Fuel tank sender

Jack21 · #1 (**permalink**) 05-25-2003, 07:14 PM

Finally took the fuel tank out to replace the sender. It was in pretty bad shape, rust wise. Tested replacement wired up before installation. Added chassis ground directly on sending unit connected to main wire ground. Works fine in, and out of tank. Fuel gauge is damaged, and will have to replace it also. Replacement gauge (Autometer "Z" series 0 - 90 ohm) has much higher damping than the original in that it moves a lot slower.

Soo, Mr. DV, I have your loaner, and a few extra components you may find use for. Will UPS them off shortly. Thanks for your help.

Double Venom · #2 (**permalink**) 05-26-2003, 05:49 AM

Jack,
Not a problem Sir!
DV.. Don't UP's em, bring em' !

Art Burtt · #3 (**permalink**) 05-30-2003, 11:55 AM

Hey DV,
I have Stewart Warner gauges. Will the sending unit in that came in the tank work? If not, the gauge came with it's own sending unit. Shuld I just put that one in anyhow?
Art

Double Venom · #4 (**permalink**) 05-30-2003, 08:27 PM

Art,
Not sure, but almost positive, the "OHM'S" for the SW vs the GM OHM reading in the tank are NOT the same!

In this case, pull the stock sending unit out, remove the "swing arm" and then reinstall.

Drill the appropriate size hole next to it, following the directions for the swing arm measurement and your all set.

Before you aske, you will need both units as the SW unit does not come with a fuel pick up tube.
DV

peterd · #5 (**permalink**) 05-30-2003, 08:47 PM

I have done a major rebuild which included going to EFI. Everything is up a running with the exception of the fuel guage. When I turn on the ignition the fuel gauge read 1/2 full the goes to empty. I measured the sender voltage and it peaks to 13 volts then goes to zero. I am assuming that the sender is simply a variable resistor that outputs a voltage propotional to the fuel level. The battery is at 11.5 to 12.0volts why do I read 13volts at startup.

petek · #6 (**permalink**) 05-30-2003, 10:23 PM

Yes, 99.9% of fuel level sensors are simple variable resistors (if you want to get real picky it's actually a rheostat -- just a fancy name for a potentiometer wired with one leg tied to the center leg -- a potentiometer being just a fancy name for a variable resistor -- a variable resistor just a fancy name for a coil of resistive wire with a wiper).

Anyway, 99.9% of 'em are variable resistors, the rest are fancy things that work by variable capacitance or ultrasonics.

There is no way for a resistor to create voltage. When you read your battery voltage are you using a DVM or the dash mounted voltage gauge? (Dash mounted gauges are not as accurate as a DVM.)

Something you might try: disconnect the output of the fuel level sensor and measure the output to ground using the Ohms scale on your DVM.

A quick search on the internet says that a Stewart Warner sender shows around 30 ohms when the tank is full, 250 ohms empty. A GM sender shows 93 ohms full, 0 ohms empty. Ford sender shows 70 ohms full, 0 ohms empty. (Data from http://www.stewartwarner.com/Tech/faqSc.html)

I've a feeling that either the tank (i.e. the level sender) isn't grounded properly or that you've got something shorting to the sender output. Or ... and this one would be a real long shot ... the power to your gauge is dropping out.

With a few lengths of wire you could isolate the gauge and sender from everything else. Power the gauge straight from the batter with a wire, connect the output of the sender to the gauge with another wire ... all separate from your current harness. If the gauge works ok, then connect the power from the harness to the gauge and see if everything is still working. Finally, connect the harness connection between the sender and the gauge.

best of luck!

Double Venom · #7 (**permalink**) 05-31-2003, 05:25 AM

Petek,
What did you just say?

DV

I have question for you; would you be interested in taking the wiring schematic in the Roadsters manual and making up a "true" schematic that guys like me could actually follow and understand?

We want to bring it up to date, eliminating a lot of "stuff" in the harness and hopefully making it better. Let me know.

petek · #8 (**permalink**) 05-31-2003, 09:30 AM

DV,

Sounds like fun! The schematic in the manual is less than exceptional (i.e. it's cr@p), I'm sure we could do much better! I'll send you an email.

Double Venom · #9 (**permalink**) 05-31-2003, 08:04 PM

When you do, send me a proposal! Yep an estimate. I know it will take some time and I know you will do it right!
DV... 1+ hour and it DVSF III Month!

peterd · #10 (**permalink**) 06-01-2003, 10:25 PM

I did further testing on my unkown fuel sender and determined its bad. I get a reading of 268 ohms. My sender sit on top of the tank and has 3 wires with 2 pots for adjusting empty and full. I pulled it out of the tank and it has a tube about 12" long. My fuel cell is foam filled and therefore cannot take a sender with a swinging arm or float. I measured the resistance and assumed that as I pull the sender out of the tank (simulating fuel level) I would get a change in resistance. Can anyone help identifying the supplier of the sender.

petek · #11 (**permalink**) 06-02-2003, 09:24 AM

peterd,

Your fuel level sensor sounds like one of the capacitive type. I think there was a thread about that sort of sender over at gt40s.com.

If I remember correctly the output of the sender goes to a "black box" that converts the changing capacitance into a signal that the gauge can work with.

Surf on over to gt40s.com and do a search on their forum.

Art Burtt · #12 (**permalink**) 06-03-2003, 10:41 AM

Don,
Just called Cobra restorers they.Stewart Warner makes gauges
that will work with Chevy sending unit??? They are swapping with me. I didn't want to take tank out and flush it drilla another hole in it!!
Art

dscott · #13 (**permalink**) 06-03-2003, 03:06 PM

Art, That's nice of them. The sending unit in the CR is a GM 0-90 ohm unit. The new gauge should be set for that range. There is also one that goes to 240 ohms so you may want to check with them and make sure you get the proper one.
Don

Snake · #14 (**permalink**) 06-01-2004, 11:23 PM

Hey Guy's! I changed out my SW gauges...Just to much trouble..Fuel gauge not working...tach not working and so on!

I bought a set of gauges from a B&B owner. These are Faria gauges...Very Nice! But the fuel gauge needs a
standard Ford 30 - 240 ohm sending unit.

Don do you... or DV have one? This gauge stuff is driving me totaly nuts! Also I need a correct length speedo cable..just another one of my new problems!!

Besides filling up at the gas station every few hours to make sure I don't run out of gas. This Classic Roadster Cobra is a DREAM..I love it.
Snake/Gerry.

dscott · #15 (**permalink**) 06-02-2004, 08:18 AM

Snake, the sender we use is a 0 to 90 ohm. You can get the correct one from the gauge mfg. You may need to do some work to install it.
Don