I've seen a number of threads about fuel gauge issues but not this exact one I'm experiencing. Please forgive me if my forum searching skills are lacking.

I have, from what I can tell, a standard tank in my '74. When the fuel gauge shows empty, I can add about 10 gallons. The gauge then reads full. Any clues what could be wrong?

 

Have you checked out the Willcox tech help site? http://repairs.willcoxcorvette.com/?yt=&s=fuel+gauge

When you say it reads full, is it right at full, or does it appear that the needle is pegged at the full stop (a ways past the full mark)? If you have a good quality meter, the tank sender should read around 90 ohms (between the sender output and ground) when the tank is full, and down near zero when it's empty.

 

Perhaps your fuel tank is not really empty when the gauge indicates that it is. It's very possible to have a fuel sending unit that operates correctly in the upper part of the range, but reads empty (zero ohms) at any point with less fuel.

 

Thanks. My gauge reads exactly full, not past it like the examples on the Willcox site. I'll get my tester out and verify. I'm assuming at this point that it's something with the sender.

 

 

The sending unit just offers a resistance measurement to the fuel gauge. Full reading should be around 90 ohms; empty reading 0 ohms. If you let your fuel get below 1/4 tank, you can test the sending unit out by removing the fuel cap and using a stiff wire shaped with a J-hook on the end to lift/lower the float while reading the output resistance of the sending unit. To do that, remove the signal wire from the sending unit (at fuel tank) and connect ohmmeter (+) lead to that terminal on the sending unit and the (-) wire to chassis ground. The ignition does not need to be ON for this resistance check to be done.

LIft float to upper limit (it will reach its upper limit and you should not pull excessively when that happens) and you should read about 90 ohms. Gradually drop the float while watching the ohmmeter. The reading should reduce smoothly, without any jumping on the dial (or readout). It should do that all the way to 0 ohms. If my hunch is correct, yours will drop to about 45 ohms, then stay at that reading, even if you remove the hooked wire.

If you find that it drops to some ohms reading, but thereafter the float will not longer go any lower, that could also be your problem. Some of these aftermarket parts can have defects, such that the float binds up at a spot or the resistance wire has damage to it.t setting up these senders with the correct amount of bend in the float arm.

Good luck with your diagnosis. Please report back on what you find. Your info may just help someone else with a similar problem.

 

I have to disagree - if it stayed at 45 ohms, then the gauge would never read below a half tank. C3Newb stated that he is reading 'empty' on the gauge, but the tank is still half full (he stated he can only put in 10 gallons at that time). This means that for some reason, the gauge is seeing zero ohms from the sender before it should. Either the float isn't floating (unlikely, since it does read full when tank is full), or it's just defective. Or perhaps the sender just has a bad spot where it is reading 0 ohms - if you reach in with a (clean) stick to press the float down slightly further, you might see the gauge actually go up if there is just a bad spot on the sender.

 

bradleyb66..........

You can disagree all you want, but there are several failure modes with that sending unit system. I didn't list them all in my post; I just stated that manufacturing/assembly problems could cause the PO's problem. The fault is most likely due to a problem with the sending unit. The question is whether it is damaged, or whether it is just poorly adjusted. Testing, as mentioned, will determine what is wrong and how to best achieve a solution.

But, thanks for sharing your negative opinion...of my opinion.

 

Sorry, but it wasn't a negative opinion of your opinion, just disagreeing that the sender would be reading 45 ohms while the gauge showed empty? As you stated, if tank is full, the sender should read 90 ohms and gauge will show full. If the tank is physically half full, then sender will read 45 ohms, and gauge will show half tank. If the tank is empty, sender will read 0 ohms, and gauge will read empty. Since OP's gauge IS reading empty (but with a tank that is not empty), it makes sense that the sender is probably reading 0 ohms, and not 45 ohms?

I agree that the sender is probably bad.

 

All that is true....IF the sender is working properly. Apparently, it is not.

 

 

True.......this is what happened to mine at one time. Erratic reading, didn't make sense. It seems that resistor can go haywire several different ways. A new sending unit solved the problem.

 

I thought the OP already indicated in his first post that the tank wasn't empty when the gauge reads empty? He said he could only put 10 gallons in it (so it's approximately half full at that time, but the gauge reads empty). The way I read it - the tank is physically half full, but the gauge is reading empty. Perhaps C3Newb can clarify if the tank is physically full or not after adding those 10 gallons?

7T1vette noted above exactly what I think is happening (operating correctly only in the upper range), I just couldn't understand why you later posted that you thought the sender would stay at 45 ohms when moving it lower by hand - maybe you meant to say that it would go to 45 ohms, then drop to zero for the remainder of the travel?