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By Charlie Nowlin


So far, In Review.......

The basis for relays, is the simple electromagnet. The simplest relay, is the Single Pole, Single Throw (spst) relay. It is nothing more than an electromagnetically controlled on-off switch. This is desireable because we can now use smaller diameter wires, to control the current flow through a much larger wire, and also to limit the wear and tear on the control switch.

Memorizing the contact terminal numbers, and their associated components, will aid in troubleshooting.

Double Pole Double Throw (DPDT)

This is the relay we have all become familiar with at one time or another.

Here, we see a second set of contacts, and some different terminal numbering,

81a, and 31b, are the coil, 56 is the input terminal,

56b, is the default position, and 56a, is the coil energized contact.

This is the wiring of the complete headlamp circuit, although simplified, showing one light for each side,..... typical in an S-III XJ6.

Notice that the entire current flow for the low beams, is routed through the light switch.

Not very efficient, IMHO, however it works. Upgrade available, bottom of this page


When the switch is "ON", current flows from battery, thru the fuse, to the switch, and directly through therelay's secondary contacts to the device, which in this case is the Low beams. So when ever the light is switched onthe low beams are on...

Almost!! There is one other contingency......and that is the dip switch. Because the type of relay that is used in our headlight system (Jaguar) is a Latching type, it will retain the last mode used. It does not default to the primary contacts on shutdown. So it stays where it was last used. If you have used the high beams, and shut down without dipping to low, when you next turn them on they will come on in the high beam mode.

The dip switch, energizes the coil , period. The magnetic properties of the coil attract the actuator, to which the contact is connected.

To understand how this works, one must visualize the working end of the relay.


In the first pic, its default position, connects terminal 56, and 56a. The dip switch is activated. The coil is energized, the actuator tip is working against the pivot. It flips the contacts, to the new position.

In the second pic, the dip switch released, terminals 56 and 56b, are in contact. The coil is de-energized, the actuator tip returns to it's rest position.

The next time the dip switch is activated, the actuator will ride down the ramp, and act on the pivot in the reverse direction, ready to connect terminals 56 and 56a again.

Photo of S III headlight relay, removed from within it's cover,

showing the actuator, coil, contacts, and latch mechanism described above

The picture below, compares the good relay (on the right), with one that had corroded/dirty contacts (left) causing high resistance on the high beam side of the circuit.

This led to the production of heat, which eventually melted the mechanism.


Photo by Mark Stephenson 11/25/2000

Mark writes:

TThe headlights totally died in the '85 XJ6. A quick check of various
components revealed that it was not the infamous bullet connectors, nor the
troublesome fuses, but instead it was the dip relay. We can not blame
Lucas(ifer) for this one. This is the silver Hella box that is bolted to the
left side wing (on Federal Spec cars) right behind the small 5-fuse box.

Putting my camera to the test, in macro mode, I have pictures in the Snaps

The problem I've had with three of the relays is that the plastic melts. in
the case of the two pictured together, the left one is completely melted, to
the point that the molten plastic began to drip before the relay died. With
the one on the right, the low beams functioned, but not the highs. I
discovered that the plastic had begun to melt on that one too, but not to
the same extent. It had, however, formed a bead that prevented the cardioid
pivot arm from touching the high beam contact -- right side contact in the

Using a utility knife I trimmed the now hardened plastic away from the
contact, however I still didn't have a contact. To understand why, a little
explanation of the operation of the dip relay is in order.

Pulling the dipper arm sends current to the large coil in the picture which
pulls the metal bar down. The hook at the end of the arm slides down the
point of the cardioid swing arm and strikes the bottom hard enough to flip
it to the other side, the other beam. Current flows into the switch on
terminal 56, the center one at the end with three. It travels up the center
contact in the end shot, which makes contact with a metal dimple (not to be
confused with a dimpled chad) at the pivot point of the swing arm. There are
contacts on either side of the lower part of the swing arm which will
contact one or the other of the vertical terminals on either side.

That little pin with the spring around it puts a little pressure behind the
swing arm on either side so the contact is solid.

Back to why it didn't work on the first try. The cosmic reason is that I
violated my usual rule of testing things before assembly. It looked like the
high beam side was making contact so I put the can back over the insides,
crimped it down, and put it back in the car. Then I tried it. The universe
smited me for being so cocksure of myself.

The physical universe reason it didn't work was that there was a little
melting around the center of the swing arm, so that when the arm swung to
the high beam side it caused the center contact to lift off the dimple
behind it.

I carefully trimmed away the extra thickness and compared the resistance
across each side. It was about .4 ohms on the low beam side, but higher and
fluctuating on the high side. Knowing that resistance = heat, I wanted as
good a connection as I had on the low side. After trimming it to the point
that I could see a gap between the center contact and the plastic, and still
not having a good, consistent contact, I decided that the spring on the pin
might be a little weak on the high beam side. That is why, if you look at
the picture, the contact on the right side is bent in slightly. That was
just enough to let the spring hold the contact against the post.

Moral of the story: Perhaps it would not be a bad idea to add to your
regular maintenance schedule, about every 50K miles, remove the relay, clean
the contacts (4 sets), and check the conductivity. Resistance in the
contacts *will* melt the plastic swing arm. Both the cars that have had
trouble have 130,000+ miles. The 39,000 mile '86 XJ6 hasn't had a problem.

So, if you happen to lose one complete set of beams, especially high beams,
the relay is the first place you should look. Bullet connectors, although
problematic, will generally cause you to lose one beam on one side or the
other, because each light has it's own wires. The fuses are also wired so
you won't lose all your lights if one blows. So that leaves you with either
simultaneous fuse or electrical failures or the relay, or if you're not so
lucky, the light or dipper switches.

I hope that helps, and I hope the pictures are worth at least a thousand
words each.

"Mark 1" Mark Stephenson
1952 XK120 Roadster #S673129 (w/XK140 engine and C head)
1958 3.4 Litre Saloon / 1984 XJ6 4.2L / 1985 XJ6 VDP / 1986 XJ6 VDP

Photo and text reproduced with author's permission


There is another type relay I will cover here. This is the start relay.

The start relay is a triple pole single throw (TPST)

Now before you get all shook up.....

this nomenclature, if you haven't yet figured it out, is nothing more than a description of the relay, and how it works.

A single pole means that there is one contact which to make contact with.

Double pole means that there are two contacts. Triple pole.... you get the idea..

Now for the throws....there is single throw which means the contact can be made or broken

Double throw, is either of two contacts can be made at one time. Think of it as a pendulum.It can swing in two directions, left and right, but it cannot swing both ways at the same time.So the same holds true with our relay that has a double throw.

There is yet a third type of relay, found in our Jags. This is the double pole double throw (DPDT) non latching type. It's operation, is to make contact with either one contact set or the other. It, by default, does return to the preset contact when power to the coil is removed. This Relay, has a terminal marked 87a., and one marked 30/61. The "Red" relay is one such device. Until the coil is energized, the relay provides an internal path between the terminals marked 87a, and 30/61.


This type of relay is also (sometimes) used to connect an alarm system. It can also control another , or a string of relays, but that is another topic.




My XJ6, had a 12 gauge wire feeding the switch / relay leg.

Now 12 gauge wire, is more than sufficient for two 50 watt lamps. However, I am upgrading to 7 inch H4 bulbs, and do not like the idea of all that current, running through the headlight switch. The headlamp switch contacts, are small, and have a high resistance, which means they'll get hot. With two 50 watt lamps, the current flowing through them is An addition of a 30 AMPERE $3.00 relay, will give my 17 year old switch some relief, from the increased current, and the higher resistance, and accompanying voltage drop of the small gauge wiring.

You can do the same. The new relay is wired in as shown.

I have replaced all wiring to the headlights with a 10 guage wire, and also added a fused 8 gauge cable from the battery, to the area near the radiator. This will provide a high current capacity, low resistance line, for the wiring of the newer relay's I will be adding.

The control circuits, have been left as is for now.

Did you memorize the terminal numbers? If not, ........go back, and read again,


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