Strange Aux Voltage input reading

[NME308]

Hi guys,
I am totally new to motec, and have an M4 which I had sent to the dealer for latest firmware and checking over.

The issue I have is the Aux Voltage input is showing 1242 or thereabouts out of 1250 when not even connected. The Aux Voltage is setup as function 10 for 0 – 1250 linear reading, and I want to use a relay to send 5v from the ECU 5v line activated by a button for boost control.

The setup is a twin turbo 350 chev in a HQ holden mainly aimed at drag racing. My intention is the transbrake button will activate the Aux Voltage as above and restrict the boost to 5psi until trasbrake release at launch where the normal boost control parameters will take over again.

Back to the 1242 input showing with no connection again… when the transbrake button is activated the reading jumps up to the expected 1250. Even with the relay removed I still have the 1242 input reading.

Is there something I may be doing wrong, or some other feature I am using that may be interfering???

I have Aux Output 1 doing boost control, Aux output 2 doing Aux Table (actually to start methanol injection pump), Aux Temp connected to a K-type thermocouple, and Aux Voltage as above.

Hopefully it is just something dumb I have missed… Any suggestions?

Cheers,
Jason

[DarrenR]

Hi Jason,

I'm not sure i understand your setup fully, but it sounds like you are switching +5V via a relay to the AuxV pin. With no load on the pin the voltage floats when the relay is open giving you the reading 1242. There are two solutions for this.
You can leave your setup as it is and just add a resistor (1kOhm) from the AuxV pin to 0V. This will pull the voltage down when the relay is open giving a reading close to 0, and still 1250 when the relay is closed.

The other (better) option is assuming you are using a Thermo Couple Amp. You can move this to the AuxV pin and connect the relay to the AuxT pin and switch to 0V, not +5V. The internal pullup resistor on the AuxT pin pulls the voltage to +5V when the relay is open, and 0V when it is closed giving a reading of 1250 and 0 respectively. You will also have to change the sensor setup and boost comp to suit.

[NME308]

Thanks heaps DarrenR,
Option 1 has instantly made the reading show about 55 or thereabouts before the transbrake button is depressed dragging it straight up to 1250!
I have used pull up resistors heaps of times before... just didn't think to reverse the concept to a pull down resistor...

Cheers,
Jason