Jim Hinde
The topic for this session is relays:
how they work, how the factory used them in the 356, and popular
additional applications for them on the cars today.
The car on the lift at today's session has a starter relay kit
installed, and we will take a detailed look at it.
What is a relay, and how does it work?
A relay is an electrical switch that is operated by
an electromagnet. When the electromagnet's coil
receives current, the magnet pulls a spring-loaded hinged plate away
from its normal position, causing one or more pairs of electrical
contacts to open or close. These contacts are called
NC (normally closed) or NO (normally open) according to their position
when there is no current moving through the coil. A
relay can have only NC contacts, only NO contacts, or both types.
The photo below shows the inside of a 356B's light signal relay
(LSR), which has both NC and NO contacts. When its
coil is energized, the hinged plate is pulled downward by the
electromagnet, opening the NC contacts and closing the NO contacts.
The numbers shown on the terminals on the base of the
relay are the same ones that are stamped next to them on the base board.
The terminals on the relay are wired internally as
follows: 57 and 31 to the two ends of the coil, 30/56
to the hinged plate (which holds the lower contact of the NC pair and
the upper contact of the NO pair), 56a to the upper NC contact, and 56b
to the lower NO contact.
How did the factory use relays in the 356?
One use for relays in the 356 is to allow the state
of one circuit to control another circuit. An example
of this is the LSR shown above, which controls whether it is the high or
low beams that are activated when the turn signal lever is pulled toward
the driver in a T5 or T6 car, based on the position of the light switch.
In this case the first circuit includes the relay
coil and the light switch. When the light switch is
in the park position (pulled out one stop, parking lights on but
headlights off), power is supplied to the relay coil by a wire running
from terminal 57 on the light switch to terminal 57 on the relay.
When the light switch is in any other position, this wire is dead
and no current reaches the coil. Terminal 31 on the relay is connected
to ground.
The second circuit includes the signal/dimmer
switch, the relay contacts and the headlights.
Strictly speaking, it is one of two different circuits, depending on the
position of the contacts. When the lever is pulled to
flash the lights, power is supplied to relay terminal 30/56 by a wire
running from terminal 56a on the signal/dimmer switch.
If the relay is in the normal state—that is, if the lights are
off altogether or the headlights are on—the NC contacts remain closed
and the current flows through the NC contacts to the high beam lights
via a wire running from relay terminal 56a to the fuses for the high
beams. If the relay is activated—if the parking
lights alone are on—the NC contacts are opened, the NO contacts are
closed, and the current flows through the NO contacts to the low beam
lights via a wire running from relay terminal 56b to the fuses for the
low beams. (The rationale for making the lights behave
this way is complicated and makes sense only when you consider that the
car may be equipped with fog lights: the goal is to
prevent flashing of the headlights from cutting out the fog lights,
which are prevented—by, yes, another relay—from being on at the same
time as the high beams.)
A second reason to use a relay is to reduce the
amount of current running through a switch that controls a high-current
device. An example of this on the 356 is the horn relay,
where only the NO contacts are used.
The horn relay's coil is connected to the battery at one end and to the
horn button in the steering wheel on the other end.
When the horn button is pressed, the second end is connected to ground
and current flows through the relay coil. The relay's
NO contacts are connected to the battery on one side and the horns on
the other. When the relay coil gets power and the
contacts close, battery current flows to the horns.
The important thing here is that the current flowing through the horn
button is only the very small amount needed to power the electromagnet
in the relay, while the current flowing through the relay contacts and
the horns is many times greater. This saves wear and
tear on the horn button, and it serves as a model for latter-day add-on
applications.
The photo below shows the horn relay and LSR in
their factory location for a T5 356B, which is on the right-hand end of
the bulkhead behind the dashboard. In T6 cars the
relays are located in the trunk directly above the fuse block.
Why do present-day 356 owners install additional relays in their
cars?
Because the control switches on the 356 are
expensive to replace, because their internal contacts will deteriorate
more slowly over time if the current load on them is reduced, and
because worn contacts in a switch create resistance that will reduce the
amount of current that reaches the devices controlled by that switch, a
couple of strategically placed relays can pay for themselves many times
over in the long run and improve the performance of the electrical
system. The most popular applications are the
headlights and the starter solenoid. High quality
kits for both of these applications are available from Joe Leoni at
www.356electrics.com.
Headlight relay. As we
have seen, on 356B and later models, current for the headlights goes
through both the light switch and the turn signal/dimmer switch.
The latter go for over $200 when they are available at all.
If the original headlights have been replaced with halogen units,
the current is even higher, more than the switches were designed to
handle in the first place and therefore even more dangerous to their
long term health. The 356 Electrics headlight relay
kit includes two relays, one for the low beams and one for the high
beams, that are installed directly upstream from the two
pairs of headlight fuses. This has the advantage of
retaining the original fuse protection.
Many owners install relays in order to cure the
problem of dim headlights. If the reason for the dim
lights is resistance in the switches due to wear, installing the relays
will make the lights brighter. But dim headlights can
also be caused by other problems that installation of relays will not
solve, particularly by bad ground connections to the bulbs.
The ground for each headlight is a brown wire that emerges from
the wiring harness along the side wall of the spare tire well and is
attached to that wall with a screw and nut. If that
attachment does not make a good electrical connection, the headlight on
that side will be dimmer than it should be.
Starter relay. Two wires
power the starter: a big, fat one that runs directly
from the battery and a small on that runs from the ignition/starter
switch. The small wire carries power to the solenoid,
and when the starter is engaged this wire carries up to 30 amps, all of
which goes through the switch as well. With a relay
installed, the current through the switch is reduced to what is
necessary to power the relay coil, less than ½ amp.
Installation of the relay is simple, because the relay is mounted
directly on the starter and all the necessary electrical connections are
right at hand:
- One end of the relay coil is connected to the wire from the starter switch
which was originally connected to the solenoid.
- The other end of the relay coil is connected to ground (on the body
of the starter, which is grounded via the transmission case and the
ground strap connecting the transmission to the car chassis).
- One of the relay's (NO) contacts is connected to the solenoid
terminal that was originally connected to the wire from the starter switch.
- The other relay contact is connected to the battery via the fat
cable connection on the starter.
With the starter relay in place, when the starter switch is engaged the relay
coil receives current and the NO contacts close, connecting the solenoid directly
to the battery.