You would need to get a trouble shooting manual to see if you can follow where the signal comes from.
On my 2000 400 the gear for the speedometer was on the left hand side on the transmission, but it was mechanical where yours is digital so there could be a sensor down there with some wires that could of been damaged.
You might be able to look at a parts diagram from a online parts source and see if you can tell where the signal comes from.
I did a google search and came up with this video. The person doing the video is saying that the ground is lost on the wiring going to the speed sensor. They were saying that their problem was the same as yours and while is isn't the correct fix it may point you in the correct direction or if you try what they did it may be alright for you.
I should of also said that to fix it right you would need to obtain a electrical circuit diagram of the speed sensor to find where it gets its ground from and then correct that problem. You could also just follow that black wire from the connection and see just where it does go to to see if you can correct the problem.
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