Back feed causing trans not to shift

[Ishmael]

For my 4l65 to shift I have to have 0 volts on the computer side of my brake switch wiring. I have 9.5. I have an aluminum LS2. My battery cables are tight. I have removed the paint under my grounds. I am using braided ground straps for all the grounds in the engine bay except to the battery. The battery is connected to the block and the frame and a smaller wire to the rad support. The engine has a ground to the computer, the frame in two places and the firewall. The firewall grounded to the frame. The rad support is also grounded to the frame.
My wiring harness is a classic update kit so it is new and has blade fuses. I disconnected the headlights - no change. I pulled each fuse in the fuse panel and the only one that changed 9 volts to zero was the brake lights but then the engine doesn't run at all but will obviously turn over. I cleaned up the sockets for the brake lights and the marker lights have ground clips (68 firebird). I pulled the tail lights right out - no change.



I don't have a ground from the alternator. Don't know if that is a good idea or not.
What do I have left? Any ideas to chase this down? Know of anything that would back feed 9.5 volts or is that just the amount that is back feeding?
Other than walking around the car talking to myself, what should I do?
thanks
Scott

[Peter Mc Mahon]

Are the brake lights the only thing on this circuit?

[Ishmael]

The brake lights and some kind of connection to the computer.

[Peter Mc Mahon]

So you have voltage to the trans that signals not to shift because the brakes are on? Are the brake lights on? If they are not on then I assume there is no voltage there. If no voltage there, I would think you have another wire in there somewhere

[Sean Cooke]

Are your brake lights ground?

[Ishmael]

There isn`t an extra wire as far as I know. The brake lights work but I pulled them right out of the sockets and still got the back feed and the voltage didn`t change when I turned on the blinkers. Could they still need a better ground than their housing?

[Peter Mc Mahon]

If the voltage is always at the trans and therefore won't shift, your brake lights are always on. Is this the case?

[Ishmael]

No, tail lights are working properly.

[Peter Mc Mahon]

So the brake input is not your issue. How did you diagnose that a brake input is not allowing the trans to shift?

[Peter Mc Mahon]

The engine ecu also controls the trans? If you unplug this connection at the ecu and then use a voltmeter, is the power coming from the ecu or from the wire? I would think if this was a basic circuit you would be more like 12 volts

[Ishmael]

There is a brake switch with wires going back to the ECU/TCU. If it doesn't change values the trans doesn't shift.
If I unplug the computers that back feed goes away. I contacted the company that made the harness and we are making sure its for the right version of my trans.

[Peter Mc Mahon]

Back feed goes away" but measured where? I don't think the brake wire is your issue but simply unplug it at the switch and then retest

[Ishmael]

Turns out I have the wrong harness for my trans. 13 pin trans, 15 pin harness.