Yanmar 3jh2-tce low oil pressure

[HueyHuey]

Hi just bought a new cat with twin yanmars, whilst delivering boat oil alarm sounded, turned motor off, checked oil, ok. Looked at forums which identified the sensor as a possibly faulty unit, tried to remove to swap but broke off unit, bogged up the end and sealed with emergency tape. Started again, then turned off motor and removed oil fill cover and stuck finger inside to check for oily film. All dry, so I assume the pump is not pumping oil. Did not use for remainder of delivery, now awaiting the mechanic next week maybe.
Does this sound like the oil pump is not working and if so how can I check it and or remove it? Where is it located?
I have no knowledge of diesels, but am quite compatent on outboards and motorcycles etc. If someone can point me in the right direction it would be greatly appreciated.
I would atleast like to know if it is the pump before the mechanic arrives, if atleast I look like I know abit about these motors I may avoid geting ripped off.
Also neither motor revs above 2500 rpm. And at 2500 rpm both smoke abit ( black).
No abnormal sounds, both seemed smooth etc.

[HueyHuey]

Sorted it out, swapped oil sensors all ok, ordered new oil sensor, oil seems to be pumping up to the valves. The turbo was seized, sat around too long unused and the fins got stuck with build up around them, once the housing was removed they spun freely. The elbows have alot of build up though, does anyone know how to clean these?