Jorge, The new PS-3 upgrade kits are different from each other and older PS-2 3V upgrade kits. So you need to be clear what you are working with. The reference post in the top of this forum have much information that can help.
For a Diesel PS-3 upgrade kit, it is a real PS-3 board just like in MTH production models. All LEDs and it comes with a 40 pin and 8 pin harness with all the necessary parts. This COULD NOT be used as a direct replacement for a PS-2 3V or 5V Diesel engine. RATHER, you would gut all electronics and lights and then use the kit to do a repair. The smoke unit, speaker (for 3V) and couplers can remain.
The PS-3 Steam Upgrade kit is not a true PS-3 Steam upgrade. Instead the PS-3 diesel board is used, but modified to install an adapter board that allows light bulbs to be used instead of LEDs. It is called a PS-32 board. This does use the original PS-2 3V harnesses.
This board is also the replacement board for PS-2 5V and in some cases PS-2 3V boards. But special flash code is required and for some engine it won't work because the special code is not released yet. For example GG-1 with operating pantographs, the COORS train, units with small single element smoke units, etc...
The PS-3 Steam kit is the closest to the old PS-2 3V upgrade kit. Everything the same except the new PS-32 board and no battery or battery harness.
The PS-3 Diesel is a completely new kit. G