Speedo maxing out when ignition is on

[Jerry DC]

Hello everyone, this is my first post and I hope someone can point me in the right direction. I have a 1988 Carrera Coupe originally a US spec car which I have imported to Canada. I switched the speedo to one with KMS and it worked fine until I had some major electrical work done recently. The speedo worked for about a week after the repairs and then ‘maxed’ out whenever the ignition is on. I switched back to the original MPH one and the same thing is happening to this speedo after about a week of use. I am reluctant to have the speedos fixed as I fear the same problem will happen and I will have wasted money. The car did have cruise control which didn’t work so my workshop removed it. Before I send the car back in for a little more work, I wonder if anyone has had this problem and has a solution or a suggestion as to what my workshop should look for first. Any help will be gratefully accepted Thanks!