Transmission ECU – Brake Sense Circuit

GPI4 (Brake sense)

This circuit is located behind the center of the Ampseal connector. This circuit is built as a digital input, but we must bring the nominal 12V brake light voltage down to the 5V that the processor pin can handle (~6.5 Volts absolute maximum). R33 forms one leg of the voltage divider, and also limits the current through the Zener diode if the voltage is greater than 5.6 Volts.

  1. Install and solder a 15K Ohm, 1/8 Watt resistor {brown-green-orange, 15KEBK-ND} in R33.
  2. Install and solder a 10K Ohm, 1/8 Watt resistor {brown-black-orange, 10KEBK-ND} in R34.
  3. Install and solder a 5.6 Volt Zener diode {1N4734ADICT-ND} in place of C20, with the banded end towards heat sink. You will have to space it up off the board by about ½” (12mm) to get the leads to fit.

Thats you done!

Megashift brake sense circuit

12v brake sense circuit




If the input voltage is 14.5 Volts, the voltage divider will reduce this to 14.5 * 10K/(10K+15K) = 5.8 Volts, and the Zener diode further bleeds this off to 5.6 Volts (nominal) with a current of 14.5/15×103 = 0.97 milliamps (the Zener is rated for 1 Watt, which is much higher than we need which is ~1 milliamp * 0.2 Volts = 0.0002 Watts).

If the input voltage is 11.5 Volts, the voltage divider reduce this to 115 * 10/(10+15) = 4.6 Volts, and the Zener diode does nothing. 4.6 Volts is till plenty to trigger the input pin (the acceptable voltage range for ‘high’ on a HCS12 input pin is 3.25 Volts or higher), which means the supply voltage from the brake signal could drop as low as 8.1 Volts and still trigger the input successfully.


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