I had the same problem with an 84 300D's CCU. It's probably a failing CCU unit. I'd try to re-flow it, as others have stated, adding solder where needed. If that doesn't work, you might want to look into a rebuilt / remanufactured unit off Ebay - one can be had for under $300.

Once you get it working, I'd also highly suggest putting a 1A fuse in-line with the auxiliary water pump. When those things blow, they have a nasty tendency of taking the CCU out with it.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]