An inexpensive hazard warning system and turn switch for any MG

TTT2 issue 13 Aug 2012 featured a panel which I constructed from parts from my deep spares bin to incorporate turn signal switches, a low oil pressure warning lamp and a buzzer. My TA has a loom with turn signals wiring and all lamps are LED. The system had limitations; the pull on push off left and right switches for turn signals were not very practical and the buzzer I used and the operating turn signals unit were not loud enough. (An electronic LED unit is normally silent in operation; units purposely made with a loud tick are available).

As an update, I have made another panel with hazard switch, turn switch, low oil pressure warning and a louder buzzer. This is screwed to the bottom of the inner dashboard.

The hazard warning switch is based on a Durite switch 00-484-50 which is push on push off with an illuminated red triangle. It has seven 6mm spade terminals, if required, a removable base is available.

The turn signals switch has three positions with the middle position as off as shown in the dashboard image, unfortunately not self-cancelling. The type of turn signals flasher unit used depends on whether LED or tungsten lamps are fitted. The tungsten lamps version works when the lamps load is applied to the flasher unit and some LED or electronic version run continuously if its 12v supply is present. The latter types are polarity sensitive. Some LED relays have no earth connection.

In the diagram in black ink the basic hazard warning switch circuit for LED lamps negative earth is shown. The circuit connects to a turn signals unit, to the left and right indicators lamps wiring on the vehicle, to a buzzer if required, and to an indicators ‘on’ lamp. A 12v feed via the ignition system and a 12v battery feed are used to enable hazard switch operation when the ignition is off. I am using the green 30mph lamp as a dashboard turn indicator as my TA speedometer does not have an outlet. Also, bright green LEDs are fed from Left and Right contacts on the turn switch located the top left and right of the turn switch via diodes for isolation. The series resistors used are 47K ohms for the green LEDs.

If a turn switch is added the additional wiring is in green. A flasher unit for tungsten lamps has a fairly loud tick but the electronic version is silent. A special electronic flasher unit with a loud tick is available. If a buzzer is used, further additional wiring is shown in red, the diodes prevent interaction. In this circuit the buzzer sounds when the turn signals are used but not when the hazard warning is operated.

For positive earth cars the diodes are reversed, the connections to the wired in left and right green LED indicators are reversed. The low oil pressure LED warning lamp connections are reversed. The buzzer connections are marked + and -. For LED vehicle indicator lamps, diodes rated at 1 amp are required and for filament lamps the rating is 4 amps. The correct polarity LED flasher unit should be used for the vehicle earth polarity.

The oil pressure switch is as described in TTT2 issue 13. As this switch (SW1) is open for no oil pressure and closes as the pressure rises, a transistor is used to effectively change this state to operate the lamp and buzzer when the pressure is low. The transistor type for the oil pressure circuit depends on the vehicle earth polarity, for -ve earth a TIP122 or similar npn transistor is used. For +ve earth TIP127 or similar pnp. Circuits for both are shown (in blue).

Hazard switch, Durite 00-484-50. £5.51 plus VAT and postage from AGM Component Solutions Ltd.

Base for hazard switch if required, connectors, wiring and buzzer from Vehicle Wiring Products 01159305454

The three position rotary selector switch with knob Item ID 151819788748 from eBay.

Also, transistors and diodes from eBay.

Indicators switches, positive and negative earth flashing indicators units, LED bulbs all bases, and buzzer units from:

Bob Butson

Any enquiries via email to: email(at) {Please substitute @ for (at)}