Transmission shift issues?
Thread Starter
Cruising
Joined: Jul 2014
Posts: 12
Likes: 0
Transmission shift issues?
Tough to explain, but....
I have a TH400 in my 75 that seems like it shifts late....?
At WOT, it seems as though the RPM's climb high before it shifts and it is a soft shift, not very crisp. Not great acceleration....
So if you accelerate onto a highway, at WOT, the engine is climbing in RPM's, but seems like there is something holding the car back....
Would the modualtor valve have anything to do with that? Ive never had to change one out before.
I have a TH400 in my 75 that seems like it shifts late....?
At WOT, it seems as though the RPM's climb high before it shifts and it is a soft shift, not very crisp. Not great acceleration....
So if you accelerate onto a highway, at WOT, the engine is climbing in RPM's, but seems like there is something holding the car back....
Would the modualtor valve have anything to do with that? Ive never had to change one out before.
Race Director
Joined: May 2006
Posts: 18,743
Likes: 2,585
what is the exact shift rpm?
what is the maintenance history of the transmission, fluid and filter changes?
are you sure it's not the engine having an issue? when was it tuned including fuel filter or filters?
what is the maintenance history of the transmission, fluid and filter changes?
are you sure it's not the engine having an issue? when was it tuned including fuel filter or filters?
Thread Starter
Cruising
Joined: Jul 2014
Posts: 12
Likes: 0
It seems to have changed since I replaced the intake...installed an Edelbrock Performer. I also set valve lash, using the running at idle, backing off rocker until clacking, then tightened a qtr turn PAST when the rocker stops clacking.
Thread Starter
Cruising
Joined: Jul 2014
Posts: 12
Likes: 0
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Shift "timing" is controlled by the vacuum modulator on a THM-400. ALL shifts will be very 'late', unless that modulator is working properly and has an unrestricted vacuum supply. First, check to make sure that the vacuum connections from manifold to downpipe...and from downpipe to modulator...are in good condition. If so, then pull the hose to the vacuum modulator off and see if any transmission fluid runs from it. If so, the modulator is defective and needs replacement.
Any car parts store will carry a modern replacement (much smaller than the original modulator) and it will work satifactorily. They are less than $30.
Any car parts store will carry a modern replacement (much smaller than the original modulator) and it will work satifactorily. They are less than $30.
Burning Brakes
Joined: Mar 2014
Posts: 904
Likes: 46
From: Crestline CA
Symptoms almost sound like a bad torque converter stator sprag but before jumping to that conclusion there's a few things to check. As others have said, check the vacuum line to the modulator, make sure its connected(sounds like it is) and that there's no oil in the line. Next, check the kickdown, easiest way is to turn the key on engine off and slowly open the throttle to WOT and listen for a very subtle click from inside the trans at around 2/3-3/4 throttle. Also, make sure to check the fluid level(if haven't already).
