Testing a fuel cell sending unit....
 

Do you need 12V power to the sending unit before you can check resistance?

NO! You won't measure resistance because you'll likely harm your meter if you contact a Hot wire while on the ohms scale. Is your fuel gauge inop?

The fuel gauge is 2.5 hours away....along with the car....just trying to test a sending unit before I go home and install it.

I put an ohmmeter on it....one side on the sender wire post and the other on the ground post.....moved the float but didn't get any resistance change.....

Just wondering what I was missing.

That should have worked. Did you first short the leads to test meter/leads for 0 ohms?
Is this a new or used sending unit?
Are you sure it's not a capacitive type unit?

I shorted the leads first.....it read like .4 ohms.....close enough for what it's for. It's a 70-10 sending unit.....

I took the cover off to make sure no parts were broken or wires pulled apart....it looked fine. It has like a little rectangular coil-like looking thing with a metal piece that skims the top of it that's externally connected to the float. I didn't see a capacitor.

If it's not working, I really don't know why....it looks fine inside.

I was hoping I was just checking it wrong....I'm a lot more mechanically knowledgable than electrically....

Was your ohmmeter reading somewhere between the 10-70 ohms, or was it reading 0 ohms, or an open circuit?

it was hovering around .5-.6 ohms I think.

If its .5 or.6 ohms from sender to ground, it's shorted .
I assume you moved the float arm up & down while watching the meter?
The coil thingy is the resistor winding part of it, and the piece that moves over the coil when the float arm is moved, is the wiper arm. The sender wire is connected to the wiper arm. Check continuity between sender terminal and wiper arm. Should be low or 0 ohms. Check resistance of windings (resistor) from one end to the other. Should read 70-100 ohms. One end of resistor should be connected to ground. Ck with ohmeter. Other end should be open (not connected to anything) As the float moves, the wiper arm makes contact with a changing number of windings to ground, thus varying the resistence to ground measured at the sender terminal. Try these cks and see what you get.

If the resistance is that low, it may be that the sending side is partially shorted to the ground. You might check the little rubber or fiber washer on the sender stud is in place so that no part of that stud is touching anything else that might ground it.

Thanks guys....I'll try these things tomorrow and post back...hopefully it will be something easily fixed....

OK...here's what I found out.

I could measure resistance across the resistor...it was fine.

Still measuring across the resistor, I put the little arm back on and moved it around....it changed the resistance. Good.

I checked continuity from the resistor to the post. Good.

I checked continuity from the sender post to the ground post. It was short.

There was a fiber bushing on the top....but for some reason there wasn't one on the bottom of the flange....and it was shorting everything out.

But here's the thing.....When I insulated the bottom part and put it all back together, I'm getting really high resistance values.

The minimum I can get is 46 ohms. The max is 169. It corresponds to the positions on the resistor......Of course I can get as low as 5 and very high by just checking resistance across the resistor....so I guess the little arm is fallling within those values.

So the big question now is.....Do sending units have comparatively high resistance? For example, if I buy a 70-10 gauge and a 70-10 sender, does the sending unit actually have 10 ohms on the low side and 70 on the high side.....or is it a little higher to compensate for the length of wire it has to go through plus the gauge itself?

I guess what I'm getting at is....are these values correct for a 70-10 sender, or are they too high.....like high enough that they should be for another gauge?

Blykins:
I'm not familiar with the 70-10 designation, so lets just ignore that for the moment.
When you measured from one end of the windings to the other, what resistance do you get?

Sounds to me like maybe the arm is not positioned properly along the resistor. If the positioning is correct, you should be within a couple of ohms of the 70-10. The total value of the resistor could be somewhat higher. That resistor could be made for several applications depending on the location of the little wiper. If the wiper arm can't be moved in a position to get 70-10 ohms, my guess is it isn't the right one.

Actually, there are different units depending on what meter they match up to. The 40-169 ohm range is a fairly typical sender unit if you have the correct meter calibrated to it.
Check the manufacturers listing for these parts and hopefully you can match up the year or model of meter this sending unit goes with.

For some reason the forum won't let me edit my last post. The 40-169 ohm resistance you measured may not be exactly what the rating of the sender is required but is close enough for the meter to recognize.

When I measure across the resistor by itself (one lead on one side of the resistor, one lead on the opposite side), it shows approximately 192-193 ohms depending on how close I can get the leads to the very edge.

Now with that being said, I can put the ohmmeter across the sender post and the ground post at the top of the flange.....then take the wiper arm, position it down in its little pivot point and manually twist it with my fingers while measuring resistance across the two posts.

I can get it down to about 1-2 ohms on the low side.....then back up to about 194 ohms on the high side.

So I guess this is sufficient since it covers the range. I don't know why it wouldn't do this for me before.....maybe I wasn't holding my tongue right.....

My next step is hopefully just plugging it up to the gauge and positioning the float arm (it's adjustable on a rod) to where the gauge says full and empty at the correct places.

OK....now that I try and recreate my results, I notice that the arc wherein the 10 and the 70 ohm readings lie is very, very short. Considering I have a 9" deep fuel cell, I don't think this is gonna fit the bill. It seems like the float would only move about an inch or so vertically between the two readings.....

Sydney has a good point. What I would do at this point is clip-lead the sender to the car wiring and move the arm while observing the fuel gauge. If it operates the gauge, then you could check the calibration if you know the total travel for the float arm in the tank. If the wiper arm is adjustable you can adjust it while its out of the tank. The amount of float travel in your tank may only be the portion of the windings that are in the 10 to 70 ohm range.

I've got another question.....Fuel sender types are usually listed as follows:

240-33 ohm
70-10 ohm
0-90 ohm

Is there a standard there? Does the first number mean the amount of resistance at empty and the second number represents full?

If so, then I need to put the float on the other side so I can get 70 ohms when empty.......

I have a 240-33 ohm sender. It has 241 ohms at empty. Motorhead is right, your float arm should have an adjustable lenth so you can adjust the radius to be able to get within the 70-10 ohm for the depth of your tank. If I remember right, I think I set mine up so that an empty reading was about an 1 1|2" from the bottom of the tank.