My 1987 XJ12 recently encountered a problem with its tachometer. Each time I start the car, the tachometer won't work immediately but delays for a few seconds. After about five seconds, it will start to work properly.
Any of you experienced the same problem and/or know a solution of it? Thanks,

 

I've seen it a few times with both the Smiths and Veglia instruments used on the Series III cars but never bothered tracking down the defect.

On general principles I'd check the connections at both the engine end and the instrument end. If nothing stands out I'd have to assume the problem is in the instrument itself.

Hopefully others will chime in will something more concrete

Cheers
DD

 

where the tachometer signal from? the crank position sensor, or the distributor?

 

From the ignition amplifier, small box on LH side inlet manifold. Look for the white wire with spiral blue or blue/slate stripes.

No crank sensor on your model

Cheers
DD