Dash wiring

How to track down and cure in­stru­ment and sender unit faults

Practical Classics (UK) - - CONTENTS -

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.

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