Summary:

The 100C/V (0-11V) output of the [EGT amplifier] can be used with some care, if the ECU was not ordered with the sufficient number of EGT inputs or analog inputs without pullup. See "GOOD granularity" below.

----

When ECU is purchased with 1,2 or more (4 or 6 is sometimes requested, some logger was ordered and delivered with 8 EGT inputs),

* EGT amplifier outputs are connected to inputs with no pullup

Confirmed:

* after the home-install, the cause of the higher reading was the internal ecu pull up

** 51K (really ? Surprisingly weak pullup. Which input ? Usually strong, 2k7 pullup on analog inputs, or no pullup at all.)

----

Unfortunately NO input with no pullup resistor in this ECU.

* is there a way to scale it mathematically based on the pull up?

** yes, since the function is monotone, it is possible to calculate. However...

Rough granularity/resolution

The EGT amplifier (internal voltage divider for convenience =) 233C/V (= also called "5V output") has an output protection equivalent to

* 1 / (1/75+1/100) = 42.86 k Ohm resistance

** 2.7 / ( 2.7+42.86) = 0.0593 ... this means 2k7 pullup causes a strong bias, and resolution is reduced

* say 20C => 1102 C (at 0.02V measured on the 0-11V output )

* say 1170C => 1170C (at 5V)

** The theoretical formula for that case is:

** EGTconv1=1170 - (1170-20)/(1170 - 1102) * (1170 - EGT)

** EGTconv1=1170 - (1170-20)/(1170 - 1102) * (1170 - 1147) = 781 C for example

** Note: this is rough estimate, not based on actual measurement or calibration.

** even if calibrated, the granularity would be rather rough.

Best to use input with no pullup.

* input with 2k7 pullup will likely make the EGT reading granularity annoyingly rough, above.

----

Achieving GOOD granularity, using the 100C/V (= also called 0-11V) output and 2 resistor voltage divider:

* reasonable formula (VT ini can be tweaked to output real calibrated result) becomes possible if

Connecting the analog input (with internal 2k7 pullup) via/to:

* 5.3k to the 100C/V (pin7: 0-11V) output of the EGT amplifier (since the traditional EGT amplifier has 1k resistor on pin7 100C/V output: 1k+5.3k = 6.3 kOhm in the calculation).

* 4k7 "pulldown" to GND

This acts like 0-11V => 2.5 - 5V output divider

* note: 6k3 and 4k7 effective resistance: 2k7=1/(1/6.3 +1/4.7) which means that half of the voltage range is available, which allows precise measurement after simple calibration.

** Something like 2*EGT-1170 conversion will be sufficient (refine it at least with the low temperature reading) and good granularity:

** 0C => 585C (2.5V on input) => 0C

** 1170C => 1170C (5V) => 1170C

Needless to say: take care when using the 100C/V output, do NOT short to GND or 12V (only connect to the 1 end of the 5.3k resistor, use heatshrink, do not leave it uninsulated).

* when using a DVM to measure voltage of the 0-11V output, make sure the DVM is in DC voltage mode (not current mode, which shorts the signal to GND, and might cause damage)

----

Orig report:

Engine off, ambient sensor temp (~20C)

* Tested Pin7 DVM output (1V/100°C) and getting 0.7 volts

** corresponding to 70C

** not 6 volts, roughly corresponding to Pin6 2.5 volts.

* I'm getting 2.5 volts out of Pin6 (0-5V output, 1V/233.3C)

** Thus getting 580°C instead of 20°C.

Hint: The output is almost certainly connected to an input with pullup (possibly strong pullup).

* Diagnostic: disconnect anything from output, and measure DC voltage with DVM (the DVM has 1 to 10MOhm input impedance).

* Solution: make sure to connect pin6 output to an input without pullup resistor

Sensor connection (seems good):

EGT sensor is K type yellow-red wires:

** yellow = green (plus wire) to board GND

** red = white (minus wire) to the board cathode on corner