There are three major known issues with the 90+ C4 instrument pod.
1. Poor day time illumination. This is a lighting issue caused by the failure of the photo cell as discussed above.
2. Fading of the central lcd display. This is caused by failure of the lcd display itself and can result in the display characters virtually disappearing. This is the issue the second OP likely has.
3. Incorrect tach readings. Sometimes the tach needle will peg itself to the top or bottom of the display but generally just a noticeably inaccurate tach. Caused by failure of the control chip for the tach.
As noted polarized sunglasses can have an affect similar to #1 and 2. However that is easily eliminated by simply elvaluating the display without wearing them.
These items can all be resolved by the company Paul noted above. They can either do the work for you or sell you the parts to do it yourself.
H
