Headlight Motor Operation
Part 2

by James K. Thorusen

   Here is the second part of my headlight motor control system article:

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 1/2 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 volts).

 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 it's 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 there 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.

 Good Luck,           Jim T.