Is there a primary fuel pump on '86 300sd? If there is, how can I check if it works? thanks in advance.

If you're asking about a manual PRIMER pump, there isn't one on the 86-up Diesels. The manual says to crank for a minute or so until the engine starts. HA! Give your battery a break. Loosen the fittings at the pump or injectors and crank until fuel leaks out of each one. Tighten the ones that have fuel and continue to crank.

Sixto
91 300SE
87 300SDL
83 300SD