Why does my Tachometer's needle dance to the beat of my turn signal when it is activated?

How do you fix this horrible breach of electrical etiquette?

 

Voltage variations due to bad wiring and a very poor (cheapo) tach drive circuit.

 

Check the grounds. Tach ground, engine to body ground (rear of engine to firewall) and battery to body ground. Poor grounds will do strange things.

Marty

 

Can they be forgiven?

 

I have to agree with 73custom.. Had a truck a few years ago that the faster I went the dimmer the lights got and the further off the gauges were..Well 2 altenators later and lots of frustration, and old timer at a swap meet told me to check the ground on my cab...Regrounded cab and never had another prob.
Hope this helps.
Dan

 

The power fluctuations should not affect the tach drive circuits unless they are very poorly designed.