I just had my cylinder replaced in my '97 because my old switch blade remote key would not turn the cylinder. Fortunately I also have a new flat key which would turn the cylinder so replacement was easy and inexpensive. Now, with the new cylinder, both keys work well. I think there was just too much wear on the old key and old cylinder. If you have a newer key, try it. It might even be worth getting a new key from the dealer because replacing a cylinder that will not turn, can be expensive as it has to be drilled out. Since yours is a bit older than mine, I am not sure how much of my experience applies.

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'97 E 300 D