Take a look at the back side of the steering wheel hub- there should be a heavy circular ring with a 90* finger pointed straight out. This finger is what trips the cancelling switch. If it is missing or incorrectly indexed the signals will not self-cancel.
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As the wheel spins to return to center, that finger contacts one of these two parts of the signal switch, tripping it off.
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So if the finger is missing or clocked way off, it will not cancel.
It is a beeotch to move (just held in place by spring tension), but I have seen it missing.
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