When I found an economy gauge from a 1985 280CE at the local pick-a-part, I did the same thing. I tee-d into the transmission vacuum line and it functions like the gauge on a gasser. Gassers produce about twice as much vacuum at idle than what's recorded at the VCV on diesels, so the gauge only reads about halfway (that registers about 12-13 in.Hg vac) at idle. It'll drop as you give it more foot, but it's not good for anything besides eye candy and another moving gismo in the cluster.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.