The TH350 or 400 (don't know which it has) automatic transmission on my 1980 Corvette went out, so I will be ordering a 700R4 soon. However, one of these days I will be upgrading the engine to a 400+ c.i. small block.

My question is how do automatics know when to shift? I assume it is "programmed" into the transmission, rather than the transmission simply being able to measure airflow or something to determine when to shift. If it is programmed in, won't I have to have the transmission pulled and reprogrammed once I upgrade my engine? What are all the variable that are actually programmed into a transmission?

Thanks for the help.

 

The trannies you mention use a throttle valve which is linked to the throttle body via a cable.

When the throttle opens, the cable pulls on the throttle valve inside the transmission valve body and that's how the trans "knows" what the engine is doing.

Except for the TC lockup switch, the 700R4 is fully mechanical.....no "programming". Late model cars have electronic transmissions and the TV cable isn't neccesary.

 

Hey, I answered to this too , on the same exact post, must have been double

This is better explained though.

 

Actually, the TH350 doesn't use a TV valve. It uses a modulator hooked to manifold vaccuum to accomplish that. The cable that attaches to the throttle body is simply a kickdown valve (actually acts more like a "switch [ON/OFF]" than a "valve [PROGRESSIVE]") to tell it to drop a gear when you floor it.

And on 700s, the TV valve doesn't control when the transmission shifts. It only controls the pressure so that at higher RPMs it has higher pressure in order to hold the clutch packs against the increased torque, quicken the shifts, etc.. What controls the actuale shift points is known as a governer. It's a gear driven centrifugal valve..er.. it has weights that are held closed with springs. As it spins faster the weights fly out and opens a valve that causes the next shift. You can change the shift points by varying the spring strengths or weights on the governer.

 

There are 1-2, 2-3, 3-4 shift valves in the 700R4/2004R overdrive units and there is a governor. In the TH350/400 there are 1-2, 2-3 shift valves and a governor. The TV valve in the 700R4/2004R controls the upshifts & downshifts at what ever the throttle opening is & relative road speed is, and these work together with the shift valves & governor. In the TH350/400 the upshifts are controlled by the vacuum modulator, upshift valves & governor, the downshifts are mostly controlled by the downshift cable (TH350), and electricaly on the TH400, along with the modulator. Torque converter lockup is electrically controlled on these units that have TC lockup. This is the basics on this. Glad to help.

 

One thing I forgot to mention is the TV valve (700R4/2004R)is controlled by the TV cable and its adjustment is critical to the life of the transmission.

 

Thanks for the help guys. Two more questions:

1. With the valves and governers and what not, it seems that there are some modifications that would need to be made to the transmission when changing to a new motor with different characteristics. Are these something that the tranny must come out to change? If so, I suppose it would be best to order the tranny specifying the shift points, etc. that the new motor will have, rather than the old one. Seems funny though...

2. When I am ready to order a transmission, does anyone know all the variables I should have ready to go into the transmission? Like shift at 6000 WOT, 4000 POT, down**** at certain points, etc.


Thanks again for the help.

 

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