I'm sure the real experts will be along shortly, but until then...
If you have not downloaded the TM's (technical manuals) for the 802a you should do so, and spend a little time trying to read them. They're challenging until you get used to how the military wrote them, but they are excellent, through resources.
Have you ever had/seen this unit running before?
Is there oil in the set?
When you did short it to start, did you register any oil pressure on the gauge? The gauge runs on a different circuit than the one that triggers the warning light, so it's unlikely they'd both be bad.
I just went through a oil pressure light issue. Haven't actually figured out the root cause, but it was either a corroded contact in J&/P& connection, or it was the fault panel. Either way, the wiring harness burned as a result. You don't want that happening I assure you. Once I got that replaced, I found the fault light panel was also bad, so I had to replace that as well.