myf16

1990 LS400 with no options.

The trans was acting weird. High rpm but poor acceleration from low speeds. I think it was stuck in high gear.

Brake light wires on left trunk hinge had failed a few days before and I had not yet repaired them.

The two problems were related!!

I fixed the broken wires but the shifting problem persisted. I opened the wire repair and found there was no 12 volts on the brake wires with the brake pedal pressed. Aha! I checked the wiring diagram. If the other brake lights work but the trunk lights do not, it must be a bad brake light failure sensor box.

I got another box for $15 on ebay. Plugged it in and the brake light worked. And the shifting was back to normal.

I can't explain why the trans cares about the brake light failure sensor, but it very much does.

dicer

Where is that box? And what year car is it?

myf16

1990. The box is beneath the front lining of the trunk at the extreme left side. It has one connector about 1.5 inches by 0.5 inches. All brake light wires pass through this box. Inside the box is an integrated circuit and three calibrated low resistance hoops, used to compare to the resistance presented by the different brake light sub-circuits and the tail light circuit. I must have blown out the integrated circuit because no damage was visible.