I just replaced a bad altenatora and serpentine belt. When I took the truck for a test drive I had extremely low oil pressure at 20 on the gauge. The truck just shut off on me when I was doing 50mph.

This just started happening the same time my alternator went out.

I checked the oil when the truck was hot and cold. There was the proper level oil when cold and it was beyong the full mark when hot.

Any ideas what would cause low oil pressure like this and why my truck just shuts off when I am driving?

When cranking it was really bogged down but the truck did start and I think that might be because the battery is not fully charged yet.

 

Coulda broke the oil pressure switch

 

Yeah! the oil pressure swith may be the problem but take a look at the fuel pump or the clutch plate.The fuel pump creates a heck of problems when the trucks is old.And worn out clutch plate also creates problem but its managable.You have provided very less information about your truck.Need more information!!!!!!!