by Mark Bramfitt
Originally published in the Austin-Healey Magazine, December 1988
The Lucas electrical system has been the focus of jokes since the beginning of time. Most of the time, however, problems can be corrected through a little decoding of the Maintenance Manual and its mystical wiring diagram. Here, Mark Bramfitt shows how by talking us through the indicator systems.
Big Healeys up to early BJ8's have rather complicated turn indicator systems. Because these cars have single tail lights, an electrical control relay is fitted so that the indicator lights override the brake lights.
Because of the added complexity of the system, it is somewhat more difficult to determine the cause of faults when compared to the more common arrangement where the rear brake lamps and indicator lamps are in separate tail lights. When the turn signals stopped working on my BJ7, I flipped through my shop manual to the section covering the repair of the system. The troubleshooting procedure in the manual was not terribly clear to me, and reference to the wiring diagram for the car did not shed much light on the subject.
I found that to understand the operation of the system, I needed to draw a new wiring diagram for the indicator system alone. Using this diagram as a guide, I was able to systematically work through the wiring circuits to determine and repair the cause of the failure.
How the System Works
The indicator system shown in the diagram covers the brake light circuit, the turn indicator circuit, and the control relay that governs their operation.
Let's run through three separate operations of the system: operation of the brake light circuit alone, operation of the indicator circuit alone, and operation of both circuits in tandem.
Operation of the brake light circuit is very simple. A green wire runs from terminal 4 of the fuse unit to the brake switch. The brake switch is located at the brake line union on the right side of the car in the engine bay. Pressure in the brake system forces the switch closed. The other wire(green/purple) from the brake switch runs back to the left side of the engine bay to the control relay. The relay is located under the left fender, above the lever shock and behind the horn. The center contact in the control relay is now "hot". When the indicator circuit is not in operation, both contactor sets in the control relay are positioned against this center contact, allowing power to go to terminals 3 and 7 of the control relay. The right and left brake lamp filaments, respectively, are connected to these terminals.
Operation of the indicator system is a bit more complex. It's helpful to view the system as two circuits: one controlling the relay, and another accomplishing the actual "flashing" of the indicator lamps. Both circuits start at the same place as the brake light circuit: terminal 4 of the fuse block. Another green wire runs from here to terminal B of the flasher unit, and then on to the indicator switch. The wires to the indicator switch run up the steering shaft, starting at the steering gear housing at the front of the car.
Turning the indicator switch to the right or left allows power to run to terminals 4 and 8 respectively on the control relay. This activates the corresponding electromagnet in the relay, pulling the left or right contactor set over to the outer contacts. The right contactor set connects terminals 1 and 2 of the control relay; the left contactor set connects terminals 1 and 6. Once either contactor set is activated, the flasher circuit begins operation. Power from the fuse unit goes to the flasher, which allows power to continue through it in an on-off cycle to terminals L and P.
The on-off signal from terminal L goes to terminal 1 of the control relay, and then on to either terminal 2 or 6. Terminal 2 is connected to the right front flasher and to the right rear flasher element, causing these lights to flash on and off. Terminal 6 is connected to the left side lights. Activating either the left or right flashers also allows the on-off signal to go from terminal P of the flasher to the direction indicator warning light on the dashboard.
Now for operation of both circuits in tandem: When you brake the car while the indicator circuit is operating, the brake circuit is connected to one rear light and the indicator circuit is connected to the other. For example: activating the right indicator circuit allows the flasher signal to go to the right rear light, but because the brake circuit isn't connected to terminal 3, the right brake light filament is not energized. The left contactor set is not activated, so the brake circuit is connected to terminal 7 and the left brake light filament is energized.
Once you understand the operation of the system, it is easy to apply a logical troubleshooting process to any system failure.
There are really only two ways that something can go wrong: a component can fail, or a wire or connection can fail. You only need a test lead or two to check the system; I use a three foot section of insulated wire with alligator clips on both ends. Another shorter test lead is helpful, also. The test strategy is simply to go around wires, connections, or components using the test lead, until you get the system to operate. The wire or component you bypassed then must be the problem. The only trick is to determine the logical series of tests that will isolate the failure. Also, remember that the wiring diagram doesn't show connectors, and doesn't indicate where the components and wires are in the car (though the colors on the diagram do match the wires in the car, a small bit of help.)
Before listing the diagnostic procedures, a word about technique. A lot of failures are caused by poor or faulty connections, so when running the procedures, be sure to connect your test lead directly to the terminals when testing components, not to the wire connectors. When testing the indicator switch, you will have to make use of the connectors in front of the radiator. Now, turn the power on in the car and run the procedure for the type of failure you're experiencing.
Single Front Lamp Failure
If just one lamp fails to operate, check the lamp filament. If the lamp filament looks good, try the lamp in another place to make sure.
If you have a good lamp and it still doesn't work, check the terminals in the socket to make sure they are clean and coming in contact with the lamp base. Then test the ground wire by connecting the test lead to the grounded terminal of the lamp and a good bare metal ground. The last thing left to check is the wire back to the control relay (wires usually don't fail, but look for worn or burnt insulation that may allow the wire to ground out). Replace the wire if it's faulty, or thoroughly clean the connections to the terminals.
Single Rear Lamp Failure
If both the indicator and brake lamp filaments fail to operate on just one of the rear tail lights, run the procedure described above. Also check the connectors just below the fuse unit. There are also connector sets at the back of the tail light assemblies in the boot. If you have some type of wiring problem, some parts suppliers offer a replacement wiring harness just for the rear lights.
Brake Light Failure
If the brake light system fails, start at terminal 4 of the fuse unit and work towards the lamps, checking wires and components along the way.
At the fuse unit, check that the green power wire is firmly held in the screw terminal. If that is not the problem, connect your test lead to the terminal and then to the brake switch. This will indicate if the wire or connection at the brake switch is faulty. To test the brake switch, connect the two terminals with your short test lead. If the circuit works, you have a faulty switch. Spray the switch with penetrating oil and remove it. You might consider bleeding the brake system after installing a new switch. If the switch is not the problem, leave the short lead in place so that you don't have to get in and out of the car to test the system.
Check the brake wire from the brake switch to the control relay. To check the relay itself, connect terminal 5 to either terminal 7 or 3. If the relay is faulty, the left or right brake light respectively will light. Generally speaking, the control relay shouldn't fail on both sides. If one lamp works and the other doesn't, the corresponding contact set may need adjustment. If the relay test doesn't work, the only things left to check are the wires from terminals 7 and 3 to the lamps, the lamps and sockets, and the ground wires.
The commonest failures for the brake light system are brake switch failure or poor connections.
Some failures are easy to diagnose:
If the correct lamps light but don't flash, the flasher unit is defective.
If the direction indicator warning light on the dash doesn't work, check the lamp, socket, and wiring.
If any single lamp fails, check the lamp, socket, and ground wire.
If the lamps on one side fail to operate, you will need to check the indicator switch, the control relay, and the wiring from the control relay to the lamps.
If both the indicators and brake lights fail, check the fuse!
The more problematical failure is when the indicator system isn't working at all. Again, start at terminal 4 of the fuse block. Remember to turn the indicator switch to the left or right while running the checks. Is the power wire firmly screwed into the terminal? Check the power wire back to the flasher unit.
Check the flasher unit next, as it is easier to test than the indicator switch. Connect terminal B to terminal L; if the lamps work (they will light but not flash) the flasher unit is faulty and will need replacement. If you are not concerned with originality, you can buy a flasher unit with spade terminals at any auto supply store for four or five dollars. An original flasher with screw type terminals costs about twenty dollars.
The indicator switch is difficult to test because the contacts are a set of wire connectors in front of the radiator. Unfortunately, this switch is often the problem. Just to save time, you can isolate the switch as a source of the problem by running tests at the control relay.
Connect your test lead to terminal 4 of the fuse unit to terminal 8 or 4 of the control relay. If these tests cause the lights to flash, the indicator switch or wire connections are faulty. If this is the case, be sure to check the connector set. These connectors are often corroded as they are exposed to rain and mist coming through the grill. The wire insulation may also be damaged by heat from the radiator.
If the wires are good, then the indicator switch will have to be replaced. Because a new switch is very expensive, and because installation is rather involved, the procedure for replacing the switch won't be covered in this article.
We've checked the flasher and indicator switch; all that's left is the control relay. Connecting terminal 1 to terminals 2 or 6 of the control relay will reveal if there is a problem. Again, there is little chance of the relay failing completely; one contactor set or the other may be out of adjustment, or the ground may be bad. If the problem is the control relay, I recommend that you make every effort to adjust or repair it as new units cost anywhere from ninety to one hundred and forty dollars. The cover comes off of the unit easily, and the shop manuals have a good description of the adjustment procedure.
I hope you'll find it easier to diagnose problems in the indicator/brake light system using the wiring diagram and troubleshooting procedures. It's important to have the system working to ensure the safe operation of the car.
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