Tank indicates halfway when empty

My son's 1999 D90 TD5 has had an irritating fault since he bought it. The fuel gauge shows full correctly and reduces linearly as fuel is burnt, but only gets as far as half full when the tank is empty. After this caught him out the first time, he is careful to fill up in good time, carries spare fuel and is frankly not bothered by it. Given that the pump is otherwise fine and that getting at it in a Defender is not straightforward, if this is a sender failure then ignoring it until the pump needs changing anyway seems reasnable.

However, the engineer part of my brain hates having an unfixed fault. So while I don't intend to persuade him to empty and drop the tank to let us access the pump, I do want to check that there is not an alternative solution, so I thought I'd open the symptoms to the experience and wisdom on the forum.

If the sender unit were not free to move properly but sticking at halfway, then the truck would still have half a tank when it first reads half and continue to indicate that until empty. In fact when it reaches halfway it is pretty much empty. I have searched here and looked through RAVE and there does not seem to be an intermediate voltage regulator between the sender and the gauge, so if I check the gauge and it is OK I can blame the sender and continue to ignore it. Is there a simple way to test the gauge?

Does the low fuel light come on also?

Good question, apparently not. As it is seperate to the gauge I guess that means that two discrete outputs are showing the fault, which suggests the problem is at the sender. Now I just need him to check that the low fuel light illuminates before start (he didn't know it had one), and I have a diagnosis.
Cheers nobber.

I'm not familiar with them, to me if the fuel lamp doesn't come on that would suggest a sticking float. Fuel lamp on suggests a problem with the gauge or windings.

Either way you will still have to get in there to test it. You could drain the fuel and put a meter across the sender unit.

nobber said:

Either way you will still have to get in there to test it. You could drain the fuel and put a meter across the sender unit.

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@nobber if the OP measured the ohm reading when guage at full then at half would that not indicate a stuck float with no need to drain tank not sure of ohm rating of defender but they're normally around 200-30 but the reading at full would give starting point..

No, you would have to measure at full, half full and empty. We know it works from half to full.

Shippers, that sounds like it is worth a go. Does anyone know what the relative resistances should be, or where I could find them?

The low fuel warning light is a red herring as when I spoke to Jonny this morning he made the point that he judges when to fill up on mileage and he always does so with about 15 litres remaining in the tank. Therefore the warning light would never come on as it does not get activated until about 9 litres remain.

nobber said:

No, you would have to measure at full, half full and empty. We know it works from half to full.

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That would be true if the gauge was acting normally between full and half and then not moving as the second half of the tank drained. Unfortunately when the gauge gets to half the tank is almost empty. Hence why this does not sound like a classic sticking float.
If I knew what the readiings should be from the sender I could compare with the observed readings and that would tell me if the sender was sending crap signals or the gauge was misreading them.

nobber said:

No, you would have to measure at full, half full and empty. We know it works from half to full.

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I read it as ...full on guage full of fuel...half on gauge empty of fuel...therefore if reading low ohm on half full float not stuck ,but if reading mid ohm range stuck at mid way...a reading at 3/4 full would therfore if float was stuck read mid ohm value I.e a half full tank of fuel...might be reading it wrong though but had the same on the 101...

Shippers said:

I read it as ...full on guage full of fuel...half on gauge empty of fuel...therefore if reading low ohm on half full float not stuck ,but if reading mid ohm range stuck at mid way...a reading at 3/4 full would therfore if float was stuck read mid ohm value I.e a half full tank of fuel...might be reading it wrong though but had the same on the 101...

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Correct. The issue is, does the sender deliver the correct range of readings (gauge faulty) or does it only deliver half the required range (sender faulty).
I just need to know what the full range should be.