You really dont need a multimeter, but it wont hurt to have one. The only other thing I can think of that it could be is the wiring or a plug between the starter button and the relay. The 2 small terminals on the relay need 12VDC to activate it. That takes a positive and a negative to give you 12VDC. When you hit the starter button that sends 12VDC + to the relay. 12VDC - should already be there possibly all the time. If you lose either the + or - that will keep the relay from engaging. Another test you can try with a meter is to ohm out the small terminals on the relay. These terminals should read as a "short" or connected, or if you have a meter that beeps when you have a low resistance, the meter should beep.