I know this is probably a really easy question to answer, but how do I figure out what ohms my fuel tank sender is?
I've sat out there for about half an hour trying to figure it out and given up... it's been a long day

Does it have anything stamped on the top? & do you have a multimeter to test it?

Nothing aparent stamped on it.
Yeah, I've got a multimeter

Connect the meter accross the terminals or if there is only one terminal connect the other to the metal casing.

Set the meter on the ohms scale & measure the resistance at the lowest point the sender will go & the highest.

I'm glad I'm not going mad. That's what I did and the readings were all over the place.
I'll have another look tomorrow and see what I get when I can keep my eyes open.

Also, I have a gauge but dont know if it's matched to the sender, is there any way of testing that too? It's a white faced VDO one with some numbers that dont seem to tie into any on VDO's website

Should be ITRO 1-200 ohms

Of course if its faulty and your holding the probes with your fingers it could be measuring you, which will give variable readings

New Euro guages such as Telemetrics/Smiths/Green Guages et al use 240 to 33 while older imperial senders/Guages and the American stuff use (around) 18 to 180 so if you have the missmatched sender and guage (as i currently have) your guage will never read completely full or empty and read near full when near empty and vice verse

Well at the moment (last night when I was testing) the gauge wasn't reading anything, and was getting some weird resistance from the sender.
I'll nip out in a bit and have another play.

I've just had a thought... if the tank wasn't earthed would it give strange readings?

[Edited on 28/3/09 by thefreak]

If you only have 1 wire from the sender to the guage then yes, its worth having an earth from the tank anyway.
If your sender (like mine) is sealed in the hole in the tank with a rubber ring then the sender could do with an earth aswell.
I also have a chrome steel tube between tank and filler neck, this is also earthed.

Ok. this is starting to drive me up the wall...

The tank is earthed, good earth straight to the chassis. the sender has 1 connector but I'm still getting random readings.

when the tank is empty (float down) it reads around 20ohms sometimes, then can jump up to 70, if I lift the float the resistance goes up to 90, or stays at 20, or gives a -- reading.

Could it be I have a flaky sender? or a dodgy multimeter? I've been searching the house to try and find a resistor I can test the meter on but no luck so far without ripping the TV to pieces

the body of the sender is definitly connected electrically to the tank ?
may be the gasket is acting as an insulator

You may be on to something... looking at the tank the person looks like they've used sealant to hold it in place. I'm guessing from that the mount should be directly in contact with the tank?

Also (sorry if I'm teaching you to suck eggs) you should be carrying this out without anything else connected to the sender.
i.e. all wires off and no gauge connected.

To eliminate this I earthed the body of the sender to the chassis, made a new connector for the sender and hooked the meter up to that. Then I was getting readings of around 170-190 whether up or down.

Think tomorrows job is to pi it out and test it on the bench... Bloody thing

forget earthing to chassis, making new connectors etc.

do the following.
1. disconnect all leads to sender.
2. set multimeter to ohm, 0 - 200 range (or nearest)
3. place one lead on sender tab
4. place other lead on sender body
5. measure resistance with float up and float dwon.

^^^ this is assuming sender has one tab; they usually do, the earching is made via the body - whcih you do by placing lead there, as above.

some senders have 2 tabs and it is quite clear that one tab is insulated from the body and other is spot welded to sender body.

... and some senders have 3 tabs - the third is for low fuel lamp; you are unlikely to have this.

is the sender new?

Did pretty much all of that last night, but I'm going to have another go this afternoon with it outside and in the sun.