Ground control
QMy FSO 125p has a fuel gauge with a warning light. The other day I noticed that, unusually, the light started to flash while the gauge was showing half-full. I filled up as a precaution, but which should I believe?
Franciszek Landowski, Peterborough
Ed says:
AThe warning light is operated by two fixed contacts on the sender unit which touch when the float drops below a certain level. It's very simple and unlikely to be telling lies. The gauge, on the other hand, relies on a current flowing through it, the size of which varies according to the position of the rheostat in the tank sender. There are a number of things that can throw this offkilter. In general, the more current, the higher the reading – so a bad connection to the tank would be more likely to cause less current and an artificially-low reading. Your FSO is relatively unusual, though. Most dash gauges are simple current measurement devices with two terminals. Some European cars – including yours – have a gauge with a +12V and earth connection, then a middle terminal which goes to they sender. In this case the gauge can also be affected by a poor 12V connection (too-low reading) or a poor earth (too-high reading).