Thread: TH350 shifts fine into third when on power, but not when accelerating slowly?

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Are you sure it's the tranny and not an engine surge? A mixture problem at mid-range cruise might cause surge as it's being held waiting to shift? Just a thought. How much vacuum are you pulling, and are you sure the line from the manifold to the modulator is like new and not leaking? Assuming the line is good and that you have good vacuum at the cruise range where you're seeing the problem I'd buy a new modulator and eliminate that from the list of potential problems. Sounds funny that you backed out the adjustment screw with no effect.

Since I'm not comfortable messing with automatics much I'd probably stop by our local independent tranny guy's shop, talk it over with him and see if he'd take it for a quick test drive for me. But then we have a fantastic old school tranny guy that's honest as the day is long, and I trust him 100%. I'd avoid the "big name" national chain shops like the plague, unless you know for sure that the guy running the place is more than a shyster fleecing the unknowing.