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How do i find the resistance value of an internal pullup when its resistance is unknown? Is there a way i can measure it without opening up the ecu?
Sure, first figure out what voltage the pullup is attached to. Leave the input disconnected, and see what the ECU measures as the voltage. This is the Reference Voltage (Vref)
Next, put a fixed resistor of a known value (FixedR ohms) between the input and the sensor ground. I suggest you choose something close to a typical pullup value (say 1-2K). See what the ECU measures as voltage (V).
V = Vref * FixedR / (FixedR+PullupR)
This can be re-written as:
FixedR + PullupR = V / (Vref * FixedR)
And to get just the PullupR:
PullupR = (V / (Vref * FixedR)) - FixedR
Plug in your V, Vref, and FixedR and you can calculate the PullupR