Date registered: Feb 2010
Vehicle: 1972 280SEL daily driver. 904 Dark Blue with cognac interior. euro 4.5 engine
Location: Williamsburg, VA
Mentioned: 3 Post(s)
Quoted: 39 Post(s)
Relays come in two flavors, normally open, normally closed. A four prong is always a normally open. A five prong can be wired either way depending on which pins are connected.
On the bottom of the relay, the connections are numbered: 86 is power to the coil (the thing that turns the relay on/off) and 85 is coil ground. 30 is power to the relay for the component you want to control (fuel pump, lights, etc) and 87 and 87a are power out of the relay (actually goes to the conponent). 87 will have power when the relay is energized. 87a will have power when the relay is not energized.
You can use a 5 pin relay in a 4 pin slot because the 87a pin is not connected to anything in the harness.
You just have to make sure you are buying the right relay (round pins for the old MB relays and spades for modern relays).