The 1988 Prelude Si has the optional B20A5 DOHC EFI engine. The standard B20A3 2.0 L SOHC carburated engine was not used in the Si. So you can tell by the fuel system, which engine you have. The standard engine had Carbs and the Si engine had FI. B20A3 (12-valve, SOHC, dual side-draft carburetors) Found in: 1988-1991 Honda Prelude 2.0 S HorsePower: 104 @ 5800 (MT) 105 @ 5800 (AT) Torque: 111 @ 4000 B20A5 (16-valve, DOHC, PGM-Fuel Injected) Found in: 1988-1991 Honda Prelude 2.0Si Power: 135 hp
It's the "master relay" located under the dash. It clicks when you turn the key and you hear the fuel pump pressurize. Part no is usually RZ-XXXX and you can get em at the Honda dealer.
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.