new fuel tank sending unit

i bought a new sending umit for my underseat poly tank.just for fun i put a ohm meter on it to see if it was the right sending unit.it reads 87 on the empty side and 18 on the full side.is this going to read right with a auto meter gauge or is an adjustment need to be made? seems to me it will read higher than what"s in the tank. bob

 

The sending unit, ohms, needs to match the ohm rating for the gauge.

 

the gauge i'm looking at is an auto meter 3515. 73 empty,10 full. those numbers are lower than the numbers i get with the ohm meter. bob

 

It's possible your gauge will not read quite empty and not quite full. They are pretty close and you may not even notice any diff. I don't think your so far outa range resistance wise that its gonna do any damage.

 

when i get my gauge i'll try it again,on the bench, and see how it reads with the gauge.bob

 

ok, i got my new gauge. i connected everything to a battery on the bench. just as soon as the float arm comes off the stop at full on the sendig unit, the gauge starts to fall quickly. when it gets to halfway of it's throw, the gauge reads empty,then you would still have a 1/2 of a tank to go.then you would have to figure out how far you could go after the gauge reads empty. i don't see how to make the sending unit send a siginal it's full range. maybe i'm missing something :? bob

 

It does seem that something is wrong. These gauges aren't precision pieces of equipment, but you would expect the Autometer gauge to read about (73 - x)/63 where x is the sender resistance, assuming it expects a linear resistance with float swing.

Half way on your sender should be about (87 + 18 )/2 = 52.5 ohms (you can check that with your ohmmeter with the sender set for half way) and if the autometer gauge is working correctly that should read about 1/3 of a tank, not empty. This would indicate that the Autometer gauge isn't
working correctly.

Sometimes the tank might not let the sender go through its full range. If you can, you should test mount it in the empty tank so you know the true empty resistance. Sometimes the rod the float is mounted on can be you can adjusted to get the resistance in range.

 

That may be an oversimplification. The instructions for the Autometer gauge should specify what range of resistances the gauge expects at full and empty. You need to match the sender to the gauge.

Are you using the factory sender? The FSM should give the range of resistances that the sender should give. Test it with your multimeter. The sender should vary smoothly through the specified range from full to empty.

 

It still sounds like the gauge and sending unit are not matched. They both need to be same ohm readings.
What is the ohm rating of the gauge? Does it match the tank sending unit ohms rating? When you purchase a new gauge, you need to know the ohm rating for the sending unit to get the right gauge.

 

ok. I brought the sending unit in the house with me. At full stop it's reading 12 ohms at half full
it's reading 44 ohms, at empty it's reading 86 ohms. the papers with the gauge call for a sending unit that has resistance of 73 ohms empty and 8-12 full. it's a autogage 3515. maybe my connections weren't tight . bob

 

ok, i think we can put this to bed.maybe tim can explain this. i went to the faq section at autometer and at the top of the page was my exact problem. they say that if you replace the original sending unit , even the same resistance, it won't read in the middle of the scale. this is because some aftermarket sending units operating with 73-10 ohm range now utilize a linear movement instead of the original non linear movement. it reads correct at full and empty, but not the middle. they suggest finding an orignal sending unitR) i'm not sure what to do at this point,i'm in pretty deep.$$$$$. bob

 

my summit catalog lists sending units for ford/mopar with the 73-10ohm. just make sure you get the right range, as mine reads backwards (empty is full and vice versa), but have the factory unit with a GM gauge (90/0ohms).

 

You don't understand the meaning of the linear vs. non-linear statement? A linear scale means the amount of resistance is exactly same as the fraction of movement, ie at half-scale, the resistance will be half of the total... so 73-10 gives a 63 ohms difference between full and empty. If we round this up to 64 ohms, it will make the math easier. Thus each 1/4 tank of gas adds 1/4 of the resistance or 16 ohms:

Full = 10 + 64 = 74 ohms
3/4 tank = 74 - 16 = 58 ohms
1/2 tank = 74 - 32 = 42 ohms
1/4 tank = 74 - 48 = 26 ohms
empty = 74 - 64 = 10 ohms.

Non-linear means that the scale changes from full to empty in some more complicated way. It does not say how it changes, except that it can't be linear.

It boils down to finding a sender that's compatible with the gauge, or vice-versa. There's no simple way to make non-linear and linear compatible with each other.

 

well i have a poly tank, i ordered a sending unit from the same people i bought it from. i bought a gauge that is in the range of resistance of the sending unit. i feel that i have done all i can do. i spent that money on a set up that works correctly,not something i still have to guess how much gas i have. if i would have done some homework i wouldn't have gotton in this mess.bob

 

Post the link to the FAQ you're reading.

<edit> It would also be helpful to post links to the parts you have, ie the sender and the gauge.

Also, I read your previous posts as the sender ranging from 18 to 87 ohms, and the gauge requiring 10 to 73 ohms... it's true that these gauges aren't precision instruments, but the difference between 10 and 18 is 80% - that's a lot of difference.

Sounds like you tested this setup on the bench, and that you have a multimeter. Another inexpensive test you can do requires a part from Radio Shack. Buy a 100 ohm potentiometer and set it with your multimeter to one of the ohms readings I gave in my post above. Then substitute that resistance for the sender in your bench circuit to see what the gauge reads.

 

my poly tank and sending unit came from mts. i'm calling autometer techline tomorrow to see what my options are. it's obvious that the gauge (sport comp 3515) will not work with a mts (cjsu-4) sending unit.i didn't mean to ramble on about this after i figured out this combination wouldn't work,i just wanted other people be aware that some gauges don't work with aftermarket sending units. bob