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Discussion Starter · #1 ·
Trying to wire in a TACH out of my SXE10 3SGE ECU. To my understanding pin D26 is the TACH output pin. I can put a multimeeter on it, set to AC, and i get a constant 14.4V.

When revving the engine I see no change in Voltage. Is the ECU pin dead? Am I missing something?
 

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I'm no expert, but:

1. According to this, which is allegedly an Altezza/3sGE ecu pinout, you've got the correct pin (D26)

https://www.sq-engineering.com/wp-content/uploads/2018/11/3S-GE_Wiring_Diagram.pdf

2. According to that document, you should expect to see a 16V waveform while idling

3. Is your multimeter capable of displaying a waveform? Most multimeters will display a static RMS (root mean square) value of a waveform, which is why your meter will display a steady 120V if you probe your home's electrical outfit. It wouldn't surprise me in the least the 16V peak-to-peak waveform from the tach output might be about 14.4V RMS on a typical multimeter, though I would expect it to change at least some when you rev the engine.

4. Which leads me to suggest you use an oscilloscope to monitor that signal...

Again, I'm no expert... I'm just musing.

On a slightly unrelated note - are you trying to get OEM tach in the instrument cluster to work, or an aftermarket one?

EDIT: Just spent a few minutes educating myself more about RMS stuff (I'm a gears/springs/levers engineer, not an electrons guy). Anyway, RMS voltage will always be 0.707 times the peak voltage, and peak voltage will always be 1.414 times RMS voltage. Additionally, frequency does not effect RMS reading - so that could potentially explain why the multimeter reading does not change with respect to engine speed. However, your reading of 14.4V implies a peak voltage of 14.4V RMS * 1.414 = 20.36V peak, which is a bit higher than the 16V the schematic indicates. But this all pre-supposes a sinusoidal waveform and now that I think of it, your waveform is likely a square wave...

I still say you need to put a scope on that signal...
 

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Discussion Starter · #3 ·
The engine itself is on an 86 Toyota Cressida, ive got a Dakota Digital box in it and am trying to wire it to the OEM cluster. Ive got an output voltage running the dash and the TACH signal.
It makes sense the multimeeter cant read the waveform. The Dakota Digital outputs about a 20V signal through the input. Im no wiring expert in this area so its difficult
 
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