Series 3 fuel gauge

1984 ex-army 109 canvas top. The fuel gauge reads full on both tanks as soon as the ignition is turned on. I was advised to get new fuel tank sender units but that made no difference. So I had the senders checked and they work okay according to the electrical resistance thingy they tested them with.
Then someone said it might be the voltage regulator but since the temperature gauge works and it apparently uses the same regulator, it can't be that.
Another pundit said to check the earth on the instrument panel but that must be okay because both the temperature gauge and the instrument lights work.
Another suggestion was that perhaps the gauge itself was faulty but I'm doubtful about that, too, when it reads empty with the ignition off and rises slowly to full as soon as I switch on. Or is there more to it than that?
Any suggestions will be very welcome. Thanks in advance.

On my 101 last week fuel gauge reading empty when ignition on...checked everything and it was v.reg so by passed it and it now reads ok...an earth fault will show a full tank on ign on...if you take supply of the sender with ign on does the gauge drop?...

if live wire from gauge to sender gets earthed before the sender, gauges will read full, switch over valve is a common place

Agreed, short to earth will read full. Could be rubbing on an edge where it passes under the bulkhead.

Shippers said:

On my 101 last week fuel gauge reading empty when ignition on...checked everything and it was v.reg so by passed it and it now reads ok...an earth fault will show a full tank on ign on...if you take supply of the sender with ign on does the gauge drop?...

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It does.

Then as James said probably earthing out between gauge and sender...

jamesmartin said:

if live wire from gauge to sender gets earthed before the sender, gauges will read full, switch over valve is a common place

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Same on both tanks so the changeover switch has got to be on the list. Thanks.

Shippers said:

Then as James said probably earthing out between gauge and sender...

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It's the same on both tanks, so at least we know where to start looking - where the two join, presumably.

Can be a pain to trace sometimes but at least you know the problem...good luck...