I had an '80 TD and the valve 'box' cracked, allowing antifreeze to flood the electric motor and kill it.
Like the previous lister, I bypassed the box with the first thing that fell to hand, which was a 1/2" brass ball valve.
Pop the hood, and: Open in winter, closed in summer.. easy

I replaced the vacuum actuated valve on my Audi with a manual unit from the F.L.A.P.S. for about $5-$10. Same deal. Open in winter..
Best of luck.
Mark