I'm searching for some way to manipulate a feedback signal from an engine ECU. The signal is AC at 2000hz and varying in voltage from approx. 2 to 5 VAC according to how the engine behaves. I wan't to lower the voltage from the feedback signal using a resistor.
A 300 ohm resistor connected to the signal and ground gives me the feedback signal i want. But i want to vary the feedback signal by controlling how much current that flow through the resistor instead of just switching it on/off with a relay.
What i have tried so far:
-Controlling a transitor by Arduino PWM and connectet to the feedback signal. 100% PWM signal works fine, but every value between 0 and 100% messes up the signal.
Why i think this goes wrong:
-Unfortunately i haven't got access to a oscilloscope at the moment, but i think the transistor is messing up the AC signal, as only positive or negative current will flow to the 300 ohm resistor depending on what transistor i use (NPN or PNP) Furthermore i think the stock 490HZ Arduino PWM signal messes up my 2000HZ feedback signal.
What i need:
Some component or way to couple two transistors so that i can vary AC current flow. I've read about Triax and SRC but from what i understand they both continue to lead current even after gate voltage have dropped.
I hope some of you clever people can lead me in the right direction.
Thanks
If you have the base of the transistor connected to PWM you are turning it on and off at the PWM frequency. At 100% duty cycle you get your signal but any other duty cycle your signal with the PWM on top. Maybe a digital potentiometer would be better. Can be controlled by !2C or SPI and will only chang the voltage not frequency.
nonosusno:
i basically just want to PWM control an AC signal with the Arduino. But the signal must stay at 2KHz and only vary in voltage.
That dosn't make much sense. PWM, is Pulse Width Modulation, it is already an AC signal. If you apply a PWM to your signal you will get beating and the result will be a mess.
A digital pot sounds the right idea but watch for the maximum current rating.
A 300 ohm resistor connected to the signal and ground gives me the feedback signal i want.
From this I understand that you have used a 300Ohm resistor to load down the output of the ECU, you have got your desired level by pulling more current from the ECU than it was possibly designed for, that's a no no and could cost you LOTS of money, an ECU is an expensive, complicated device.
You have not told us what your desired levels are.
I gather what you are doing is breaking into a feedback line and you want to change its level and feed it back into the line.
A potential divider would do the trick, but we would need to know the full story.
So again, DO NOT load the signal down the way you have.
Without knowing the source impedance and maximum load ratings of the ECU you need to use
a high impedance voltage divider to reduce your signal, resistors in the tens of K or more would
be safe with any conceivable output driver stage.
Anyway how about some actual concrete details - which ECU? which signal from the ECU? is there
a datasheet or manufacturer's data about this thing?
Okay if you want the details about this project then read along:
I have a VW Passat with an 1.9 TDI AFN engine which i want to increase the horsepower and torque of. I know the proper way is to flash the ECU with new maps, but i haven't got the equipment for this.
My tuning idea is instead based on fooling feedback signals to the engine ECU to increase injected fuel and increase boost pressure.
The amount of fuel injected in the engine is calculated from engine load, intake air flow, boost pressure and other parameters. The ECU regulate the fuel quantity adjuster according to the calculated value. To ensure the quantity adjuster has moved correctly, a piston movement sensor gives feedback to the ECU about the exact position. If the feedback value from the sensor is lowered, the ECU thinks the piston hasn't moved enough, and the moves it further which increases the amount of injected fuel.
Page 7 in this PDF describes the piston movement sensor called G149 -> http://pics.tdiclub.com/data/529/13523tdi-technik-eng.pdf
The sensor is connected with three wires Pin 1,2 and 3 to the ECU. As i haven't got access to a scope at the moment, all i know is that two of the wires (pin 1 and 3) are AC signals. They are both 2000Hz, one stays at the same voltage all the time, the other changes in voltage according to piston movement. I don't know what pin 2 are, but if i connect a 300 Ohm resistor between pin 2 and 3 the voltage drops, and the ECU is fooled and increases injection amount. All i want is to be able to regulate how much more fuel that should be injected. Connecting the 300 Ohm resistor is kinda on/off, so i want to regulate it to get a smooth increase in fuel quantity as the accelerator pedal is pressed.
Increasing the boost pressure is not as complicated, because the feedback consist of a DC signal.
Loading it down is the wrong way to do it, as pointed out, for the reasons given.
Using a high impedance voltage divider with a digital potentiometer, possibly followed by an Op Amp is the right way, but without knowing the exact composition of the signal, we are limited in the advice we can give. Does the signal only go from ground to some positive voltage? Or does it go below ground? Based on the limited information given, it will need a driver circuit (Op Amp or similar) and I'm guessing it is a full AC signal, but it could also be pulsating DC.
