TH400 kickdown question?🤔
December 29th, 2020, 07:35 PM
#41
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Location: Chesapeake, VA
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Guys, I really appreciate all the input and advice, but here's the real reason I started this thread: I'm not really that concerned about the fact that there is no kickdown at highway speeds. Would I like it to happen? Sure, but it's not an issue that concerns me enough to go digging into the tranny or making adjustments that may or may not yield the results I hope to achieve. It upshifts fine, it downshifts fine, and when I'm on the carpet and it grabs second, you know it. Besides, I don't do a lot of high speed passing anyway, and when I do need to gain some ground, I let the secondaries do the work.
All I'm trying to do is gain understanding about the process and hopefully be able to pass the knowledge on to the next guy who asks.
And...I'll ask this...based on what I've posted, what would you do? Call it good or dig deeper?
All I'm trying to do is gain understanding about the process and hopefully be able to pass the knowledge on to the next guy who asks.
And...I'll ask this...based on what I've posted, what would you do? Call it good or dig deeper?
December 30th, 2020, 04:05 AM
#42
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Join Date: Jan 2010
Location: central Indiana
Posts: 5,267
If your trying to get every last bit of acceleration, and don’t want to mess with using the shifter to hold each gear, you have no choice but to play with the governor and valve body.
If you don’t want to play with the governor and valve body, either accept the car as is or use the shifter to hold the trans in each gear until your ready for it to shift.
If you don’t want to play with the governor and valve body, either accept the car as is or use the shifter to hold the trans in each gear until your ready for it to shift.
December 30th, 2020, 05:09 AM
#43
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Location: Chesapeake, VA
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If your trying to get every last bit of acceleration, and don’t want to mess with using the shifter to hold each gear, you have no choice but to play with the governor and valve body.
If you don’t want to play with the governor and valve body, either accept the car as is or use the shifter to hold the trans in each gear until your ready for it to shift.
If you don’t want to play with the governor and valve body, either accept the car as is or use the shifter to hold the trans in each gear until your ready for it to shift.
December 30th, 2020, 05:11 AM
#44
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Dave, I've been following your thread from a distance. Transmissions are not my favorite cup of tea - at all. My '71 CS convertible is a numbers matching 350 sbo + TH350 + 2.56:1 open carrier differential. I have not had to work on any aspect of this car's drive train. Whomever performed the work on the drive train in the past must have been a genius. It shifts flawlessly & will (aggressively) downshift up to 70 mph. At 65 mph the downshift is phenomenal & very aggressive. That's about all I can say. I don't like working on any of the internals of a transmission.
Question for you though. If I recall in your previous discussions (other threads over the past couple years), wasn't your car previously setup as a race car by the previous owner or by one of the owners? They may very well have made adjustments to the governor & transmission modulator.
Question for you though. If I recall in your previous discussions (other threads over the past couple years), wasn't your car previously setup as a race car by the previous owner or by one of the owners? They may very well have made adjustments to the governor & transmission modulator.
December 30th, 2020, 05:40 AM
#45
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Join Date: Mar 2007
Location: Northern VA
Posts: 47,357
A few things come to mind after looking at the TH400 hydraulic diagrams in the CSM.
First, kickdown timing balances oil pressure from the governor against oil pressure from the vacuum modulator. The modulator internally balances manifold vacuum pulling on the diaphragm against a spring pushing on that diaphragm. When manifold vacuum is near zero (ie, at W.O.T.), the spring pushes the diaphragm all the way out, which provides maximum modulator circuit oil pressure and thus the highest possible upshif RPM. If that internal spring is weak or broken, the modulator can act as if manifold vacuum is higher than it really is, causing upshifts and kickdown at lower than intended RPM. Unfortunately, the only test for that spring in the CSM is to use a special Kent Moore gauge tool and a second, known good modulator to see if the spring rates are the same or not. Frankly, it might be prudent to just get a new, adjustable modulator.
The governor controls maximum shift point RPM by regulating line pressure in the governor oil circuit as a function of trans output shaft RPM. It is common to tune the governor to change this max shift point RPM by swapping weights and springs. Depending on the original application, the governor could limit shift points to RPM levels anywhere from 4000 RPM to over 5200 RPM.
Finally, the detent solenoid valve itself can be a source of the problem. The valve is normally closed. When you floor the throttle, the electrical signal opens the valve and dumps the detent oil pressure that is preventing the shift valve from commanding a downshift. If the solenoid is sticky or if the orifice in the valve is partly plugged, that oil won't vent when intended and the trans won't downshift.
Personally, I still think the behavior of your trans sounds pretty normal, but these are possible things to look at. As I noted before, tuning the governor and modulator are easy things to do that don't even require dropping the pan (though oil will come out when you remove either one, so have a pan ready).
First, kickdown timing balances oil pressure from the governor against oil pressure from the vacuum modulator. The modulator internally balances manifold vacuum pulling on the diaphragm against a spring pushing on that diaphragm. When manifold vacuum is near zero (ie, at W.O.T.), the spring pushes the diaphragm all the way out, which provides maximum modulator circuit oil pressure and thus the highest possible upshif RPM. If that internal spring is weak or broken, the modulator can act as if manifold vacuum is higher than it really is, causing upshifts and kickdown at lower than intended RPM. Unfortunately, the only test for that spring in the CSM is to use a special Kent Moore gauge tool and a second, known good modulator to see if the spring rates are the same or not. Frankly, it might be prudent to just get a new, adjustable modulator.
The governor controls maximum shift point RPM by regulating line pressure in the governor oil circuit as a function of trans output shaft RPM. It is common to tune the governor to change this max shift point RPM by swapping weights and springs. Depending on the original application, the governor could limit shift points to RPM levels anywhere from 4000 RPM to over 5200 RPM.
Finally, the detent solenoid valve itself can be a source of the problem. The valve is normally closed. When you floor the throttle, the electrical signal opens the valve and dumps the detent oil pressure that is preventing the shift valve from commanding a downshift. If the solenoid is sticky or if the orifice in the valve is partly plugged, that oil won't vent when intended and the trans won't downshift.
Personally, I still think the behavior of your trans sounds pretty normal, but these are possible things to look at. As I noted before, tuning the governor and modulator are easy things to do that don't even require dropping the pan (though oil will come out when you remove either one, so have a pan ready).
December 30th, 2020, 08:05 AM
#46
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Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 13,793
Those vehicle speeds are typical for a 2.56 rearend - both my 1970 Supreme from my school days and the current 71 vert both shifted at similar speeds with 2.56 rearends. Changing to a lower rear gear ratio decreased the vehicle speed at which the trans upshifted and downshifted. A 3.08 gear will result in lower vehicle speed shift points than a 2.56 gear, with no other changes made.
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