Rev counter going haywire!

[MikeBradbury]

Just brought my imported 1981 FI Spider home after an extended trip in a container from Colorado and 8 months in storage in UK while bodywork stripped down and new paint job applied. Car looks great and runs fine, but takes a lot of turning over to start so, given the lengthy time in storage, I thought I would follow Brad Artigue's advice and give it the 'Italian Tune-Up'. Bit scary driving with the rev counter on the yellow line for extended periods, but I persevered. Filled the tank at the nearest petrol station and then put car in the garage. When idling I noticed the rev counter was going haywire - flickering around at random between 3,000 and 6,000 revs, even though the car was
idling at a steady 1,000 rpm.

Anyone seen this before? Any ideas what's causing it? Also, if there are any suggestions about how to improve starting, they would be well received. Maybe the two problems are related?

Mike

[Penny]

Because the tach is electronic, it gets the input from the pink wire going to the coil. First things first, I’d inspect it for being clean and tight and that the grounding post for the tach is also clean and tight. Naturally, this goes for all of the car’ contacts too,.. this may help your starting issue too.

[MikeBradbury]

Thanks for the tip. I'll start checking the wires and the security of the fixings. Funny thing, though - I haven't been able to recreate the problem. Tacho now behaving perfectly. Maybe the Italian tune up drove the electronics crazy!

[spider2081]

it gets the input from the pink wire going to the coil.

I'm thinking the pink wire on the coil is connected to the B+ terminal and is switched power from the ignition switch.
The tachometer signal is connected to the D or - terminal of the coil and is often a brown/white wire. There are 2 wires on this terminal one is for the Fuel injection ECU and the other is for the tachometer.

[GeorgeT]

I had something like that when the battery developed a bad cell in my '75. Replaced the battery and all was good.