This is rather specific and the probability of a good answer is low.
I have recently been given a motherboard out of an old 386 with a 486 upgrade socket on it unfortunately if I insert a 486 processor it fails to recognise the chip or initialise at all.
I guess a jumper setting needs changing to make it work.
The board is labelled m359p80 and has an opti 82c495sx chipset the original 386 processor was an AMD 386-40. The board was made in thailand I guess in about 1993 according to the bios th only other clue to its origins is a label with tam/40-p8 marked on it.
If anyone can come up with a description of what settings the jumpers should be on then I'll be pretty amazed/grateful.