Can someone help me with tachometer of my 205, because the car injector pump rev limiter has been tampered by the i**** previous owner.

When I took over a tractor other day, the car has pause round 4,000rpm, it doesn't feel like is limited on right rev of 5,540rpm what my manual book said, I have a multimeter tool clip the positive red cable to the alternator signal and the black negative to the battery. Idle speed on cold and warm is prefect, but when I tried to raise the max rev, my multimeter screen number has disappeared after 2,000rpm. I could not check the max rev numbers. I was thought that my multimeter was faulty when I purchased a new and better one than the old, it is done the same numbers is disappeared after 2,000rpm.

So one more chance, bought a laser point the crank plate comes inaccurate speed of 960rpm! Tried the alternator reading of 540rpm, the multimeter is set accurate of 775rpm +5

Why is the multimeter numbers disappeared after 2,000?