| I have attempted to analyze the
headlight operating circuit, and, unless I have made an error (always a possibility), it
works like this:
Summary: The headlight
control system is an electronic logic system that has two inputs: one, the headlight
switch on the dashboard, and two: the cam-operated switch associated with each headlight
motor, which detects the actual position of the headlights.
+12 volts from the control lines will be applied to the motor
control relay near each headlight when there is a "disagreement" between the
physical position of the headlight mechanism and the position of the headlight switch (on
the dashboard). This causes the relay to pull in and the motor to operate until there is
"agreement" between the position of the headlight mechanism and the headlight
switch. When this is achieved, the +12 volts is removed from the relay by the cam-operated
switch until the headlight switch (on the dashboard) position changes.
Thus, the headlight position follows the position of the
headlight switch on the dashboard.
Detailed Theory of Operation:
The headlight switch itself contains a portion that is a DPDT
(Double Pole Double Throw) switch. The center contacts of this switch are connected to two
different sources of +12 volts (one switched with the ignition, and one "hot"
all the time. When the headlights are switched on, +12 volts from the ignition switch is
applied to the "on" contact of the headlight motor control portion of the
switch. There are two control lines running to the front trunk from the headlight switch.
Both have steering diodes in series with them. One is for the "extend" command,
and one is for the "retract" command. The +12 volt source from the
"on" headlight switch contact is routed to the front trunk via the
"extend" command control line and is connected to the cam-operated switch on the
headlight mechanism. When the headlights are in the stowed position, the cam operated
switch selects the "extend" command line from the "on" headlight
switch contact (energized when the dashboard headlight switch is in the "on"
position) to energize the relay. When the relay is energized, it connects +12 volts from
the battery via fuse S12 to the motor, causing it to run. The motor rotates in the same
direction to both raise and lower the headlights. The operating crank must rotate ½ turn
to do this, and is directly connected to the cam-operated switch. When the raise function
has been accomplished, the cam-operated switch disconnects the "extend" command
line +12 volts from the relay, and instead connects the relay to the "retract"
control line from the "off" contact on the headlight switch, which is not
energized while the lights are switched on.
This disconnection drops the relay and
removes power to the headlight operating motor as the headlights reach full extension.
When the headlight switch is turned off, it applies +12 volts (hot all the time) to the
"retract" control line. The "retract" control line is selected by the
cam-operated switch as the source for the relay control voltage when the headlights are in
the raised position, so the relay operates, causing the motor to run and retract the
headlight. When the headlight has reached full retraction, the cam-operated switch again
operates to select the "extend" control line (voltage not present when the
lights are switched off), which disconnects the relay from its source of control
voltage, causing it to drop out and stopping the headlight motor. In this position, the
headlight motor is now ready to repeat the cycle the next time the lights are turned
One other trick is employed: to ensure
that the motor does not overshoot the fully raised or stowed position of the headlight
mechanism, the relay pin 87a is connected to ground. This puts a short across the motor
when it is de-energized. A permanent magnet motor (which these are) can also act as a
generator, and if its output is shorted, it is equivalent to putting on a very
effective electric brake.
Basic Trouble-Shooting Procedures:
There are two main types of electrical
faults that can occur in the Headlight Motor Control System: Motor drive power loss and
control faults. There are other types of faults that will cause the headlight system to
fail, but these I would clasify as mechanical faults, i.e. bad motors, broken or jammed
mechanical drives, etc.
The first step is to determine in which
electrical system the problem is located. An important clue is whether both headlights are
affected, or just one. If both headlights are affected, the fault is highly unlikely to be
caused by a bad motor or a bad relay.... it most likely is in the part of the system
common to both headlights.
I. To make this determination, remove a
headlight motor control relay from its socket and set it aside. CAUTION!!! Be
sure that the relay has been removed from its socket before making this test!! If
you fail to do this, you will fry the relay!! If both headlights are affected, it does not
matter which one you choose for this test, otherwise, choose the defective one of course.
Now take a reasonably good size jumper or clip lead (#16 wire
or bigger) and short pins 87 and 30 of the SOCKET together. The motor should run and the
headlight should cycle repeatedly between up and down.
II. If this test operates the motor, then the problem is in the
control circuitry (go to paragraph IV). If not, while the short between pins 30 and 87 is
still present, measure the voltage on either pin. It should be +12 volts. If considerably
less or zero, then you have a supply problem.... check fuse S12 and associated wiring. Be
sure to spray the fuse spring clips with contact cleaner and rotate the fuse in its
clips a few times. European fuses are notorious for corroding into high-resistance
connections. Also, remove the fuse block and remove the wiring push-on terminals
associated with fuse S12 one at a time and spray the contact surfaces thoroughly with
contact cleaner. If the terminals are loose on their male flags, squeeze them slightly
with pliers to restore proper fit.
III. If there is +12 volts present, and the motor still does not run,
check the grounds, and if they are OK, then replace the motor.
IV. If the motor runs with terminals 30 and 87 of the relay socket
shorted, the problem is in the control circuitry. Remove the short between pins 30 and 87
of the relay socket, and then install the relay. CAUTION!! Be sure you remove the short
FIRST, and then install the relay... if you do it in reverse order, you can kiss the relay
V. Now, turn on the ignition and the lights. If the headlights do not
extend, measure the voltage between pin 85 of the relay and ground. There should be +12
volts here. If not, go to paragraph VI. If +12 volts is present on relay pin 85, check the
voltage on relay pin 86. It should be zero. If anything much above ½ volt or so, check
and clean the ground connections; this point is (or should be) directly connected to
ground. Once you have +12 volts across pins 85 and 86, the relay should be operated,
causing the headlight motor to drive the headlights to the extended position. If this
still does not take place, replace the relay. This should be the culprit.
VI. If you have no control voltage on the relay, then the problem is
further upstream. Using a long probe, or a short piece of bare wire, probe into the wiring
harness connector to the headlight system on the left side of the car, where the headlight
motor system connects to the car wiring harness.
Specifically, probe first the gray wire (extend control line).
If the headlights have been commanded on by the dashboard switch, and the ignition is on,
there should be +12 volts here. If there is, but not on the motor control relay pin 85,
then either there is a bad connection (corrosion, bad crimp) between the point you are
measuring and the motor, or you have a defective cam-operated switch in the motor.
Likewise, if the problem is that the lights have been
commanded off, and there is no retraction, probe the green / black wire (retract control
line). If there is +12 volts here, but not on the motor control relay pin 85, then either
there is a bad connection (corrosion, bad crimp) between the point you are measuring and
the motor, or you have a defective cam-operated switch in the motor.
VII. If you have no control voltage on the control lines, then the
problem is further back still, toward the headlight switch. Measure pins 56K (extend) and
30b (retract) of the headlight switch for +12 volts present when the switch is in the
appropriate position. If these voltages are present, there is a defect in the wiring
between the headlight switch and the front of the car. If the voltages are not present,
then go to paragraph VIII (missing extend +12 volts) or paragraph IX (missing retract +12
VIII. If the extend +12 volts is missing on pin 56K of the headlight
switch, check pin 15 of the switch. If the +12 volts is present here, then the headlight
switch is defective.
IX. If the retract +12 volts is missing on pin 30b of the headlight
switch, then check for +12 volts on pin 30. If the +12 volts is present here, then the
headlight switch is defective.
X. If the headlight overshoots its position, i.e. retracts and
then extends slightly, or extends and then retracts slightly, try changing out the
headlight motor control relay. Also check the ground on pin 87a of the relay socket.
These instructions were prepared from 74 and 75
wiring diagrams, but earlier years should be pretty close.
Some Notes on Relay Operation:
All of the relays in the 914 (the ubiquitous little round
black cans) are identical. They consist of a SPDT (Single Pole Double Throw) switch [
thats one set of form "C" contacts for you electronics buffs ], operated by an
electromagnet. There are a total of five connections to the innards. These are as follows:
- Pin 30 is the wiper, or center of the switch. It alternately connects to either of two
other pins, depending on whether or not the coil is energized.
- Pin 87a is the normally closed contact... that is, it is the pin that pin 30 is
connected to when the relay is in the de-energized, or relaxed condition.
- Pin 87 is the normally open contact.... that is, it is the pin that pin 30 is connected
to when the relay is in the energized, or operated condition.
- Pins 85 and 86 are the coil of the electromagnet which operates the switch.
Briefly, when their is no voltage across pins 85 and 86, pin
30 connects to pin 87a. When 12 volts is applied across the coil (pins 85 and 86) the
electromagnet is energized, operating the internal switch. This disconnects pin 30 from
pin 87a and connects it instead to pin 87.
Copyright Date of Origination by James K. Thorusen This material may be reproduced by
anyone without charge or notification.