[HO455]

First thing done on the TH350C was to put a drain plug in the pan. (Photo 1) There should be a special place in hell for the bean counter who decided that it wasn't necessary.

The TH350 I installed is an early 80's version which has a lockup converter like a 2004r or a 700r4 would. There was no computer control of the earliest versions so it is a simple control circuit to operate the lockup. The transmission will automatically lockup the converter in 3rd gear once pressure rises high enough. Usually around 45 MPH. There is also solenoid inside the transmission that has to be energized to allow the lock up to occur.
The lock up needs to be released when braking and when accelerating. To get braking cut out I replaced the factory brake switch with a brake switch for a 72 C10 with cruise control. Amazingly this switch is still readily available at NAPA. (Photo #2)

The acceleration cutout comes from a vacuum switch that opens a set of contacts at about 6 inches of vacuum. So the contacts are closed when at cruise when manifold vaccum is high. Then when the throttle is punched vacuum drops and the lock up is released. I found that the original GM switch has been discontinued for quite some years and there are 2 aftermarket options available. TCI makes one and since I needed to order a new O2 sensor and Summit had both of them I got the TCI version.

It was much smaller than I imagined from the photos. It also doesn't look to be robust enough to live under the hood without some protection. This necessitated finding some sort of box to put it in. (Photo #3) I robbed the box off a set of under counter lights I was planning on putting in the kitchen but didn't. There were a couple of holes to plug with snap in plugs with some 5 minute epoxy to seal the gaps and keep them in place. 5 minutes with a Dremel and I had modified the original power jack so the vacuum hose and wires would pass through.
To mount the switch I used double sided tape. I noticed that there is a small hole in the back (Photo #4) that if I were to cover with the tape might prevent the switch from working properly, so I used 2 pieces of tape to leave a gap for air around the hole. If you look close at the front of the switch you can see a small Allen head that gives some adjustability of the set point of the switch.

I should have painted the box before I mounted it. Dhoo! The PO''s rattle can paint job of the firewall expired over the winter. So things are really looking sorry under the hood.

The box got bolted to a metal plate that got bolted down using 2 of the brake booster bolts.

For power I used the 12v positive lead that originally was used to power the TH400 kick down. It was ran to the brake switch then out through firewall and down to the transmission. There is a 4 prong plug on the transmission but only 2 of the pins are used in this application. The forward driver's side pin is the 12 positive from the brake switch. The back driver's side pin is hooked to a good ground. Before going to ground I ran the wire through the vacuum switch. I ran a vacuum line from behind the carburetor teeing into the line that goes to the modulator on the transmission.