Ta­cho prob­lem

Car Mechanics (UK) - - Help! -

My stan­dard Smiths tachome­ter stopped work­ing af­ter fit­ting an elec­tronic ig­ni­tion dis­trib­u­tor on a 1969 MGB. It worked OK be­fore this and I’ve tried fit­ting an ear­lier ta­cho with the loop wire at the back in the pos­i­tive feed to the coil, to no avail.

Richard Carr

The orig­i­nal volt­age-sens­ing tachome­ter is con­trolled by the coil and the pulse is not suf­fi­ciently strong to trig­ger the cur­rentsens­ing unit with the elec­tronic ig­ni­tion dis­trib­u­tor. The ear­lier unit may op­er­ate, but due to the low sig­nal used by the dis­trib­u­tor, the loop wire should be moved from the SW (+) side of the coil to the CB (–) side. You should also en­sure that the earth con­tacts are clean. Of course, this is as­sum­ing the tachome­ter is work­ing cor­rectly as they do have a built-in ca­pac­i­tor that’s prone to fail­ure.

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