How to track down and cure instrument and sender unit faults
Incorrect dashboard readouts can be more than just a nuisance. They can cry wolf about low fuel and overheating – or they can keep schtum about these issues and lead to a long walk home.
Various systems are commonly found behind classic dashboards, and various things can go wrong with each of them. The good news is that faults are generally easy to track down and straightforward to put right. This feature shows you how to check the whole system from dashboard to sender and looks at many of the common problems you might find. It complements the feature in the February 2017 issue of Practical Classics, which demonstrated how to repair the instruments themselves.
Older Lucas systems use a ‘voltage stabiliser’ – a small box on the back of the instrument panel with two connections: B (battery) and I (instrument). This sends a steady 10V input to the instruments to iron out differences in the dynamo output voltage between tickover and higher engine speeds. Check the output voltage with the instruments connected as a first step when diagnosing wrong readings or if all the instruments are dead. The regulator must have a good earth (usually through the mounting screw). Use a multimeter to test it – don’t use a test lamp to spark-test it by earthing its output.
Most sender units have two connections – one to the gauge and one to earth (or it may be earthed through its fitting, bolted to the engine, etc). We refer to the ‘input from’ or ‘output of’ the sender in this feature. This isn’t scientifically correct, but it’s an easy concept to work with.
A wiring diagram can be a handy faultfinding aid.