TH400 vac modulator
- Thread starter WUTSAV8
- Start date
John Kirkland
New Member
- Joined
- Jun 22, 2001
Thanks Bruce, I needed this too. I replaced a modulator that was leaking ATF into the vacuum line during the winter. I am just now starting to drive that car, a 64 Riviera, now that the weather is getting nice. Thanks again for volunteering your time to answer our questions!
WUTSAV8
Member
- Joined
- May 24, 2001
Actually it's in a truck, that soon tow my regal around 
I'm using a Edelbrock 1406 carb and tee'ing off of the full time vaccum port.
I haven't gotten around hooking up the electrical detent. I haven't gotten it much out of first yet tho either.
What's the best way to retrofit a TH400 switch intro a truck?
Thanks.
I'm using a Edelbrock 1406 carb and tee'ing off of the full time vaccum port.
I haven't gotten around hooking up the electrical detent. I haven't gotten it much out of first yet tho either.
What's the best way to retrofit a TH400 switch intro a truck?
Thanks.
Y
yanniz
Guest
I am playing with my TH-400 now and Ihave found that modulator changes shift points to a small degree. The biger adjustments come out of the governor by changing springs and/or weights.
John Kirkland
New Member
- Joined
- Jun 22, 2001
My understanding of TH400 shift point tuning is that the vacuum modulator controls the part throttle shifting for normal around-town driving, and the governor weights and springs control the WOT shifting when eating rice. Is this right Bruce? Don W? Greg Meanbuicks?
MeanBuicks
Scaring the neighbors!
- Joined
- May 24, 2001
These two elements work together to determine shift points.
Let's look at it this way.
The governor is just a road speed indicator, telling the transmission only how fast you are traveling. It generates a pressure curve, starting at 0 psi, that rises as you accelerate.
The shift valves in the valve body are held closed with shift-valve springs. The governor pressure works against the opposite end of the shift valve train, and strokes the shift valve when the force is sufficient to overcome the spring force. Problem with my simplified model is, it wants to upshift at the same road speed regardless of driving conditions. Enter the modulator.
The modulator is the transmission's load sensor. As the engine vacuum signal diminishes modulator pressure rises. This pressure is sent to the main pressure regulating system where it boosts line pressure to meet increasing torque demand (i.e. clamping force on the clutches). Modulator pressure is also distributed to the shift valves where it works against governor pressure in trying to keep the valves in the down-shifted position. That is why your shifts get later & more spread out as you stand on the load pedal.
That being said, once you are at WOT, the modulator signal is static; atmospheric. Now the governor characteristics are going to be the determining factor as to when the shift occurs.
When adjusting WOT shift points, you usually want to make changes to the smaller, secondary weights and springs. It's a centrifugal device so lighter weights & weaker springs translate into higher shift points.
Class dismissed.
Let's look at it this way.
The governor is just a road speed indicator, telling the transmission only how fast you are traveling. It generates a pressure curve, starting at 0 psi, that rises as you accelerate.
The shift valves in the valve body are held closed with shift-valve springs. The governor pressure works against the opposite end of the shift valve train, and strokes the shift valve when the force is sufficient to overcome the spring force. Problem with my simplified model is, it wants to upshift at the same road speed regardless of driving conditions. Enter the modulator.
The modulator is the transmission's load sensor. As the engine vacuum signal diminishes modulator pressure rises. This pressure is sent to the main pressure regulating system where it boosts line pressure to meet increasing torque demand (i.e. clamping force on the clutches). Modulator pressure is also distributed to the shift valves where it works against governor pressure in trying to keep the valves in the down-shifted position. That is why your shifts get later & more spread out as you stand on the load pedal.
That being said, once you are at WOT, the modulator signal is static; atmospheric. Now the governor characteristics are going to be the determining factor as to when the shift occurs.
When adjusting WOT shift points, you usually want to make changes to the smaller, secondary weights and springs. It's a centrifugal device so lighter weights & weaker springs translate into higher shift points.
Class dismissed.
