2002 Freightliner fld120 12.7 Detroit got probalems low oil pressure

Bad sending unit it sounds like?

 

Put a test gauge on it and go from there. Don't rely on the dash gauge for diagnosing. If test gauge indicates low oil pressure, diagnose the engine lubrication circuit.

My guess is if it was fine before going into the shop and it got new main bearings and the oil pressure is low after getting it back, they could have put the wrong size bearings in. Sometimes during an out of frame overhaul they need to grind the crank. This means going to an undersize bearing. If the guy did his job and plastigauged the bearings he'd have know if the bearing clearances were good or not.

 

Was anything else done?
If not something is put together wrong or like AModel said. I saw a 3304 crank with one main turned .050 and if you put a standard bearing in it you would have low oil pressure.
Wondering why you had the work done?

 

You can't buy the crank bearings until you know what size to get.

Listen to @AModelCat

Even if you did not realize that, the mechanic should understand that.

 

Thing is unless you've owned the truck since day 1 you really have no way to know if the engine has had any major work like a reground crank (unless the owner who had it done and each successive owner was nice enough to pass that paperwork along). Not saying that is your issue 100% but typically when something goes to a shop ok and comes out with a possible issue it usually is because of the work that was done.

 

Is it a DDEC V? If so did he have the rocker shafts off of it?

I can tell you good oil pressure cold that falls off that much warm is an internal leak or too much clearance some where