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The second is this overdrive servo boost valve, which Glassbrenner replaces in all '01-and-newer 4R70Ws. The original valve is steel (left, upper) and wears out the bore it fits in (left lower), while the new valve (the black one) is anodized and fits into a hardened sleeve. This valve helps control pressure to the overdrive band. When it goes bad, the band will wear excessively, ultimately resulting in the complete loss of overdrive.