Setting up a sending unit

renaissance man · #1 (**permalink**) 05-16-2002, 06:26 AM

I have Ford Motorsport guages. before I got the proper sending unit that coorolates to the proper resistance for the guages I had a standard Mustang sending unit. Problem was (and still is) the tank reads "Full" properly but I'm actually "Empty" when the guage reads about 3/8 yet?????

I don't see an adjustment on either the guage or the sending unit. Any suggestions?

Clois Harlan · #2 (**permalink**) 05-16-2002, 07:47 AM

Hey Ren Man,

I have a similar problem but mine will say empty when I have a quarter of a tank left. I am replacing my gauges with Autometer including the fuel tank. I believe the sending units on the tanks measure the fuel left based on the amount of OHMS they read. I hope that a better quality gauge and sending unit will solve this problem with my fuel reading.

By the way, are you coming to Tulsa for the Mid America Ford meet at Hallett and TIR? If so look me up.

Clois Harlan

renaissance man · #3 (**permalink**) 05-16-2002, 07:52 AM

My sending unit came with the Ford Motorspot guages and should be tuned to the set with the proper ohm's. At least you have more gas than it reads. Mine has left me stranded twice. I can't go by mileage either as if I'm tuning the car for a half hour in the garage I burn up ???? gallons of fuel at idle. I've had 135 miles on a tank and 99.5 miles on a tank. Maybe I need to call Ford Motorsport?

xlr8or · #4 (**permalink**) 05-16-2002, 08:07 AM

Some sending units have a mechanical adjustment so that you can set the proper tank depth. I have seen this before where the depth was set incorrectly and the float was sitting on the bottom of the tank but the readings were as you have described.
To verify if this is the problem pull the sender out and manually move it up and down and watch the guage. If you measure the tank depth and move the unit to that length it should read empty on the guage. If not and you move it farther will it go to empty? If it does you need to adjust the length of the sender to match the tank.

Just my .02

Scott

renaissance man · #5 (**permalink**) 05-16-2002, 08:11 AM

Scott,

Thanks for the input. I never thought of just pulling the unit and moving it manually to see what the guage indicated. I know I had to shorten the length of the sender itself as it was adjustable for deeper tanks. Perhaps, like you say, the float is bottoming out.

Mr.Fixit · #6 (**permalink**) 05-16-2002, 07:52 PM

Even after you get the gauge to accurately read "FULL" and "EMPTY" the other lines on the gauge are not what the fractions next to them say. So many cobra owners say, "by the time the gauge reads 1/2 I only have a quarter of a tank!" Thats because the cross sectional area of the gas tank is not uniform. The bottom half tapers on many cobras.

Jack21 · #7 (**permalink**) 05-18-2002, 09:43 AM

Thread on this on CR section too. Goofy fuel tank readings, and indicators bouncing all over. Mr. Fixit is right though. You're getting a linear interpretation of the level of fuel in the tank (if you're lucky), not the number of gallons, unless you have a rectangular tank. Most Cobra fuel tanks are slanted on the bottom. If you had (for instance) a 16 gallon tank, slanted on the bottom, at a gauge reading of 1/2, you would have 4 or 5 gallons remaining, not 8.

Technically, it is correctable, but would be cost prohibitive. The tank probe at your local gas station measures fuel level in inches, and converts it to fuel in gallons. The tank is cylindrical, the probe is 10 feet long, and costs $1,000. The "box" that performs the calculations runs $2,300 - $3,200 depending on the brand.

Bob Putnam · #8 (**permalink**) 05-18-2002, 03:34 PM

"If you had (for instance) a 16 gallon tank, slanted on the bottom, at a gauge reading of 1/2, you would have 4 or 5 gallons remaining, not 8."

And that still doesn't take into account that The sender's resistance is proportional to the angle of the float, not its height. Of course you could just make the face markings on the gage wildly non-linear.