring diagrams…Great look car…

[GC427]

Blas,

I checked the fuel sending unit wires. The wires are in excellent condition, no shorts at the terminal posts, and no loose connections where connected to the sending unit. Fred (SP0884) mentioned his problem was the voltage stabilizer. Should I replace that next? Is there a simple way to send 10 volts through the fuel gauge first to see if it's working? I haven't traced the wires from the fuel sending unit all the way to the fuel gauge. Are there anymore terminal connections along the way I should check? Is there an in-line fuse in the fuel gauge circuit that may of blown?

Gil. Battle Ground, WA

[Blas]

Hmmmmm....
Check to see if you have power at the tank sender....Then check to see if you have power at the step-down unit on the drop down fuse panel. (upper left hand corner) Power to step down is a green wire powered thru fuse #10
(The 2nd fuse down on right side fuse bank) #10 also powers the Tach...So if the Tach works, check power to and thru the step down. Keep me posted on what you find...

[GC427]

Blas,

Thanks for sending the image (photo) for the voltage stabilizer. The photo helped me locate the voltage stabilizer in the Fuse Box Tray. I did not see it at first glance since it's under a tie-wrapped bundle of wires. With the image I knew what to look for. It's smaller than I thought it would be. I cut the tie-wrap, checked the connections, then turned the key to see if the fuel gauge now worked. Still not working. Per our conversation, I'll check for voltage at the source and work my way around the circuit using your recommended procedure. I loaned my volt meter to a friend, so I'll check voltages tomorrow evening. Thanks for the advice.

Gil, Battle Ground, WA

[madmaxx]

disconnect the wires from the sending unit and and check resistance, shake the car back and forth so the gas moves the float up and down and see if you get a resistance change, this will tell you if the sending unit is working

[Blas]

Madmaxx - That's a good idea to try too. You can use an OHM meter to do this test if you have one...
Gil - Remember to also check the connections on the back of the fuel gauge since the problem started after you hit a few large bumps in the road...could be a loose wire connection there.

[GC427]

Blas & Madmaxx,

The fuel tank sending unit reading is 38.2 ohms (see attachment) with car at rest. When I rocked the car back and forth, the ohm reading increased to 41 ohms. The wire terminal connections on the back of the fuel gauge are excellent, no loose connections. The voltage stabilizer input reading is 12.32 volts (see attachment), and the voltage stabilizer output reading is 9.90 volts (see attachment). Although there is a resistance change across the fuel tank sending unit when rocking the car, isn't the resistance supposed to be much lower than these readings? Blas, I believe you indicated we should be looking for readings between 3 to 9 ohms. Also, when I turn the key on, I get a voltage reading of a steady 24 volts to ground from the brown wire at the sending unit, and a fluctuating voltage to ground from the green/white wire at the sending unit. I was expecting to see a nice steady 9.90 volts to ground at the supply side of the circuit to the fuel tank sending unit. At this point, do you think it's the fuel tank sending unit? With 38.2 or more ohms as a reading being sent to the fuel gauge, that would cause the fuel gauge to read empty. I think you indicated the lower the fuel level, the higher the ohm reading. Or do I have that backwards?

Gil

[Barnsnake]

The higher the fuel level, the lower the resistance.
With the key on, connect the two wires at the sender to each other. That should peg the gauge at full. If it doesn't, the problem is in the gauge or wiring. If it does peg the gauge, the problem is probably the sender.

[Barnsnake]

The resistance range of the fuel sender varies with the gauge manufacturer. Many brands use 33 ohms at full and 240 ohms at empty. The older Smith's used 20 full and 260 empty. I think the later Smith's are 20-240. Some brands use 10-180.
Since your's is a 10 volt gauge, it is likely Smith's.

[Blas]

I believe I said about ~10V out of the step down (VOLTAGE STABILIZER or VOLTAGE REGULATOR) ...(9.9V IS ACCEPTABLE)

The voltage range across the tank sending unit I needed to look up...(~8 VOLTS FULL & ~2.5 VOLTS EMPTY FROM MY NOTES)

CLAIRIFICATION: MY GROUND WIRE COMMENT WAS RELATED TO A SEPERATE GROUND WIRE CONNECTED DIRECTLY TO THE TANK SHELL.

RESISTANCE IS 20 OHMS WHEN FULL / 67 OHMS AT HALF FULL / 240 OHMS AT EMPTY - AS PER THE MANUFACTURER.

Good Idea Jim - Try connecting the two wires together and see if the gauge pegs full. It should...

You went thru the process of trouble-shooting very well.
The voltage stabilizer in & out numbers are excellent, you have power to the tank sender, so that wiring to the tank is ok, you now know what is working properly. Did you check the wire connections on the fuel gauge? I'm not sure what your 24V comment is about...you don't have 24V anywhere on the car...

REVISED WITH UPDATED INFO FROM MFR. (CAPS)

[Jerry Clayton]

Why not???? the fuel tank should be grounded, the sending unit housing should be grounded and the fill cap should be grounded----

On fiberglass cars and boats----grounds are a big problem

On the SPF cars the dashes are grounded poorly and causes erratic or intermitten operation things like wipers, cigar lighter, guages(non-mechanical)

[GC427]

Jim,

I connected the two wires at the fuel tank sending unit together and turned on the key. The Smith fuel gauge did not move off the empty reading. My guess is I should now check the voltage at the fuel gauge terminals to see what the voltage reading is, or?

Gil

[GC427]

Blas,

I did check the wire connections on the fuel gauge. Excellent connections. I'll check the voltage at the Smith gauge terminals on the back of the gauge this afternoon and let you know what I find.

Gil

[Blas]

Do a resistance test across the two terminals on the back of the gauge (with the wiring disconnected) a good gauge reads about 62 Ohms.

Power Path:
~12V in to Voltage stabilizer / ~10V out of stabilizer into Gauge / Power out from Gauge to tank sending unit.