Short version, I want to test the Starter Relay, I know where that relay is, but I need to know what the relays next to it are so I know if there's one that is non-vital that I can swap it with for a quick check to see if that's the problem.

The below image is what I found detailing the position of the Starter Relay.


Long version. My friends 2001 S40 stopped running the other day. Her roommate had been driving it, came home to pick something up, went to leave again and it wouldn't start, wouldn't even try to turn over and no click from turning the key. Overnight the battery died even though nothing was left on, so first step was taking the battery to a parts store to see if it would hold a charge. The battery was shot so we replaced it but the car still wouldn't turn over. We were working on this between 10hr shifts for me and in the bitter cold, so after fruitless searching for the location of the Starter Relay we decided to revisit it saturday morning. My plans for tomorrow consist of listening to see if the fuel pump turns on with the ignition, if not I'm going to try replacing the Crankshaft Position Sensor as the most likely cause. If the Fuel Pump turns on I'm guessing it's most likely the Starter Relay, as there was no indication of the starter dying, just going from being fine one minute to nothing the next. Hence my question, I'd like to swap two fuses and see if it'll start then for the most time efficient check, but I need to know if either of the identical relays in that row are non-vital.

 

If you can't find what you are looking for you can try a screwdriver across the posts on the starter solenoid (or hook up a manual remote starter if have access to one). This would tell you if the problem is definitely not the starter motor.