1980 FI Fiat Spider tachometer

[sigkadetm]

My tachometer only works when I accelerate???Goes down to zero at idle even though it is idling at 800 rpm.

[AriK]

Mine did exactly the opposite, zero at acceleration and performed ok at idle. I stumbled upon the problem a year later, loose ground at the battery terminal to chassis.

[sigkadetm]

The funny thing is it will work once the engine warms up.

[AriK]

Typical indicator of a bad connection.

[sigkadetm]

Where should I start???

[AriK]

The tach is sensitive to ground and resistance conditions. I'm sure it's something simple. Start with battery connections, make sure they're clean and test for tightness with a wrench, not just visually. Don't forget the cable connection on the battery box. Check fuse connections, sand them while you're at it and bend the fuse prongs so that they snug the fuses better. It's good practice to replace your 40 yr old fuses with new ones. Inspect the plug in connectors behind the dash and make sure the connections are clean and tight. This is regular maintenance for other electrical circuits as well.
Check the connectors at the ignition coil as one of those wires (not the pink one) picks up the tach reading. Follow the wire and test for breaks under the insulation.
Good practice for electrical reliabilty is to add a secondary ground cable from the engine to the chassis as i did (bottom center braided wire).

[sigkadetm]

Great thanks....