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It would be helpful to know the year of your transmission and other nameplate data as to what car it came out of. The nameplate is on the passenger side.
Later T400s have the VIN on the drivers side pan rail.
Also be helpful if it has the two terminal connector on the drivers side or one.

Early Buicks and Olds had the switch pitch converter and it did have a two terminal connector. That was phased out and the single terminal became standard. In 1970, the two terminal connector came back, one terminal for KD, the other for the distributor vacuum solenoid.
Internally the terminal for the KD goes to a solenoid which alters the oil flow to enable a lower gear but only if certain conditions are met. Depending on how your 400 is calibrated those conditions could prohibit KD above 60 mph.
Internally the second connector terminal for the distributor vacuum solenoid ground is controlled by a switch that closes to ground in 3rd gear. If the wires were mixed up, when the transmission shifted to third, it would blow the fuse for the kick down circuit.

There are numerous ways to approach this problem.
1. Test light or meter; Check for voltage at the kick down switch. 1966 Tempest models had two Orange wires at the switch. One comes from the wiper fuse at the top, the other goes to the firewall connector and then to the transmission.
With the ignition ON, engine not running, you should have 12 volts on one terminal. Place the meter lead to the second terminal, and press the accelerator to WOT. It should show 12 volts at WOT. If not, then your problem is in the switch. Try moving the switch actuator arm manually and check for voltage.

2. If the switch checks out, pull the terminal cap off the transmission connector. Insert meter lead inside the terminal cap and check voltage again, helps to have a helper to push the accelerator to WOT. If you have voltage then you have a bad or miswired solenoid. No voltage means there is an open in the wire between the accelerator and transmission. Possibly the firewall connector.

3. A final check would be is to take a long jumper wire from the positive battery to the transmission connector terminal. Touch the transmission terminal and listen for a click. If it clicks, solenoid is good. if not...
 

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No. They were two separate circuits.
The second circuit energized the intake manifold mounted vacuum solenoid in 3rd gear to allow vacuum to the distributor advance. In other words, the distributor only had mechanical advance in first and second gear.
Manual transmissions had a switch built into the case that did the same thing but after third in a 4 speed.
 
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