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Can I use LED bulbs on a 2006 harrier or does it do that thing where the indicators flash fast if the resistance isn't right to indicate when a bulb has failed?

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You can either fit load resistors to ensure they don't hyperflash or you can buy a proper LED flasher unit so that you don't need the load resistors. Have a look at this topic where switchback LEDs have been used to provide DRL functionality as well as flashers:

 

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11 hours ago, Herbie said:

hyperflash

I was at my Saab garage the other day and there was a 9-3 on the ramp and one of the guys was working under it. But every second or two the rear number plate lights flashed on and off. They said he'd fitted some "daft LEDs". I've a feeling the ignition wasn't even on. Is that what you mean?

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Yeah, that can happen if you fit LEDs to a car that is supposed to have halogen bulbs. The computer detects bulb failures by periodically sending a pulse to the bulb and checking the resistance across it. Because halogen bulbs take a second to light up you don't notice anything, but with LEDs they are instant so you see it flashing.

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3 hours ago, rich1068 said:
15 hours ago, Herbie said:

hyperflash

I was at my Saab garage the other day and there was a 9-3 on the ramp and one of the guys was working under it. But every second or two the rear number plate lights flashed on and off. They said he'd fitted some "daft LEDs". I've a feeling the ignition wasn't even on. Is that what you mean?

No, I was talking about the indicators because that's what the OP mentioned.

Indicator bulbs are 21W so front and back makes 42W, plus about 5W for the side repeaters and a little for the dash telltale, so for ease of explanation we'll say 50W in total, which would equate to a current draw of 4.17A.

A 'traditional' flasher unit uses a bimetallic strip to flash the indicator lights by way of the fact that current flowing in a circuit creates heat. The bimetallic strip is designed such that a current flow of 4.17A will make it get hot and bend, thereby cutting off the supply to the bulbs. As there is then no current flowing the strip cools down and bends back to make contact again, thereby restoring the current flow, which is how we get the flashing sequence, and to pass an MOT they must flash between 60 and 120 times per minute.

As I said, a normal indicator bulb is 21W but an LED is rated much, much less and draws only a fraction of the current, so the bimetallic strip never gets hot because there isn't enough current draw. The CANBUS system sees this as a fault and makes the indicators flash at a much higher rate - known as 'hyperflashing' - to draw our attention to the supposed 'fault'.

This is the reason why you have to put load resistors in the circuit - to make the circuit draw the correct amount of current to make the bimetallic strip do its job. Or you could change to a proper LED flasher unit that uses electronics rather than a bimetallic strip.

As to the other lights, such as number plate, side and tail etc., they should work perfectly well as long as you replace them with LEDs that are specifically described as 'CANBUS aware' or 'No CANBUS errors'.

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