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Basic Car Electrics for Beginners Part 10:
Relays on Corsa's

The Idea of a relay is to reduce arching on switches and to carry more current than thin wires are designed for,
and reduce voltage drops, there are 2 normal types of relays on a Corsa broadly speaking, there probably are
more, but there is the cubical one used for Cooling Fan and Fuel pump, stuff like that, then there is the small
brown one used for horn and wipers, probably heated rear window and heater blower, that sort of stuff, most
of these are mounted in the front fuse box, on my year 2000 , 1 litre, 3 cylinder there under the bonnet on n/s
front, the relays have 4 terminals on them, normally they are wired like this, terminal 30 main battery supply
for the switch contacts, 87 goes to the consumer unit, like the main beam head light's or fuel pump etc,
86 is usually live and 85 is Earth, now obviously 85 or 86 has to have a switch connected in it as they go across
the operating winding inside that activates the switch contacts..So it depends if you want to use it as live
side switched or ground side switched...

The Layout of the terminals is obviously different for the two different relays....the resistance of the winding across
85 and 86 is about 80 ohms, the relay can be tested by putting 12 volts across 85 and 86 and listening to it click, and at
same time put ohm meter across 30 and 87 and check for continuity, or make up a simple bulb circuit and see if
30 and 87 switched the bulb on when energized....In certain cases to test circuits, certain terminals on the base of
the relay can be by passed, for instance jump across 30 and 87 on the fuel pump relay and the pump should
run all the time, other tests you can do are check for lives and earths on the relay socket, a wiring diagram would
be handy when doing this....
From Fred in Essex 2016
 
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