Fuel sender woes. New sender but no power to the sender.

[73InkaTiiEuro]

So, one of my last things to do on a long restoration(besides trying to get door gaskets to glue)...I have a fuel gauge that is pegged to full.  I pulled the old sender out and one of the wires was broken...bought a new sender and installed it.  Still pegged on full.  Read up on fuel sender/gauge issues and people indicate there should be twelve volts to the sender.  Using a simple test light, I get no power reading.  Did a dedicated ground from the sender..still no power.  The lights work to the gauge.    What could be the cause of no power to the sender?     I looked at wiring diagrams but can't make it out.  I think power comes into the gauge on the 10 pin connector but not sure how the power goes to the sender.     Another "20 minute project"...;)

[TobyB]

The sender forms a resistive voltage divider to ground-

it doesn't get 12 as such, it sees one leg of the divider.

The meter then measures the center point of the divider,

and translates that to a fuel level.

 

Since you're not metering voltage at the sender wire, something's either shorted or broken

(measure resistance from the wire to the chassis)

so the next step would be to pull the pin at the gauge (remove the sender wire from the 10- pin socket)

and meter the pin itself.  If the pin shows voltage (and the gauge drops to empty) it's a wiring problem,

but if nothing changes and there's no voltage, it's a gauge problem.

 

t

divide and conquer

 

 

[73InkaTiiEuro]

Thank you!  I'm still learning electrical..

[73InkaTiiEuro]

Hi Toby,  After reading this in more detail,  I don't understand part of your suggestion.    If I have voltage at the pin for the sender, that means power is going into the gauge pod and to the gauge therefore in my mind(but you indicated that if the pin shows voltage, it's a wiring problem.   If no voltage at the pin, it's a gauge problem.     

 

I guess I don't understand how the voltage flows.    Does battery power go into the 10-pin, then to the gauge from the 10-pin?  Where does the power to the sender come from?  Is it separate from the 10-pin power?  I've taken the wiring out of any wrapping in the trunk and no breaks there so I'll need to look at the power wiring that goes back from the front of the car to the trunk, just not sure where to look for that wiring....

Hope that makes sense...really just trying to understand the whole circuit for the sender and the fuel gauge...

 

 

[tzei]

How about I put it this way: tank sender unit needs ground from chassis and then it ”sends” ground to gauge. 10-pin provides +12 volts to gauge -> gauge works.

 

How much ground tank sender ”sends” relates amout of fuel in tank.

Edited April 12 by tzei

[73InkaTiiEuro]

Perfect.  Thank you!

[Mike Self]

I was under the impression that the fuel gauge circuit was basically governed by resistance vs voltage.  As the float in the sender unit moves up and down the two resistance wires, it changes the resistance in the circuit from around 80 ohms when the tank is empty to 5-6 when it's full.  

 

If you connect an ohmeter to your sender unit (out of the car) you should see the ohms go up or down as you (carefully!) move the float up and down its wires.

 

BTW, if you can find some resistance wire that measures 6 ohms/inch you can repair the broken wire on your old sender.

 

mike

[73InkaTiiEuro]

Hmmm....so I pulled the gauge cluster and looked inside and all the wiring looks fine.   I also traced the sender wire back up under the dash and I see no breaks/etc.   I then put key on and probed pin 10 with a simple 12v power probe and no light...I did get it to light on other pins.   

 

Does the power come from the battery to the fuse box then to the pin wiring?   Not sure how to proceed now.   It is a new sender Mike.  It looks like I have about 1/2 tank of gas, maybe a little less.  the gauge either shows full empty or full full, nothing in between.  I did pull the sender out of the tank and move the float up and down while watching the gauge and it just shows full, I think this was one of the troubleshooting methods in the forums, I assume the sender doesn't have to be in gas for the resistance to work.  It is a Euro car so I have the warning hookup as well, I noticed if I let the float go all the way to the bottom of the sender, I here something at the gauge and it moves the needle to full empty(but no light).  Would these symptoms point to a gauge problem or wiring issue(no power at the 10 pin).  I'm thinking wiring problem if that pin is supposed to have 12 volts but seems like that sender wire would just be sending a resistance value from the sender to the gauge, through the 10 pin.  I do have lights to the gauge so I know power is getting to the lights via the wires that plug into the 3 pins on the back of the cluster...      

[Mikesmalaga72]

I have a regular 2002 unsure if tii has this wire but, look around near the center area of trunk floor for a brown wire that has an eyelet that should have a screw through it giving the sender a chassis ground sometimes slight corrosion could occur. Try to twist the screw back and forth to restore chassis ground. There are several ground points around the car that will enable operation of lights, signals heater etc. Inspect and rotate the bullet fuses etc. pull fuse box carefully and inspect the underside panel for abnormalities unless already done. Someone here in the FAQ I'll  quote" 90% of electrical problems are the grounds"

It works for me. A lil bit of dielectric grease helps on exposed ground points. 

Edited April 12 by Mikesmalaga72
add

[73InkaTiiEuro]

Yes I cleaned up the ground and no luck, then I did a new dedicated ground and no luck.

[Mikesmalaga72]

Inspect /Spin fuse 11 check if blown and pull fuse block and perhaps look underneath it for corrosion .Caution disconnected  battery negative cable before pulling fuse box.

Edited April 12 by Mikesmalaga72

[73InkaTiiEuro]

I have pulled the fuse box and looked underneath and it is good.  I'll spin fuse 11.   So is pin 10 supposed to have voltage or is it just getting resistance readings from the sensor?   I can't imagine why the sendor wire(brown/yellow) would have voltage...

[John76]

7 hours ago, 73InkaTiiEuro said:

(but no light). 

Ahhh...a Euro spec car with the low fuel warning light.

Are you sure the leads to the sender are on the right tabs?

Try switching the two leads other than the brown ground wire.

[73InkaTiiEuro]

OK, thought I'd close the loop on this one.  Finally found a 9-volt battery with some juice for my multi-tester.  Took the sender out, moved the float and could see the resistance was changing.  Then I measured the resistance at the gauge cluster wire, same resistance.  Then I took a wire and grounded one end and touched the other end to the nut for the Fuel gauge....presto.  All I have to do now is mount a permanent dedicated ground to that gauge.   

Thanks for all the help contributors.  On to the next thing...