What do you mean by limp mode?
A diesel will run with fuel, air, compression. (No spark required). Electronics do very little with these engines. (The EDS will adjust idle but not required, can be adjusted at the injector pump.)
Electronics is another story.

What's the condition of the battery? Does the vehicle turn over quickly? Does the engine start run, can you press on the accelerator and keep the engine running?

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92 300D 2.5L OM602 OBK #59