Hazard telltale light

I'm using LED indicator bulbs and side lights throughout the car along with an LED flasher relay. I found that when the telltale light was fitted to the inside of the standard sierra hazard switch, the LED indicators didn't flash properly. Instead of blink on then off, they blinked on then half off. I tried an LED version of the tell tale bulb and the result was the same. Reading up on similar issues, I'd seen that diodes are normally installed to prevent electrical feedback although I couldn't figure out exactly where to install them! To try and work out what was going on inside the hazard switch, I took the cover off the switch and checked the terminals with a multimeter. With the switch on, I found that the two contacts nearest the steering wheel switched 12v on and off with the relay, and the other two gave a constant 12v live.

Therefore, if I could get one of the LED bulb's contacts connected to earth, the bulb will work correctly with the other contact connected to one of the switching 12v terminals. To put this into practice, a wire was soldered onto one of the bulbs contacts and then this was covered with heat shrink so that it wouldn't come into contact with the 12v terminal inside the switch.


This wire was then routed out of the back of the switch and connected to earth. The bulb was then pushed into the switch for a test and I'm pleased that this now works with no adverse effects to the indicators.