The gauge hasn’t got a clue...
The temperature gauge on the deluxe power panel connected to my Lister LPWS4 has started flapping about all over the place. I have tried shorting the sender, but the gauge did not go to high. Any idea of the cause?
These are normally bi-coil gauges and if the coil that is earthed (negative) gets an open circuit then the gauge will flick to full scale deflection. But while an intermittent earth fault may explain the flapping about, it does not explain why earthing the wire at the sender does not give full-scale
Adeflection at least some of the time. This suggests the problem may be in the positive side or, more likely, on the cable between gauge and sender. A multi-plug connector on the main engine wiring harness may have developed a loose or dirty connection.
If the wiring has no faults, you are left with a faulty gauge – save for one thing: many more modern Listers use a relay that provides a negative path when starting, rather than simply having an earth return. If this plays up (especially intermittently, as with a loose or dirty connection) it might give odd symptoms. However, I would expect this to cause starting problems before gauge problems.
So basically, check all the wiring and connections, and if all is well, change the gauge.