What engine do you have in your1986 Honda Prelude 2.0si?
The 1986 Honda prelude 2.0 Si has a A20a3.
It should be under the dash and takes 5 mins to swap out, matushita RZ-XXXX it is called the "main relay" which does both the fuel pump and ignition. Assuming this is fuel injected. Just replaced mine in my 86 Si and it fixed problems with the pump not coming on all the time when starting. $70 from the dealer.