Tachometer and AC

I have an 85 300SD Turbo Deisel. For some reason, sometimes when I hit a bump, tach and ac compressor go off. This may last for a few feet, or many miles, no matter how bumpy the ride, then they will both come back on. They are never on or off independently of one another. I have heard that the 84 & 85 models had some kind of "electronic control box." Any input? Thanks.

[mbdoc]

Whenever M.B. builds a car w/computer they include overvoltage protection. On your car the O/V relay is in the fuse box. It could be loose or going bad, check it out!