Archive for the ‘automotive’ Category

Ford Focus Mk2 LED indicators

November 26, 2016

A bit of a technical summary of what you need to do to make them work correctly.

This all started after two things happened in quick succession:

1. Some smart-ass said my indicator was invisible during daylight.

2. An MOT assessor said the paint was peeling off the factory-fit filament lamps, and it may suggest an impending MOT failure next time around. The lamps have to be the correct colour, as well as flash at the correct speed.

The ECU (helpfully … or not) will flash the indicators too quickly if it suspects a lamp has failed. An LED lamp will not pull down the voltage on the indicator supply as much as a filament lamp. This is what the ECU detects, the voltage drop due to the low resistance of the filament lamp. There is no “flash relay” or “flash module” hidden in the vehicle.

To fool the ECU you need to provide some series impedance. The simplest way is to add a low value resistor between the indicator supply and ground. I started with an 8 ohm resistor per side. The resistor is rated at 50W, which is a little over-engineered for something you’ll use intermittently. The simplest way to implement this is in the wiring loom behind the headlamps.

[ If you have changed only the front or rear lamps, one 8 ohm resistor per side is sufficient. If you’ve changed front and rear lamps, you need a 4 ohm resistor per side. You can put two of 8 ohm resistors in parallel to achieve the same thing. ]

Remove the entire headlamp unit, and unplug that 10 way connector. It is a tight fit on to the socket, and there’s a latch to unhook (flat blade screwdriver usually works). The wires are clearly numbered in to the plug. Wire 3 (blue) is the indicator supply, and 6 (black) is ground. Connect the resistance across these wires. I used Scotchlok connectors, cheap and readily available from your local craplins.

Cocoon it in tape, and secure the resistors so they’re not flappin’ in the breeze, re-assemble and test:


The image shows one 8 ohm resistor. I added a second resistor in parallel because I did the LED upgrade rear first, then upgraded the front lamps later.