TH400 (Reverse Pattern) 3rd/4th Gear ECM Switch

[motorhead]

If it wasn't already common knowledge I am putting a reverse manual valvebodied & transbrake equipped TH400 behind my low-12 second sled of a motor. Reason being that the current non-BRF 200-4R is full of electrical woes and liquid seepage; and the cost of the TH400 combo was cheaper and less time consuming than trying to fix all the little issues with the 200-4R as seen here: Transmission Switches & Controls.

The biggest issue that carried over from the 200-4R to the TH400 was the 3rd/4th gear indicator signals from the valvebody to the ECM. Especially considering that the TH400 doesn't even have provisions for the pressure switches. So the question remained: What does one do if he wants to have the ECM recognize 3rd gear for various programming reasons?

The limitation to the solution (as I saw it in this application) was that a reverse shift pattern in the valvebody was required in order to trigger the switch as 3rd gear was now at the full rear of the shift sequence. If you would end up using a TH400 with a standard shift pattern you would have a more complex time getting the switch to trigger.

Here are the basic steps that I took to achieving my intended end state:
1) Locate a micro switch that has both normally open (N/O) and closed (N/C) prongs on it. This was a fluke on my part as I bought the switch years ago for a nitrous install in Natasha's Camaro that hasn't happened yet and is now going to require a new switch. Here you'll require the N/C prong. The other advantage is that the N/O prong can be used for another circuit.
2) Remove the centre console and physically remove the shifter and prepare to disassemble/reassemble a number of times.
3) Make a small bracket that has ALOT of designed adjust-ability in it. I used some small gauge aluminum L-bracket and slotted the mounting point for fore & aft adjustments. The bracket could also be tilted as required to match the angle of the shifter as it was pulled fully to the rear.
4) Weld a long and flat "finger" that stretches across the backside of the shifter handle base. Make sure that you leave one end open/loose in order to allow adjustment bends that will touch the roller on the micro switch and trigger the circuit. You'll want to leave the top open to allow the shifter spring to be reinstalled.
5) Dry set all of your adjustments.
6) Create the necessary wiring that leads to a ground; I used a chassis ground (at the shifter base) in this instance. You will then need to create another wiring harness (forked) that runs from the N/C prong to both C7 and C8 at the ECM. You could just run a wire to one of the pins at the ECM or both; I chose to run both so that when I hit 3rd gear the ECM sees that it has been sent directly into 3rd/4th. Also, you could splice in a toggle switch between the 4th gear signal line and turn it off for racing purposes if you know it will be an issue.
7) Test fit the shifter, wiring and replace the centre console check for clearance issues... adjust as required. Conduct final install when confident.

As you can see from the pictures this assembly fits comfortably under the stock console without any physical modifications.

You can see the switch operating in the attached PL file (I was just playing with the switch with my finger; it really doesn't act that erratically... ).

[ULYCYC]

Some of the early smog big GM cars with T400's had a two prong kickdown switch. One of the terminals was a 3rd gear signal to a vacuum/solenoid switch on the intake. It was for adjusting vacuum on the distributor to stop pinging. If you could find the switch and internal control it would do what you just invented.

[pacecarta]

many ways to skin that cat with a stock shifter , yours is the hardest way ive seen to date

[motorhead]

Originally Posted by pacecarta

many ways to skin that cat with a stock shifter , yours is the hardest way ive seen to date

Thanks!

I think?