Monday, October 22, 2007 8:08:48 AM
 

 Hazard and Signal Lights by Clifton Gordon

 

The signal lights on most British cars are also used as hazard/flasher lights. This sharing of function requires some special switching to change the lights from signal lights to hazard lights. Hazard lights should be available with the ignition switch in the off position but turn signal lights do not work unless the ignition switch is on. The hazard switch is used for both functions. It acts as a central distribution point for controlling how the lights will operate. We are going to discuss each light function on a block diagram level. Understanding how each component operates will help when you have to diagnose problems with the lighting systems. The diagrams I have prepared are based on later model MGB wiring.

 First here is a little information on British car wiring. Generally brown wires are live and will have a voltage on them anytime the car battery is connected. They are not normally fused. All voltage carrying wiring in a British car begins with brown wires. When the wiring is run through a switch or a fuse the wire colors change. The British car industry used universal wiring colors in their automotive industry. Black wires are ground/earth. Wires that have two colors have a basic color and a small tracer for the secondary color. Common colors make identifying circuits easy.  

Following is a brief description of turn signal operation. (See the turn signal diagram at the end of this article). Turn signal power is supplied by a green wire from the fuse box to the hazard switch. The green wire goes through hazard switch contacts to the turn signal flasher. The voltage up to this point is 12 volts. The flasher changes 12 volts to a voltage that varies from 12 to 0 volts. The flasher has contacts that open and close causing the signal lights to rapidly blink. A light green/brown wire goes from the flasher to the turn signal switch. When the turn signal switch is switched to turn left, the flashing voltage goes through switch contacts to a green/red wire. The green/red wire goes to the left turn lights and the dash indicator light. When the turn signal switch is switched to the right turn position, the flashing voltage goes through switch contacts to a green/white wire. The green/white wire goes to the right turn signals and the dash indicator light. Each light in the turn signal system must be grounded/earthed to function properly.  

For hazard lights operation see the Hazard light chart. When the hazard lights are turned on, several events must occur to convert the signal lights to hazard lights. The hazard switch has some internal slider contacts to do the switching.  

When in the “on” position the hazard switch disconnects the green wire contacts that supply power to the turn signal flasher. The hazard flasher has 12 volt power from a fused brown wire. The flasher converts 12 volt power to a varying flashing voltage. From the hazard flasher the flashing signal goes through a light green/brown wire to a terminal on the hazard flasher switch. The signal goes through contacts in the hazard switch to a switch terminal. From the terminal a light green/purple wire goes to the hazard flasher indicator light. A green/red wire on another switch terminal goes to the left turn wiring. A green/white wire on a third switch terminal goes to the right turn wiring. All signal lights are now connected to the hazard switch and the turn signal switch is bypassed.

 The last diagram is the hazard switch as found on most later MGB’s. The terminals are numbered and each connecting wire is identified by its color and function. Other MGs and British cars with hazard lights will have a similar switch, but there are several variations of the hazard switch. The hazard switch can cause problems in either system. The slider contacts may corrode and the original grease can harden. Sometimes rapidly switching the hazard switch on and off will temporarily solve the problem. The switch can be disassembled, cleaned and lubricated. There are some parts that can fly away so it’s best to disassemble the switch in a plastic bag to catch the parts. Use die-electric grease for contact lubrication.

 This article was written to give a general description of the operation of turn signal and hazard lights and isn’t intended to be a trouble shooting guide. However, knowing the general theory of signal and hazard light circuits should help isolate problems to a general area and simplify repair efforts.

 Safety Fast

 Cliff