TH400 downshift solenoid?

[mcintyrederek]

Hi,
Running 1986 C10 with 350 and a TH400 working great.  My question with the TH400:  I have an orange wire coming from it to the accelerator pedal.  Obviously when you full throttle it, 12 volts goes to the transmission downshift solenoid.  I disconnected this and put a foot switch so I can down shift when I want to.  I don't like the idea of downshifting under full throttle (sounds dangerous to me). 

Tell me if I understand it right:  If I'm in 3rd and hit the switch, it shifts into 2nd.  If I'm in 2nd and hit the switch, it shifts to 1st??

Is this correct?
Thanks.

[bake74]

     If your wire is indeed the downshift mechanism for your transmission, then to answer your question, YES, every time you hit the "foot switch", you will down shift.
     My question to you is why do you not trust the downshift solenoid to do it's job and what it was created for ?
     It is not suppose to downshift only at full throttle, but whenever it detects a lug or too much pressure on the transmission, then it downshifts to bring the RPM's of the vehicle back up.  Of course it you floor the accelerator all at once it will downshift.

[zieg85]

It will not over-rev the engine and like Bake said, if you are lugging the engine that is when it kicks in.  I would be more worried about accidently pulling it down into 1 when you meant to go to 2 at expressway speeds

[VileZambonie]

Hook it up correctly

[mcintyrederek]

Thanks, I understand the downshift solenoid now.  I hooked it up correctly to the accelerator pedal now with a new slider switch (the old one was broken.)  Perhaps I need to be schooled more on the TH400.  How else does the transmission detect a "lug" ??    I thought the only way it would downshift was with the electric solenoid.  How does it know it's being lugged down?  I am guessing if it was in 3rd and you tried to climb a steep hill, it's supposed to go down to 2nd.  How does it know how to do that unless it's through the governor springs and vacuum modulator ?? (I am thinking your answer will be, it's with the governor and the vacuum modulator)

I never full throttle the vehicle anyway or drive it hard the least bit, and I don't lug the drivetrain, so none of this really applies to me except just wanting to understand how it works.

Thanks.

[zieg85]

it's through the governor springs and vacuum modulator ?? (I am thinking your answer will be, it's with the governor and the vacuum modulator)

That is how I understand it to be under normal conditions unless you floor it and the governor knows by downshifting or not because of ground speed in 3rd.

[bake74]

it's through the governor springs and vacuum modulator ?? (I am thinking your answer will be, it's with the governor and the vacuum modulator)

That is how I understand it to be under normal conditions unless you floor it and the governor knows by downshifting or not because of ground speed in 3rd.

     Correct on all accounts, if you were to floor it all of a sudden is when the cable attached to the throttle comes into play.

[1979C20]

Technically, you would have to hold the switch. The switch energizes a coil inside the trans, which in turn opens an oil passage. What happens is the passage lets transmission pressure escape back into the pan, keeping the transmission from shifting, or making it downshift. The governor will In deed keep the trans/motor from over revving using centrifugal force it spins weights open at a certain speed and forces the transmission to upshift. Technically, you can leave the kickdown switch on for racing and it will ride out the gears fully, but that puts a lot of strain on the drivetrain. Personally I would leave it attached to the pedal, so when you gun it it will downshift, for passing.

[VileZambonie]

The detent solenoid is designed to respond to driver demand, the governor senses road speed and the vacuum modulator senses engine load. The detent solenoid is activated by the switch on the accelerator to enhance transmission shifting characteristics so adding a manual on off switch as opposed to the momentary switch that can be calibrated to your trucks characteristics is not a good move. You may as well eliminate it altogether and manually downshift if that's all you're after. Once you have it working correctly you'll miss it when it's not there especially in the event that you decide to put the pedal to the metal.