D21 TD27 Jump Tach -- does this look like a bad gauge or sender?

[RockoD21]

Hi you all -- My '94 with TD27 (nonturbo) has a really jump tach. It just never reads correctly, and it starts jumping when I turn on the ignition before I even crank the engine. I took a video of it here: Video of my Jumpy Tach

I've tried tracing from the sender to the back of the engine bay and the wiring seems ok -- I mean I wiggled the connections, etc, while the engine was running, and nothing changed. I am trying to decide whether to replace the sender, or to try to take the cluster apart and re-flow the solder joints in the tach. Anyone got any advice for me?

 

[tweak'e]

the simple fact its jumpy without engine running says its not a sensor issue. should be able to tell by unplugging the sensor.
my thoughts at the mo is its a cluster or power feed issue.

edit: check that there is no noisy electronics that turn on. electrical noise can get into the circuit and make weird gauges movements.
i don't have a manual for that early model.

 

[RockoD21]

Thanks tweak'e, your thoughts were a huge help! I will take out the gauge and take a look at the solder joints.

Much appreciated!