What is a relay Relays control the power supply to the various electrical ancillaries, specifically those that draw larger currents. The obvious benefit of using a relay, an electrically operated switch, is it allows a high current circuit to be controlled by an isolated, low current circuit.
This enables all the wiring handling the highest currents to be located within the engine bay and controlled by low current wiring routed from the dash area. Removing the high current wiring from the dash reduces the potential fire hazard. Source: E-type restoration project (great site)
Relay types on the Silver Shadow Relays on the SSI can be either, 3, 4 or 5 pins relays.
Each relay is labeled on its cover with a 5 digit number eg 33222.
On the underside of the relay you find that the pins (terminals) are labeled C1,C2,C3,C4 W1,W2 depending on the number of pins.
Relays are fully described in electricity section the Workshop Manual by several characteristics among those : Lucas part number :the 5 digit code Contact arrangement : this describes if the relay is "normally open" or "normally" closed, when not energized. This information is very important for testing a relay. Key to the terminal markings : this is exactly what you can see on the underside of the relay Duty rating : shows if the relay is triggered by a switch (manually operated) or not. The second case applies for safety relays that are energized in the case of a fuse blow for example. Cut-in voltage : Drop-off : Source: Diverse
How does it work ? Relays are made of basically 3 elements : a coil winding, a moving contact and a fixed contact.
In the energised state the coil produces a magnetic field which draws a sprung, iron armature towards the iron core. In doing so a contact at the end of the armature (on Lucas : C2 terminal) makes a connection with a fixed contact, the C1 terminal or the C1 & C4 terminals for the double contact relay.
A relay map of the Shadow Here is a precious thread listing where are the various relays.
http://au.rrforums.net/cgi-bin/forum/discus.pl?pg=prev&topic=17001&page=15292 Source: The Rolls-Royce Forum - Australia