Speedometer needle issue

[NumZie]

I have 912E from 1976. It's a 912 by name, 911 by chassis and many parts and a 914 by engine and some parts, so hard to post the right place.

My speedometer acts wird.
When I drive and then hold still, it shows almost 30 mph.

And when I accelerate or keep a steady pace, the needle is 'jumping' up and down around the current mph. It's really annoying.

As I have been told, the car does not have a cable to trigger the speedometer but an electronic impulse sender instead.

What could be wrong and where should i start?

Both a new used speedometer as well as the impulse sender is quite expensive, so I will try to locate the error before buying parts.

[DRACO A5OG]

Most likely a dirty ground or failing sensor at the gear box. But check the cheap stuff first.

Pull the gauge and check, clean and insure a tight fit. if there is an intermediate connection check, clean and insure a tight fit.

Check the sensor is clean and no obstructions at the mount.

[NumZie]

Thanks.
A ground wire had jumped out and was burried in the wire chaos