250 ohms

[keithTVR350i]

I have cured my fuel pump problem, but am left with a curious fuel tank sender problem. I get a full deflection when I remove the positive cable (cable to the gauge) and earth the earth ? I thought there might be a problem with the sender, when measured I get a reading of 250/255 ohms, is that about right ?
Thanks
Keith

[danny hoffman]

What was the fuel pump prob?

Also what is the actual problem with the fuel gauge? Sorry, I don't know what the resistance should be (I could measure mine at the weekend)

Danny

[jmorgan]

If you are measuring the sender I would think the value will vary according to how much in the tank?
Aren't they variablre resisters on Wedges, indeed any car?

[keithTVR350i]

To be honest I don't precisely know what cured my fuel pump problem, I removed it, cleaned it, checked wiring etc all to no avail. However I also wobbled all the relays and fuses, this could have possibly brought it back to life.
My fuel gauge shows zero, even when now completely full, The resistance will vary with fuel volume, however I don't know if 250 ohms actually indicates knackered sender, rather than working condition.
Keith

[wedg1e]

Tank senders are usually rheostats, that is to say a variable resistor created by wrapping a long piece of resistance wire around a suitable former, then having a sliding tap (actuated in this case by the float) move along the wire to tap off a potential. In operation on a fuel gauge, the sender is effectively a variable resistance between the gauge and earth. So current from the battery (or in most cars a voltage stabiliser, to take out any instrument fluctuations!)passes through the meter's winding, down to the sender and thus to earth. Full scale deflection in theory is obtained by taking the sender wire to earth; any resistance in the wire (ie the sender) causes the meter to read progressively less (more resitance is introduced as the float falls). In practise, many gauges will read full scale even with some resistance in line, and often the gauge has a bias resistor built in so it will read full scale even with no sender attached. If the float springs a leak and drops then the sender will tell the gauge the tank is empty even when full.
So, if the gauge reads full scale all the time: the sender is open circuit, the wire to the gauge has a break, the sender has no earth (to complete the circuit) or the sender pivot is stuck in the up position.
If the gauge reads empty all the time, it may be open circuit, there's no feed to it, the float bowl has failed or the wiring is wrong!

Think that covers every eventuality....

Ian

[my 350i]

Easy way to check gauge is to disconnect feed wire to sender and connect it direct to earth. The gauge should read full. if not then the fault is with the gauge or its supply.
Some gauges however work in reverse, when disconected they read full and if earthed will go to empty. Depends I guess on what type of gauge you have fitted but arent all wedges fitted with the same instruments to make life simple ?