Troubleshooting 1969-1973 Sequential Turn Signals

This guide covers troubleshooting 1969-1973 Mercury Cougar, 1969-1971 Thunderbird, and 1969-1970 Shelby Mustang sequential turn signals.

The turn signal system has four parts: the turn signal switch, (located in the steering column), a turn signal flasher can (located under the dash), an emergency (also located under the dash), and a turn signal sequential flasher (a black plastic case located in the driver's side fender well under the trunk mat in Cougars and Shelby Mustangs and behind the backseat in T-Birds). The most failure prone parts are the turn signal switch, the turn signal flasher can and the sequential flasher.

Basically, the system works as follows: when the directional lever on the turn signal switch is depressed, it completes circuits that selects and feeds power to the corresponding bank of lights. Power for the lights is fed from the fuse box to the flasher can, then through the turn signal switch, and finally to the sequencer and the inner tail lights. The front turn lights are fed directly from the turn signal switch. The sequencer has electronic switches corresponding to the center, and outboard tail lights. When the turn signal switch and the brakes are applied simultaneously, the sequencer and turn signal switch cause the turn signal to override the brake lights for the side chosen. When the turn signals are not used, applying the brake causes all the rear tail lights to turn on at the same time. Operating the emergency switch causes all the rear tail lights, the front, and dash indicator lights to flash together.

Now that you understand a little about how your turn signal system works, you can start troubleshooting. You will need a VOM (Volt-Ohm-Meter) and the circuit diagram for your year car. The most common complaints are: no turn signals, one or more lights on but not flashing, all lights flash together, and one or more lights flashing. Obviously, the first place to check is the fuse under the dash in the fuse box. The next item is to check all bulbs and their sockets. If these are all good, then now the real sleuthing must begin!

No turn signals:

When the turn signal lever is depressed, no lights come on anywhere. Start by depressing the emergency switch, if the emergency flashers work, the turn signal switch is most likely bad. Disconnect the turn signal switch from the harness under the dash next to the steering column and check it according to Table 1. The table shows wire pairs that should be shorted together when the switch is in the position indicated. If any of the connections are open, replace the switch. Usually the switch has failed when the plastic around the riveted contacts in the area around the emergency button appears to be burnt or melted. The switch has failed when one of the rivets has raised up about 1/16 of an inch from flush. This can only be seen with the steering wheel removed. For more information, review the Turn Signal Switch Diagnosis Guide

For the turn signals to work, you must have power as follows at the sequential flasher connectors in the trunk:

  Left turn signal operation:      Green-Orange (LH Inner light) AND Green-White (LH Front)
  Right turn signal operation:     Orange-Blue (RH Inner light)  AND White-Blue  (RH Front)

All lights on but not flashing:

This usually means that the turn signal flasher can is bad. This is a special unit designed specifically for the Ford sequencer and is no longer available from Ford. An electronic replacement is available. Note: If any lights are bad, no flashing will occur because the flasher can is calibrated for 4 lights (3 rear, 1 front).

All lights flash together (during turn signal operation:
If an electronic flasher has been installed, check the ground connection.

The special OEM turn signal flasher can may have been replaced with a common 552 flasher. The original flasher can (red or green, labeled "4 lamp sequential trigger") is a special unit designed specifically for the Ford sequencer and is no longer available from Ford. Use D0WY-13350-AR for incandescent bulbs and D0WY-13350-L for LEDs.
On 1970 and 1971 models with front/rear LED turn signals, verify that the LED Side Marker kits have been installed.

One or more lights not flashing:

If all bulbs are OK, the trouble is most likely in the sequential flasher unit in the trunk. Ford has obsoleted this part but an electronic replacement, C9WY-13A366-AL, is available from your parts dealer.

One side flashing only:

If only one side flashes, the turn signal switch in the column is most likely bad. Check the turn signal switch connector pins in both sides of the connector carefully for burned wires

Front lights only flashing:

If only the front turn signal lights flash, the turn signal switch in the column is most likely bad.

Inner lights only flashing:

If only the inner-most rear lights flash (and the front lights), use a tester to determine whether there is power available at the red harness connector on the Green-White (left turn signal on) and on the White-Blue (right turn signal on) wires. If both signals are OK, the sequential flasher unit in the trunk is most likely bad. Otherwise, the turn signal switch in the column is bad.

Outer light on both sides not flashing or flashing very dim:

If the inner and center lights are flashing correctly and the outer light is not flashing or flashing very dim (especially with the parking or headlights on), the flasher can (located under the dash) is turning off and on too fast. The lights should all stay on about .5 to .6 seconds. Replace the flasher can with a slower unit that will allow enough time for the outer light to turn on brightly. Use D0WY-13350-AR for incandescent bulbs and D0WY-13350-L for LEDs.

No brake lights:

For the brake lights to work, you must have power on three wires at the sequential flasher connectors in the trunk:

        Green-Orange (LH Inner light)
        Orange-Blue  (RH Inner light)
        Green        (Brake signal from brake light switch under the dash)

If there is no power on the Green wire, check the brake light switch under the dash attached to the brake pedal. One side is always hot, if not check for power at the the brake light circuit breaker.

If there is no power on the Green-Orange or Orange-Blue wire, check the turn signal switch connector under the dash. Make sure all the pins are seated into the connector shells and the both connector are fully seated together. If the connector is ok, the turn signal switch is probably bad. For more information, review the Turn Signal Switch Diagnosis Guide.

No brake lights on Right or Left side:

Check the turn signal switch connector under the dash - see previous section for details. The turn signal switch is most likely bad.

Table 1. 1969 - 1970 Turn signal switch connections

No emergency signals:

This problem can be caused by either a bad emergency flasher can or turn signal switch. Note: Many cars have the emergency flasher socket pins wired backwards. In this case, remove the female connector pins using a paper clip to depress the pin retaining tang while pulling it out, and swap the connector pins.

When replacing the turn signal switch, make sure the connections follow this diagram: 1969/1970 Turn signal switch connector pin diagrams

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69seqts.htm 08/28/24