Fuel sender resistance

I'm in the middle of converting my 715 tank sender and pickup unit. I've cut the top off a 33 gallon tank from a 77 chevy van and welded it into the new enlarged hole in the stock tank. I want to test the sender since it's 30 years old. It should be simple with an ohm meter, but I'm not getting any readings. Either the unit is bad even though it was working a few years ago, or more likely, I don't know what the hello I'm doing when trying to test it. Will someone smarter than me explain a good method for testing this thing? Oh, what is the ohm supposed to be on GM vehicles so I can buy the correct guage too please?

Sometimes the wiper arm and shaft can shift and lift the contact point off the resistance wire. It shouldn't be that loose back and forth but it has happened.
Can you examine the windings? Are they intact. Can you put your meter on each end of the windings? I have carefully pried a few units open and repaired them with extremely small copper phone wire. The Ohm's value will change but the gauge will work again and not be that much different. If you have a clean break in the windings put a light drop of super glue on each side of the break. Using a nail file or very small piece of fine 500 or better sand paper very carefully clean the wire windings to prep for tinning. Carefully tin each side of the break and then make your jumper wire. Tin the ends and very carefully solder it over the break. It is important to keep all solder, flux, and glue out of the wipers path. that must be clean and debris free. You might test the tension on the wiper arm. To much will ruin the windings by wearing through. Just make sure it has light contact and cannot drift in the sending unit case removing contact. Test and reassemble. I did this to my Under seat tank in my CJ-5 15 years ago and it still works.

To test what you have put one lead on the screw terminal and the other lead on the frame of the sending unit. Move the arm slowly and the OHm's value should change. Don't touch the sending unit post while testing. Your skin will conduct and make a false reading.

If it's burnt from a bad hook up you will need a new one.

Measure from the sender terminal to frame or ground. Total empty is very close to 0 ohms. Full is close to 30 ohms. These readings taken from a new sending unit. Don't know if GM is different. This is from the 715.

Are you trying to use the stock 24V gauge? Then you will have to use a 24V military sending unit.

The GM senders go from 30-74Ohms I think. I haven't looked it up in a while so I could be way off.

The old GM senders...50's era...not sure all the years but I think before 64 or 63 or so...anyway, those have a 0-30 ohm scale like the military sending unit BUT it is the reverse of ours...I forget which but one has 0 ohms empty and 30 when full and one has 0 ohms full and 30 at empty.

The newer GM stuff, mid 60's and up, is not even close...neither is any other sender I have found including Ford, Chrysler, Toyota, Nissan and Stewart Warner.

Not only all the above, but those mid 70's GM senders were known to fail pretty regularly as well.

Al: I'm testing it while holding it my hands. Didn't know my skin could cause an errant reading. Will move to a more suitable surface.

Rboltz: That's how I tested it except I had the top of the unit as the ground since it isn't installed yet.

Tim: This is a sender for the 77 van. 12 volt.

Jon: Yes I know everyone's stuff is different. When I figure out it works or not, I can buy a matching guage--or Google it up I guess.

Randy: Thanks for that info, based on that and the fact that I can't get any reading, I'll buy a new one. It matches very well with the tank depth by a slight straightening out of the pickup tube by hand. I'll still have to add about 1 to 2 inches in length to extend it to the bottom of the tank, so I'll probably not mess with the new one other than adding whatever length is needed. The float rod I had to unbend a little with pliers. It appears to have no binding when dipping it in a tank of water.

Thank you all for the responses. Bill

If its the military sending unit, no one but the military has the same scale working the same way. To do it right, one would need a mil gauge to match.

The old GM car scale is the same range in ohms but is reversed...so when the tank is full, the stock sender with the old GM car gauge would read empty and would appear toget fuller as one drove...odd but true.

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Originally Posted by brute4c

... would appear to get fuller as one drove...odd but true.

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Hmmmmm, now if you could only get that to happen for real, you'd never have to lift a finger again.....