I always get the unlocking sound but sometimes the key wont turn and other times it will turn to the run position but not crank. so far I just take the key out and turn it over and hold my tounge in a different position and it works on the second or third try but if it fails on the wife I will be in the dog house. sounds like the tumbler to me also.

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2007 ML320CDI
1986 Jeep CJ10a 6.2 diesel