A switch is typically used for controlling torque converter lockup. Others here will have to advise you on why the trans won't shift into 4th.

Knowing the model year might help.

 

Do you have a tachometer?
If so, what rpm does it show when you’re going 70 mph.
RPM canbe a good indicator of what gear you’re in.

 

Not an expert, but I would suspect the governor. The center plunger MUST push all the way in which is pushed by the centrifugal force of the weights to overcome the TV valve output pressure in order for the hydraulic pressure port to get 4th gear. There is already a 4th gear pressure switch to put voltage on the lockup solenoid once you achieve 4th gear. You don't even have to drop the pan.

 

In a stock application, the 700R4 would lockup in 2nd, 3rd, and 4th gears. No external pressure switch or manual switch needed. Lock-up was controlled by manifold vacuum switch, and stepping on the brake pedal would unlock the converter. It was totally transparent to the driver.

 

Ummmmm... NOPE! STOCK is lockup in 4th only. Lockup in 2,3 and 4 can only be achieved (selectable) if the 4th gear lockup switch is a two pole, allowing bypass of the switch. Manifold vacuum switch (optional in aftermarket application) ONLY breaks the 12 volt signal for lockup, and will release lockup for acceleration. Same with the brake pedal switch, (except to disengage the lockup upon deceleration) wired in series with 12 volt supply. AND it's not transparent to the driver unless said driver is DUI.

 

Yeah , The 700R4 original configuration was lockup in 2, 3, 4th gears. That lockup process was changed as the years went by. Early 700 transmissions were a very confusing shift strategy to say the least

I have not seen the year, make , vehicle model of this 700R4?

How do you know what or how it is wired inside?

Has internal wire harness been modified?

Which pressure switches are installed on the valve body? Normally open switch or a Normally closed pressure switch? GM used both depending on vehicle application and year?

With that all said: There is NO NEED for a switch to operate 4th gear in the 700R4.

 

Going into 4th gear is not the same as lock up.

4th gear is in fact a gear it has a ratio of 1 to .72 which slows engine RPM in relationship to road speed.

Lock-up is a clutch on the torque converter which when applied diverts the power path around the internal elements of the torque converter eliminating hydraulic slip which isn’t much but the OEMs have been chasing lowering fuel burn.

Not catching 4th is a early warning of 3/4 clutch wear failure, it’s common especially on high mileage or hard used transmissions

Bogie

 

Ron Francis module #TC-70. Has an adjustable lock up speed.
I have used many.
Jim, School of Hot Rod Hard Knocks since 1957