72 240Z Brake Light Wiring

[72SLOZ]

Troubleshooting electrical problems on older Z's is often an art. Schematics, such as the Haymes manual are good clues, but often not exact. I thought I'd post a help note for those trying to fix brake light problems as I recently did on my 72.

The circuit for the brake lights is more than just power though the brake switch at the pedal. The brake lights share function with the hazard switch as well as the individual left and right turn signals. Therefore, the circuitry involves routing through the hazard switch as well as the turn signal controller.

Power for the brake lights is direct from the battery (it is not gated by the ignition/accessory key) through a 20A fuse located at the bottom left of the fuse box under the center console access plate under the radio. This green/red (on my car) wire feeds the hazard flasher and hazard switch. In the "OFF" position for the hazard switch, the brake lights are directly controlled by the action of the switch at the brake pedal. That switch will CLOSE when the brake pedal is depressed and removes pressure from the switch (opposite of how you might imagine it). When in the OFF position, the hazard switch makes a simple connection from the incoming power (terminal 6) to another 'outbound' wire connected to terminal 7. This wire connects to the turn signal combination switch located within the plastic clamshell cover surrounding the steering wheel.

Turn Signal interaction

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When the turn signals are not engaged (left OR right), the brake 'power' is connected directly to the left and right brake lights through the combination switch. When either of the turn signals is 'ON' a different circuit is connected through the combination switch which allows the corresponding turn signal to flash while the OPPOSITE light is controlled by the brake 'state' - ON or OFF depending upon the brake pedal switch.

If you have a Haynes manual or other schematic, study the connection logic diagram to understand in more detail.

Other than the wires that make the run back to the actual turn/signal brake lights from the combination switch, most of the cicuitry is readily accessable from under the console.

Problem areas

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Obviously, 30+ year old switches are the most likely to have an issues (corrosion, wear and tear etc.) The brake switch at the pedal is the easiest to check. A simple ohm meter can be used to check the operation of the switch once you unplug it from the green/red wires than come FROM thehazard switch and TO the combination switch. The switch will be OPEN when the brake pedal is in it's non deployed state and closed (shorted) when the brake pedal is pressed. Remember, pushing the brake releases pressure from the brake switch mechanically, but closes the electrical circuit.

Hazard Switch

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This is where my problem was. In the 'OFF' state, this switch connects the brake switch to the combination switch. It was in this 'OFF' state that I had an intermitent connection and the brake lights did not work when the brake pedal was pressed. Corroded contacts to be fixed by replacing the whole switch. In the 'ON' state, the flasher switch controls the lights directly independently of how you have the brake pedal.

Combo Switch

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This is fairly accessable, once you remove the four screws holding the plastic clamshell cover at the steering wheel column. The green and yellow wire coming from the hazard switch (through a modulare connector) is easy to test for 12V when the brake pedal is depressed (with the hazard switch 'off'). If you are not getting 12V (or whatever the battery is putting out depending on it's charge state) at this terminal point on the turn signal when the brake is depressed, you problem lies 'upstream' from the combination switch. If you are getting battery voltage at this point when the brake pedal is depressed, your problem lies 'downstream' in the wiring that feeds the actual lights. This combo switch is a weak point in the Datsun design and gets a fair amount of 'abuse' from those that follow the law and actual use turn signals :paranoid: !

Next on the list of problem areas are the numerous pin in socket connectors used to connect up the various modules/connectors. Check that these are well seated and give them a light wire brussing on the male connector if you have such when you pull them apart.

Obvious, check that your bulbs are functioning before you start debugging the switches and wires. They can be checked by turning on the hazard switch (as well as the turn signals with the ignition 'on') If the lights are working when you turn on the hazard switch, all of the circuitry past the combo switch is in order.

Last on the list is the actual wiring. 30 year old copper can corrode, particularly if the insulation has been nicked or cut in any way. My experience is that wire itself is not too likely a culprit, but rather the connectors and switches.

The fuse controls the hazard switch, so if that works, the fuse is OK. If the hazard and brake switches do not work, then check the wiring between the fuse box and the hazard switch.

Your mileage may vary...