Gear Display.
4r70e/4r75w tranny play - page III
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Last updated Aug 17, 2019
The relative "complexity" of this circuit is incidental.

Displaying the gear selection on a 7 segment display
requires a 2-to-4 line decoder followed by a 4-to-7 line
decoder stage.

A 74138 was chosen for the former, utilizing two of it's 3
inputs, while a diode matrix decodes the display.

As wired, the circuit doesn't actually display the gear, so
much as the gear solenoid selection. P,R, and N are not
represented as there is no way to do so with the
command harness.

Relays were chosen because I had them, and negated
any need for voltage level translation to and from the
'138. This is not a high speed digital counting circuit,
and input switch bounce is of no concern.

Note that this particular large display had several LEDs
per segment in series and required 12 Volts to illuminate
.
It would have been possible to matrix decode a more
generic display directly from the outputs of the 138.

A future upgrade using the '138 C input and tapping the
shift lever selector switch on the driver' side of the
transmission will allow additional decoding to display
P,R, and N.

Frankly, I was just too lazy to crawl under the car again.

Circuit function is straightforward. The transmission shift
solenoids are internally common to switched ignition B+,
and are actuated "on" by an N channel FET in the ECU
which then also closes the input sensing relays, pulling
the '138 inputs low. As an example, with both solenoids
engaged, 4th gear is selected, so both the A and B
inputs on the '138 are low, switching Q1 low as well
which is decoded by the diode matrix to display the digit
"4".
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