The map sensor detects a lower than expected value thats why the chipit unit eliminates the codes when using the blanking plate.
How does a blanking plate trigger the check light is the $1M Question
It appears from the diagram that Cowboy Dave provided the system has a EGR valve position sensor: The EGR valve position sensor ( EVPS ) detect the exact position of the EGR valve and send the information to the ECM, from this data, the computer can calculate the optimum EGR flow for the lowest NOx emissions and the best driveability, then control the EGR valve to alter the EGR flow through the EGR solenoid. The EVP sensor is a linear potentiometer that operates very much like a TPS, its electrical resistance changes in direct proportion to the movement of the EGR valve stem. when the EGR valve is closed, the EVP sensor registers maximum resistance; as the valve opens, resistance decreases untill it finally reaches a minimum value when the EGR valve is fully open.
So what is needed is like the CHIPIT unit a V in and a resistance out box Hey CHIPIT can you free up a third chanel analogue in analogue out?
Hint here is how to make a cheap unit buy a YM2760 servo motor from Jaycar affix a pot to it to emulate the EVPS helicopter7 have you measured the resistance of the EVPS I guess this would vary from 0 to 1K the servo may have to be run with a resistor on the negative if it moves too fast ie need to match the response time of the egr dc motor to the dc motor on the servo may even have to use the bigger motor if the ecu has current sensing to detect a blown DC motor. Anyway please ask questions and I will try to answer them. If the S.A. boys want to get together for an R&D session I will be in that too.
Yes, I believe you are correct about the position sensor, and I also assumed this from the information I read as provided by cowboydave.
Problem is I did not see this behaviour when I applied a DC voltage across pins 4 & 5. I could hear the EGR open, which took about 1 second and then everything stopped. Even if I increased the voltage nothing else appeared to be happening.
While the EGR valve was held open, I had also applied +5V across pins 2 & 3 while monitoring pin 1. The voltage on pin 1 [sense voltage back to the ECU] just went from + 4.5V down to +0.5v when the valve closed. There did not appear to be a 'middle point' or a variable voltage based on EGR position. It was simple, with voltage applied the EGR opened and closed when the voltage was removed.
Another possibility is that the ECU pulses the EGR to hold it in some intermediate position, which then allows for a variable voltage to appear on pin 1.
Now having said all of this, you did mentioned that:
"The map sensor detects a lower than expected value thats why the chipit unit eliminates the codes when using the blanking plate. perhaps the check engine light is triggered"
If this is true, then there is no way to fool the ECU [apart from using the chipit] by the method I envisaged as even if I design a circuit to emulate the EVP sensor inside the EGR valve, the map sensor will still be there to trigger a fault regardless.
So I guess I'm wasting my time but appreciate a more in-depth understanding as I was stumped to see what triggers the engine light when the EGR gases are blocked.
Interestingly, after the engine light was triggered in the morning, I plugged the EGR back in some time later but the light remained on all day and night, even after a few stop/starts of the engine. This morning, started the car and you guessed it no more check light.
Go figure but it seems to happen when starting the engine from dead cold.