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1996 GMC Savana, 5.7 L vortec, 4L60E trans.
In 2011 at 111K miles OEM trans repl with Jasper rebuild, at 5K mile fluid & filter change done. After market trans oil cooler installed. Used for towing 3,000# trailer ~70% of time mostly at highway speeds. Operated normally until, in Oct 2016 a routine fluid & filter change was performed at 163K miles. After oil change, trans after warm up of several miles would above 45 mph disengage lockup with slight load increase and then give a hard, lurching shift back into lockup.
Mechanic who did oil/filt inst another filter, but could find no codes or the problem. Dedicated trans shop removed & inspected valve body, inst OEM filt, checked pan for metal flakes & could find no problem. Auto electrical shop could find no codes or tell me what the problem is.
Releasing throttle prior to LU re-engaugment eliminates lurching. Heavy accel & engage at 70+ gives less lurching, but still more than normal. Trans operates normally otherwise.Need a replacement transmission? Quality transmissions are hard to find. Free estimate to your email.I suspect there’s something else at work here and it is neither really mechanical nor is it electronic either. I realize you have done a lot of work on the tranny but with the type of mileage that there is on the Savana I have to suspect that a really hard deposit has come loose somewhere in the drivetrain and has lodged where you don’t need it.
What happens is that when you have a fresh fill of oil or tranny fluid, both of which have detergent properties that will loosen enough hardened built-up gunk that will, in turn, become a free-floating (for a bit) piece of debris that will eventually settle out on something.
The way your vehicle is performing it sounds like the lockup chain has been impacted. You might try another transmission fluid and fluid refill. This might loosen things up enough again to get the parts clean and cure the problem, while the buildup is trapped by the new filter.
Let me know what happens.
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