How do you read the fuel sender ohms?

May seem like a silly question, but I don't know where the meter leads should be to take the ohm reading on my fuel sending unit. When I have them both touching the metal plate I get the same reading of 100 w/ the tank full and with it empy. When I have one lead on metal and one on the post I get no reading (full or empty).
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P.S. I've done some searching on this, but haven't found the answer yet. Thanks.

 

try inbetween the wire and the post, key on, power to wire
you might have to ground the sender with an additional wire

 

Power to the white wire? I thought power to the tank was a no no?

 

white wire looks like your ground; ground it somewhere, use jumper wire, whatever
touch one probe lead to the hot sender wire from vehicle
the other lead to the post where the hot wire attaches
your ohm meter now becomes your fuel gauge
as you move the float arm, full, half, empty, you should register different ohm readings/values
it won't matter which probe lead you use to make these connections, red or black

 

Okay. Just to make sure I don'rt blow up the garage...
"the hot sender wire from vehicle" can just be a 12V source, with the ohm meter between the 12 volts and the post? I don't know if the gas gauge is even working, so I was trying to leave that out of the equation and just check my ohms full and empty, and see if the sending unit is working.

 

Howard
have been talking to Mike over landline
thanks for the warning, he is aware of the voltage reduction

 

Yep, I understand. Thank you both for the instructions.
Okay here's what I've got:
First, upon testing the fuel gauge by turning the ignition on and puting a test light to the gauge, instead of the tester lighting up, it apparently grounded out and the gauge would peg "full". Not what I was expecting, so I believe the gauge is bad. When I purposely grounded the fuel gauge, it also pegged full, which is what I expected.

Now for the sending unit. I took it out of the tank. It is covered w/ rust and the brass float has some gas in it, so it is toast. I went ahead and hooked up the ohm meter between the gauge and the post on the sending unit anyway. With the ignition on and the sending unit grounded, I got no readings on the ohm meter. Also tried moving the float manually - nada. I had the meter on the x1k setting and I zeroed it out before hand.

Sending unit dead? Yes.
Gas gauge dead? Not sure, but something's wrong with it.

 

to Safely test your guage, make sure the power lead has a good 12 volts going to it when the ignition is on. with the meter set on volts and the ignition on, read from the terminal (with the strap that goes to the temp guage) and a good ground, you should get around 5 volts. if you do the regulator in the fuel guage is working, if not the guage will have to be replaced.

if the regulator is good; with the ingnition on, temporarily ground the S (sender) terminal on the guage, this should peg the guage, if not the guage is bad and must be replaced.

 

I was wrong
Howard straightened me out about the ohm meter being powered with it's own battery, not the vehicle power.
I was thinking volts, not ohms, sorry Mike.
is the corner available, going to stand in the corner now

 

Howard,
Here's what I have:
- It's an MB, but only some of the gauges are original. The fuel gauge is not original. I don't know if it's specifically for an MB or if it was just replaced with a generic gauge by one of the previous owners.
- I didn't verify the voltage going to the fuel gauge, I only checked with a test light that it was getting some power.
Yes, when I ran a test light from the fuel gauge lead to ground, it pegged on full.
- Correct, the tank and sender are not original and the make of them is unknown.

What started this for me was that the oil pressure gauge and water temp gauge are not working, plus it has the Ammeter gauge (that one looks original) and I was going to replace that with a voltage meter so I thought I'd just replace all four gauges with new white faced gauges. So I wanted to test the high and low ohm readings on the sender so I could get a fuel gauge that was compatable. (That much I learned from research, but I still wasn't sure how to take the actual reading). As it turns out, when I pulled the sender unit, it is in bad shape, rusted and leaking float, so I'm gonna have to buy a new sender along with the gauges.

Question: how hard will it be to match a new sending unit to this tank? Is there a stardard size opening or are there many variations?

Jim- you got me going in the right direction and your help is also very much appreciated.

 

is it possible to get another pic of the tank sending unit hole ?
I can't tell if the sender is held in with screws or a twist type lock ring.

 

Here's the hole for the sender. It's got three screws. The only thing I was really trying to use was the sender, but that needs replaced, so it's gonna be all new gauges and a new sending unit. I'm just hoping I can find a sender that will drop right in with this tank - so I can at least keep the tank.
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FYI - here's the gauges.
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I never paid any attention, but it looks like the water and oil are old Stewart Warner gauges. Anyway, when I get my new gauges I'll offer these up to anyone that wants them. And yes, that's 18,442 original miles R)

 

was this tank mounted under the seat ?
I'm thinking it's a boat/marine tank

 

It's under the rear seat, between the frame rails. The tub was cut out by a previous owner and a metal tray was made for it. It's got a funky rigged up pvc filler tube inserted into it, so yes, very possibly not even a car gas tank.
View attachment 9550

 

even though it's only using 3 screw holes, a 6 hole unit of the correct diameter would probably work.
The VDO universal senders I used on my tanks had adjustable float arms and would work in tanks from 6"-23" deep; the ohms were 10-180, empty to full.
Is your electrical system 6 or 12 volt ?
what's the diameter of the sender head ?

 

12 volt. Dinner's ready. I'll measure and post it later.

 

I've been wrong a lot lately, and other than the 3 screw sender, that tank sure looks a lot like a rear mount 15 gal CJ tank.....but don't take my word for it..
maybe walk into parts store/dealership/boat store and ask if anyone has anything like it ?