After an exasperating time with the turn signals on my TF recently, I was asked to
describe what I had learned of the inner workings of the Turn Signal Relay Box. First let me say
that I am by no means an electrical expert, but rather a backyard mechanic who has been in the
backyard a little while, and someone who has always been hesitant at tracing more than the
obvious electrical gremlins. That said, here is what I’ve been able to figure out about the Relay
Box. If there are any experts out there who can correct or add to any of the info here, .... Please
do so, because we will all benefit!
If you’ve ever looked at the Moss catalogues you already know that the relay box is a
pricey piece to be purchasing, especially if with a little fiddling you could fix it yourself. Note that
the top of the box cover has the terminals labelled 1 to 8, and I’ll refer to the terminals labelled as
such in the same positions as the box cover. The top cover of the box is a press fit and can be
pried off with a thin screwdriver. Since it is metal, it is best to ensure that the ignition switch is
turned off, and possibly the battery disconnected as well, to prevent any unwanted shorts that
could prove expensive, while removing the top cover and while making any adjustments to the
points.
Underneath are two relays and 6 sets of contact points that separately operate the brake
lights, and the front and rear turn signal lights on both sides of the car. It is a rather ingenious
setup when you watch what goes on when the system is operating properly.
The relays themselves have one end of their electromagnetic windings soldered together to
the base of the relay box between terminals 4 and 5, and dictate that the box must be properly
grounded through its mounting bolts to the firewall in order to work. The other end of the relay
electromagnetic windings go to either terminal 4 and 5, depending on which side of the car that
relay is supposed to provide its switching. This is easy seen with the top cover removed and the
ignition switched on. Putting the turn signal switch to one side will make the relay for that side of
the car operate. In my TF, the relay on the left side of the relay box operates the lights on the
right side of the car, and vice versa. (I would have thought it would be the other way around!)
With both relays at rest (and the ignition on of course) the relay box allows the brake
lights to work on both sides of the car. Since each relay operates just one side of the car, if you
have one brake light that doesn’t operate, and you know that the bulb and the wiring to the relay
box, and appropriate other wiring to the brake pressure switch and so on is correct, the top
contact point at the end of the copper coloured reed for that side of the car is the culprit. It is
either corroded or out of alignment. Cleaning with spray contact cleaner, fine emery paper, or
such, should fix the contacts if they are dirty. Bending the top contact arm slightly that is between
terminals 1 and 8 (not the copper reed plate on top of the relay!) will properly align the points to
be closed at rest, and the brake lights should light. It is often hard to see the fine clearances
between the points, to tell if they are properly closed, and a multimeter is the sure way to tell here.
When the turn signal switch is turned to one side of the car, the corresponding relay for
the signal lights on that side of the car will pull down, breaking the contact for the brake light on
that side of the car (makes sense since you would want the signal light to flash, not stay on
continuously), and closing the two other contact points to the front and rear signal lights that are
on the bottom of those copper reeds. A close look at the copper contact reed of the relay, will
show the longest part of it holding the contact for the front turn signal light on its bottom, and
the smaller paddle-shaped contact for the rear turn signal light inside of the longest part of the
reed, also on its bottom. By careful cleaning and alignment of the contact terminals underneath
the copper reeds, you’ll be able to correct improper gaps. This is best done with the relay box out
of the car, where there is room to work without doing damage to the delicate parts, and again a
multimeter may be the way to go here.
A few more items to note.
Watch both relays as you cycle the turn signal switch to one side and then the other. When
activated, both reeds should pull down the same amount. One relay on my car didn’t drop as
much as the other, which pointed to part of the problems I was having with the turn signals on
that side of the car. Bending the terminal ends underneath the copper contact reed allowed me to
match the drop of both relays, and then fine tune the contacts from there.
With the relay box in the car and properly wired, put one end of a test light to ground.
With the ignition on and the turn signal switch turned to signal a left turn, the test probe should
flash at terminal 2 for the front left turn signal. At terminal 3 it should flash for the left rear turn
signal. Likewise, with the turn signal switch set for a right turn, terminal 7 should flash for the
right front turn signal, and terminal 6 should flash for the right rear turn signal.
After many trials I discovered that the settings of the various contacts are very critical. It
doesn’t take much bending one way or another to make them either work or not. Lifting or
pushing down on one part of the reeds is all that’s needed to get a light to flash, generally. From
there its lifting or bending down of the contacts on the terminal in question, but never attempt
to bend the reed! Make no adjustments to the reeds whatsoever. Patience is what’s needed when
tackling the relay box setup. I also found that one should carry needle nosed pliers and a small
screwdriver in the tool box, in order to make small adjustments. Having everything work properly
in the garage was not the same as discovering that another minor adjustment was needed during a
road trip.
Gene