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On my 2005 Cavalier, the left turn signal indicator is permanently lit when the headlights are on and flashes at about double speed when the turn signal is actually on. The actual signal lights function normally though.

Where to start troubleshooting this?

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Welcome to the site! – Mark Johnson Dec 11 '12 at 1:52

Check your earth/grounding points for the lights. Lights illuminating when they're not supposed to is a classic symptom of a bad earth.

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There are two filaments inside a turn signal bulb. One of them is dim for the park lights, the other is bright for the turn signal. The bright filament can break on one end and land on the dim filament. Anytime the parking lights are on it makes both filaments light. The turn signal can still function depending on where the break in the filament is. This would also change the resistance in the bulb and cause the turn signal to flash faster.

As seen in the picture below, if the top filament was to break loose from one end it could land across the other filament causing the problem you describe.

First thing to check is the bulb.

enter image description here

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I have also seen it from putting the wrong bulb in the socket - often done by another shop or parts store even. Of you've had one replaced, pull out the bulb and look at the bottom. Make sure there are the same number of 'nipples' on the bottom of the bulb as there are in the bottom of the socket.

When i worked as a mechanic, it was a very common problem i saw.

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up vote 2 down vote accepted

Forgot about this question. Anyway, replacing the bulb fixed the issue, though unlike as Larry suggested, both filaments were intact. Also unlike as Drake suggests, the bulb was of the correct model. Still no idea what the cause was, but it's fixed now anyway.

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If your signal is flashing at double speed, the cause may be as simple as a burnt out bulb. The relays expect a certain amount of load, and if only half of that load is present, they'll cycle twice as quickly. Check if either the front or rear bulb is blown.

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I said the actual signal lights are functioning. There's no burnt out bulb. – Compro01 Dec 7 '12 at 16:36
Lynn's comment re load is still valid - something else has failed so the load has been reduced. – Rory Alsop Dec 7 '12 at 18:52

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