Mt643 mode??

[Soaringcook]

So i just found out i have 4.81 gears. Does this make a lot of difference or do i stioo have a 4th gear issue?

[Soaringcook]

Cadillac kid

When im maxed out if i let off the pedal the rpms drop fast.

[s2mikon]

That sounds like your converter is not locking.

[Soaringcook]

Would it be the electric modulator? I unplugged the conector and noticed oil in the actual connector. Is almost as if someone dipped the electrical harness in honey and pushed it it.

[s2mikon]

I don't think so. But they are known weak points. It is on off modulation via the ecm. Will raise line pressure against the governor. Off= short soft shifts low rpm. On= longer shift and firmer and higher rpm in each gear.

[s2mikon]

Un-plug it see how it shifts and then plug it in and see if it changes.

[EastCoastCB]

Originally Posted by Soaringcook

Cadillac kid

When im maxed out if i let off the pedal the rpms drop fast.

Sounds like your converter doesn't lock.
Are you 100% sure its a 643 and not a 545?
Sorry if this has been asked before. If not then take a pic of the transmission and post it.

[Booyah45828]

Agreed. Large rpm swings when changing throttle level at highway speeds indicates the converter isn't locked.

[cadillackid]

ill make a video of my MT643 running itself through sequence all the way to its 4th gear Locked..



there are 2 explanations for quick drop RPMs.. actially 3.


1. - the converter lockup has failed .. many reasons for this from a valve body issue to all out converter clutch failure.


2. you have lost the 4th gear clutch, the 4th clutch isnt one that burns up very often but it can.. what happens then is that the valve body calls for 4th gear and the apply piston may even go for 4th but the clutch is gone.. (or the apply piston fails to apply the clutch).. the fallback gear is 3.. so foot to the floor you essentially just spin the clutch.. when you let off the gas, the trans is in fact in 4th so the RPMS drop to the 4th gear mode..


3. your trans is actually an AT545.. (im sure you have made sure this is Not the case.. )..


your most likely scenerio is lost lockup and its a documented issue that occurs.. a broken spring in the valve body is a common reason for it.. there are test procedures where you put a gauge on the lockup test port to test for pressures..


fortunately valve body work can be done without removing the transmission. but first have to determine what the issue is..



ill try and gret a short video today so you can get an idea of how a 643 should shift out and how lockup feels and applies..

[EastCoastCB]

Originally Posted by cadillackid

ill make a video of my MT643 running itself through sequence all the way to its 4th gear Locked..



there are 2 explanations for quick drop RPMs.. actially 3.


1. - the converter lockup has failed .. many reasons for this from a valve body issue to all out converter clutch failure.


2. you have lost the 4th gear clutch, the 4th clutch isnt one that burns up very often but it can.. what happens then is that the valve body calls for 4th gear and the apply piston may even go for 4th but the clutch is gone.. (or the apply piston fails to apply the clutch).. the fallback gear is 3.. so foot to the floor you essentially just spin the clutch.. when you let off the gas, the trans is in fact in 4th so the RPMS drop to the 4th gear mode..


3. your trans is actually an AT545.. (im sure you have made sure this is Not the case.. )..


your most likely scenerio is lost lockup and its a documented issue that occurs.. a broken spring in the valve body is a common reason for it.. there are test procedures where you put a gauge on the lockup test port to test for pressures..


fortunately valve body work can be done without removing the transmission. but first have to determine what the issue is..



ill try and gret a short video today so you can get an idea of how a 643 should shift out and how lockup feels and applies..

Excellent and very informative post!
May I also suggest getting a view of what the 643 looks like while you're at it?!