How to test a fuel tank sending unit?

How does one go about doing this, I have my trusty fluke meter and have touch everything I can think of and moved everything but not getting anything I can tell if good or not. I am working on 49 pontiac. Any help?

 

You need to take the sender in hand, measure the total resistance of the sender "coil" windings. It probably will be 90 ohms I'm guessing, then check the resistance thru the "wiper" part of the sender as it moves thru it's arc. You should see it go from no resist. at one end of travel, to 90 ohms on the other end.

 

You can also just hook it up directly to your gas gauge - move the arm and the gauge needle should move along with it. (You have to have the sending unit grounded either way - it grounds the gauge.)

 

Without rippin it out you should be able see it the problem is the gauge or the sender. Ground the wire to the sender gauge should read full leave it swingin in the air and it should read empty. Good Luck........

 

If the sender is still mounted in the tank, measure the resistance between the car's body and the wire to the sender terminal, it should be anywhere between 0 ohms (if the tank is empty) to 30 ohms (if the tank is full). If it's more than 30 ohms there's a bad ground, bad connection, or the sender itself is bad.

also it could read 0 ohms when it has gas in the tank, if the float has sunk (due to having a hole in it) or if there's a short to ground somewhere.

 

Gee, If I unplug mine and leave the wire hang there collecting air the guage goes to full.

They're all different...
Some work backwards to the resistance compared to others.
Either way, if the guage works when the "hot" wire is grounded or, not if it works at all, that says/suggests sender is bad.
Mine I think the float just got knocked off the sender arm last time I siphoned gas for the lawnmower so it only shows empty...

 

the GM gages all work the same, up to about the mid 60s the sender has zero ohms empty, 30 ohms full. Later ones went to 90 ohms.

The off brand cars (snicker) did use some wierd setups, some with decreasing resistance at the higher end of the scale.

You gotta be careful siphoning gas for the lawn mower.

 

That's the way I have always done it. I have also taken them apart and cleaned up all the contacts inside.
Tom - restorit

 

I have, surprisingly, not seen any easy answers here.
1: Pull sender.
2: Set ohm-meter on resistance scale. I'm not that good with resistance, so I move it around until it reads something.
3: Attach tester leads to the sender - one lead to ground (case), the other to the terminal on the sender.
4: Move float arm. Observe tester, if it moves, the sender is likely OK.

Do not touch test leads with your fingers, as they WILL conduct.

Guage:

With ignition "ON", ground sender lead (on car) and observe fuel guage. It WILL change if it's working.

Cosmo

 

Measuring the resistance of the sender when it's in the circuit is usually easier to do than pulling the sender, which often requires dropping the tank, and will tell him what he needs to know.

But your direct approach should work too.

 

If you've changed to 12 V, you'll end up finding a 12 V gauge to fit in the cluster-the Pontiac was positive ground when new, and the gauges don't last very long when current flow is reversed. Went through 3 in a month in a '54 GMC

 

I'm trying to get my 53 Desoto fuel gauge to work. I read the sending unit is 30-90 ohm?
Anyway, basically, if I'm reading this right, if I touch the wire coming off the sending unit to the frame/ground, the fuel gauge should move?

 

What year did Chevy go 12 volt? I'm trying to fix the gas gauge in my '59 Panel. So if the tank is full and I'm reading 30 ohms, its the gauge?

 

This is probably way too much info but it does show how to test the fuel gauge and sender.......... you would just subs***ute ohm readings for what ever brand of car you're working on. And remember, some cars don't supply + voltage to the sender. The sender on them supplies a ground path only.

http://www.lbfun.com/warehouse/tech_info/gauges & instruments/C2_fuel_gauge_and_sending_unit.pdf

 

The GM pre-'66 GM gauges were 0-30 ohms (empty to full). Your gauge may be bad, but it's more likely a poor ground or other wiring issue.

 

Hey wow, it worked. Learn something new every day! Key on, rub wire on ground, flicked on and off. Now I know it is the sending unit. Now where do I get one for a 53 Desoto?

Can I just use one from a Ford?
Example: http://www.jalopyjournal.com/forum/showthread.php?t=308926&highlight=fuel+sending+unit

 

OK , I checked the wire at the tank and got 60 ohms full tank. Fixed a bad splice, chased the wire up thru the dash and found nothing hooked to the gauge so i plugged the wire into the gauge and nothing. Still had 60 OHMs on the wire. I took a switched 12 volt supply and hooked it to the other side of the gauge, still nothing. I reversed the two wires and the gauge now reads full. Im thinking its fixed but theres been conflicting info on exactly what the resistance value leaving the sender should be. Also I dont know if this is the stock '59 tank or not too..I guess I could carry a can of gas around and see if empty reads when tanks empty?