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Old 01-31-2011, 08:44 PM
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T-400 Shifting problems

Hey did you ever found out what was causing this? I have the SAME thing happening. Governor and bore look good. No plugged filters, new modulator, good vacuum. Reverse is also strong even when dropping out of third. Can override problem by downshifting to 2, then back to D while accelerating. Stop accelerating and transmission drops back out of 3rd and coasts while idling... thanks Rob **********

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Old 01-31-2011, 09:06 PM
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Originally Posted by pythonoffshore24
Hey did you ever found out what was causing this? I have the SAME thing happening. Governor and bore look good. No plugged filters, new modulator, good vacuum. Reverse is also strong even when dropping out of third. Can override problem by downshifting to 2, then back to D while accelerating. Stop accelerating and transmission drops back out of 3rd and coasts while idling... thanks Rob **********

welcome to Hotrodders.com

The thread you posted was an old thread.

Sounds like you have a vacuum problem for 3rd to drop out or the clutch pack is worn out, sealing ring area. The engine will drop to idle as the vehicle moves quicker than it would in 2nd gear which is what gear it is in when 3rd drops out.

I also removed your phone number. It is a bad idea to post phone numbers on the open forums since spiders can collect them for spam purposes like email addresses

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Old 01-31-2011, 09:25 PM
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I still have a strong reverse when this is happening. Also vacuum is normal... at engine intake and modulator. Also, it only starts doing this after temp of Trans reaches 125 degrees or more. I can move selector to 2nd and back to Drive and it will maintain 3rd until you stop accelerating.
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Old 01-31-2011, 09:35 PM
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Quote:
Originally Posted by pythonoffshore24
I still have a strong reverse when this is happening. Also vacuum is normal... at engine intake and modulator. Also, it only starts doing this after temp of Trans reaches 125 degrees or more. I can move selector to 2nd and back to Drive and it will maintain 3rd until you stop accelerating.

Reverse pressure is boosted by GM design. You can have a weak 3rd gear and a strong reverse.

Since this is worse when hot, I suspect leakage of pressure or clutch pack wear.

Is this a recent rebuild or vintage trans?

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Old 01-31-2011, 10:33 PM
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I believe I am going to find the problem at the center support. 3000 miles on overhaul from back in 2005. Going to pull and tear down in the morning. Best I can tell it's tearing up the Teflon seal. Just found out about full pressure in reverse but thank you so much for replying so quickly. Will keep you updated if you like.
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Old 02-01-2011, 05:25 AM
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Quote:
Originally Posted by pythonoffshore24
I believe I am going to find the problem at the center support. 3000 miles on overhaul from back in 2005. Going to pull and tear down in the morning. Best I can tell it's tearing up the Teflon seal. Just found out about full pressure in reverse but thank you so much for replying so quickly. Will keep you updated if you like.

of course you will.

If you think there is leakage at the center support to case transfer holes... then install 200-4r rubber tipped sealing plugs with tranfer holes in them. A common fix for center support leak.

If the teflon seal ring is being torn up, you have larger problems. Check for bushing wear on drive tube and the bushing in the center support. Should not worn much if the rebuild has only 3k miles if you properly replaced the bushings at rebuild time
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