Using the Relay Board with the PCBv3, what do terminals S1-S5 do?

You can use the schematics for the PCBv3 ECU and/or PCBv3.57 and the Relay board to trace it back. Here are the Relay Board schematics and here are the PCBv3 schematics and PCB v3.57 schematics.

Here’s the quick answer though mapping these terminals back to the corresponding pin on the PCBv3 ECU’s DB37:

S1 = 25
S2 = 27
S3 = 29
S4 = 31
S5 = 36

With the MS-I these were unused by default, available as spare inputs/outputs. With the MS-II and MS-III (legacy installation) S1-S4 are used for the stepper motor IAC, and S5 is the ignition output.

For more information on the Relay Board check out this page:  http://www.bgsoflex.com/mspower/mspower.html

Note you can do the same cross reference for the PCB2.2 ECU and the Relay Board, just use the PCB2.2 Schematics.  Though the pinout is of course the same…