Relays

What is a relay ?

A relay (also known as electro magnetic switch) puts into contact two circuits : it is a system of two independent circuits inside a box. Inside a relay there is a coiled wire which, when it is prompted by electricity, produces a magnetic field and this magnetic field pulls magnetically a set of contacts that make the current pass in the other circuit.
In general, relays are used to connect a low amperage circuit (using a wire with a small gauge) and a hight amperage circuit. This is very well explained in this article "Relay Basics" by Chet Walters.
The coil or the contacts can fail inside a relay.
One way of testing a faulty relay is, while the relay is still in place, to squeeze manually the relay in order to make the contact work and see if the device operating after the relay works properly.
If the test is positive so the device itself is not faulty, but the relay

There are four parts in a relay (low amperage coil circuit and hight amperage load circuit):

• Power to the coil
• Ground to the coil
• Power to the load
• Ground to the load

What happens when you activate a relay ?
On the first circuit, when you (1) switch the horn button, you activate an electric field into the coil (2) and this shuts down the relay switch on the second circuit (3), allowing the horn to blow (4).

Screen shot from the video by Daniel Sullivan on Youtube

Testing a relay with a 9V battery
It is possible to check whether a relay clicks or not using a 9V battery.

Testing the entire system from a relay

You see that from a relay, the two circuits are pretty inaccessible but they can be diagnosed.
There are two voltage readings and two ohms readings.

Pin number	Measure
#30	12V
#85	12V
#87	Ohm value of the device
#86	0 Ohm when closed / 1 when open

How many on a RR Silver Shadow 1, where are they located ?...

Depending on the car model there are 6 to 10 relays and they are

From RR forum Australia
I have several lethargic devices -- front passenger window, front passenger door lock, windshield wipers, and others.
I think the common element is a relay and I suspect that I am losing voltage due to worn or coated contacts.

How do I identifiy the relays ?

A first method consists in reading the relay section of the electrical manual (M10). Here you'll find the types, location and function of the relays
housed under the relay bank which is inside the bonnet just facing the driver.

For those relays which are not listed in this document, one must use the wiring diagram (for RH car > VIN 13681 see page 274 of the Wiring Diagram part of the Workshop Manuals).
And on this diagram, check for the colours of the wires that are feeding the relay.

Here for example you read that the headlamps security relay is feeded on the low amperage side by a UWP (Blue White Purple) wire
and on the hight amperage circuit by a BWP wire on the ground side and a URP (Blue Red Purple) wire on the load side towards the lamps.