refresh my memory, is it supposed to be 0-90?

I'm working on a car that read 5-100 but the gauge stops working when it gets down to 1/2 tank.

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[This message has been edited by buddycraigg (edited 05-06-2009).]

0-90 ohms. Might be a gauge problem, assuming the sender is shorted somewhere in the middle.

[This message has been edited by topher_time (edited 05-06-2009).]

thanks topher. I thought it was 0-90.
I've adjusted it and the best i can get out of it is 1-79.

I'll plug it back in tomorrow and see what the gauge does while i swing the float from empty to full.
I dont really care what it reads when it's full, it's when the tank is empty that the gauge needs to be pointing at E

quote Originally posted by buddycraigg: I dont really care what it reads when it's full, it's when the tank is empty that the gauge needs to be pointing at E

Good point.

quote [B]Originally posted by buddycraigg: I thought it was 0-90. I've adjusted it and the best i can get out of it is 1-79.

hmmmm - maybe time to make a visit to Radio Shack for a 10 ohm resistor (?)

quote Originally posted by buddycraigg: I'll plug it back in tomorrow and see what the gauge does while i swing the float from empty to full. I dont really care what it reads when it's full, it's when the tank is empty that the gauge needs to be pointing at E

After you fill or empty the tank, don't forget that the needle on the guage can be (gently) pried off with a table fork and re-applied to point anywhere you want

[This message has been edited by PaulJK (edited 05-06-2009).]

My guess is that the tank sender variable resistor has a bit of a tolerance to it perhaps as much as +/- 10% This would make it possible to see 0 - 81 Ohms or 0 - 99 or sometimes more either way. Ambient temperatures will factor into this a little bit as well.

There is a simple way to adjust for most gauge indication errors.

There are three studs jutting out the back of the gauge.
Looking at the gauge as it sits in the dash and you are sitting in the seat, the pin out is:

Left Stud: +12 volts from fuse box and one end of the Empty coil in the gauge.

Center Stud: Ground and one end of the Full coil in the gauge.

Right Stud: Sending unit connection at the gauge, the other end of the Empty coil and the other end the Full coil in the gauge.

To raise the gauge indication towards Full, put a 1/2 Watt resistor of 1 - 20 Ohms in series with the sending unit wire and the Right Stud.
(this will be a trial and error kind of thing to get it just right).

Use this to help with obtaining the correct resistor value.
R1 = Sending unit resistance measured with the tank Full.
R2 = The resistor you add for correction

R1 + R2 = R total, ie 90 Ohms.

To lower the gauge indication towards Empty put a 1/2 Watt resistor of 250 - 500 Ohms between the Right Stud and Ground.
(this will be a trial and error kind of thing to get it just right)

Use this to help with obtaining the correct resistor value
R1 = Sending unit resistance measured with tank Full
R2 = Resistor you add for correction

R1 X R2
------------ = R Total, ie 90 Ohms.
R1 + R2

This is not an exact science but it will correct most 1/4 to 1/2 tank off gauge readings. The best part is you don't have to pull the tank to adjust the gauge to read right.

Cheers,
Sidecar 2M6 SE

quote Originally posted by Sidecar 2M6 SE: My guess is that the tank sender variable resistor has a bit of a tolerance to it perhaps as much as +/- 10% This would make it possible to see 0 - 81 Ohms or 0 - 99 or sometimes more either way. Ambient temperatures will factor into this a little bit as well. There is a simple way to adjust for most gauge indication errors. There are three studs jutting out the back of the gauge. Looking at the gauge as it sits in the dash and you are sitting in the seat, the pin out is: Left Stud: +12 volts from fuse box and one end of the Empty coil in the gauge. Center Stud: Ground and one end of the Full coil in the gauge. Right Stud: Sending unit connection at the gauge, the other end of the Empty coil and the other end the Full coil in the gauge. To raise the gauge indication towards Full, put a 1/2 Watt resistor of 1 - 20 Ohms in series with the sending unit wire and the Right Stud. (this will be a trial and error kind of thing to get it just right). Use this to help with obtaining the correct resistor value. R1 = Sending unit resistance measured with the tank Full. R2 = The resistor you add for correction R1 + R2 = R total, ie 90 Ohms. To lower the gauge indication towards Empty put a 1/2 Watt resistor of 250 - 500 Ohms between the Right Stud and Ground. (this will be a trial and error kind of thing to get it just right) Use this to help with obtaining the correct resistor value R1 = Sending unit resistance measured with tank Full R2 = Resistor you add for correction R1 X R2 ------------ = R Total, ie 90 Ohms. R1 + R2 This is not an exact science but it will correct most 1/4 to 1/2 tank off gauge readings. The best part is you don't have to pull the tank to adjust the gauge to read right. Cheers, Sidecar 2M6 SE

can you do some pictures to show where these resistors would hook up at?

there are many fieros out there that run out of gas when the gauge reads 1/4 tank.

[This message has been edited by buddycraigg (edited 05-06-2009).]

quote Originally posted by buddycraigg: can you do some pictures to show where these resistors would hook up at? there are many fieros out there that run out of gas when the gauge reads 1/4 tank.

Sorry buddycraigg, but my camera doesn't seem to want to work for me these days.

I went back and read your post about the gas gauge reading 1/2 tank when the sender is at the lowest resistance you can get out of it. I would suggest you check the ground connection for the sender in the tank. This should be a black wire in the harness going to the tank. If there is resistance in this ground connection it will cause the gauge to read above empty even if the sender unit is at 1 ohm when the tank is empty.

A quick way to check the gauge for proper operation when it is in the dash and powered up is to short the center stud of the gauge (this is the ground connection to the gauge) to the right stud (this is the sender input to the gauge). If the gauge goes to empty when it is in the car and you short the center and right stud on the gauge together then you have extra resistance being added to the circuit by a poor ground at the tank. A bad ground in the gauge cluster will also cause the gauge to read improperly.

You can use an ohm meter to check if there is a difference in resistance between the ground for the gauge and the ground for the sender at the tank. Put the ohm meter across the ground connection for the cluster and the ground wire connected at the sender in the tank. you should see 0 ohms. If you don't then the difference in resistance to ground between these two ground points is the cause of your problem, I think.

The fuel gauge circuit is pretty simple but a difference in ground potential at the cluster compared to the sender ground will cause it to not read correctly when the tank is empty.

Hope this helps you.
Sidecar 2M6 SE

[This message has been edited by Sidecar 2M6 SE (edited 05-06-2009).]