Tach RPM - stopped working

[NJE]

I’m new to these forums so be gentle!
Yesterday I was going to go flying (Vans RV12) the aircraft is fitted with Skyview touch (SV-D700/SV-D1000) version 15.3.4.4867
From startup I kept get the verbal warning ‘engine speed’
The RPM remained stable but the arrow pointer was jumping all over the place. One minute it would be 2200 revs then 3000 the 700. Every time it jumped to either a high position (red) or low position (red) the verbal warning popped up.
There was no change in the actual engine revs throughout this.
When the engine warmed the dial turned green but the arrow pointer still jumped around and gave warning when it jumped to the higher or lower red sections.
I rebooted a number of times but no change.
I tried again today and it was the same, however after a few minutes the arrow pointer just jumped about a bit and has since then remained static in the lower red area.
Any ideas or suggestions would be welcomed.
Thanks

 

[Dynon]

When the engine is off, does it go to zero? Do you happen to know how much series resistance is between the Dynon and the trigger coil pickup?

 

[NJE]

Thanks for your reply.
When it was working even though it was jumping all over the place, it did return to zero. With the engine off.
However, now it just stays on zero when the engine is running.
Sorry, I can’t answer the second part of your question and have no idea how to check that.

 

[Dynon]

That's really sounding like an intermittent wire or connection somewhere that finally fully disconnected.

 

[NJE]

Thanks, I was thinking the same.
It’s going to be like finding a needle in a haystack, no idea where to start!

 

[Raymo]

Start by looking at the EMS wire mapping in the EFIS. That will tell you which pins the tach is tied to and you can then ring out the wires.

 

[NJE]

Wilco, thanks

 

[NJE]

The black earth wire had become detached further down the line from the connector. Re attached it and everything is now back working fine