Tach Only Works to 3,500 RPM, why?

[603Racing]

The tach in my VDO cluster only works to 3,500 RPM. It works fine at any RPM below that, but when I rev past 3,500 it just stays at 3,500. Has anyone seen this issue? Is something like this usually caused by a bad tach? A bad board? Other? The car is a '90 325i, and it runs good at all RPM's.

I would like some input before I go buying a cluster or tach.

[603Racing]

I now this isn't a radio or subwoofer question but I thought at least one person would ahve an idea... :P