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The Orange wire goes from the ECU to the Boost Solenoid (BS)
The Red Wire at the BS has 12v when the ignition is on
In stock form, the BS works this way: (correct me if I'm wrong) - When no knock is detected, pressure is bled off from the boost solenoid to the atmosphere (hiding the "true" pressure from the Wastegate (WG) Actuator). When knock IS detected, no pressure is bled to the atmosphere, increasing the pressure "signal" to the WG Actuator, signaling the WG to open, reducing boost and eliminating knock.
When the BS is disconnected (not in use), the WG ALWAYS operates like knock has been detected. Manual or Electronic Boost controllers easily compensate for this.
The Orange wire is NOT grounding when knock is detected as one might expect. This circuit actually works the Opposite direction! The ECU grounds the Orange wire when there is NO Knock.
This paragraph is Very-Important!!! You Absolutely MUST have some sort of resistance between the Red Wire and the Orange wire at the BS Connector to limit the current to the LED and to limit the current that will be grounded when the ECU grounds the orange wire! This resistance can be in either the form of the BS (with no vacuum lines connected - just hidden away), OR you can use a 470 - 570 Ohm 1/2 Watt resistor plugged into the BS connector... Do NOT skip this step, you COULD damage the ECU!! No resistor will cause a short circuit with a traditional LED, will at best case blow the LED, at worst case, blow the ECU!
When the ECU is grounding the Orange wire (normal condition), All 12 volts from the Red wire are "used up" by the resistor (again this could be done with the BS or a separate resistor). In this state, the LED sees 2 grounds (one on each lead) so it doesn't light up. When knock is detected though, the circuit changes! In this state, the orange wire is switched off by the ECU, the current from the Red wire has no place to go, so it searches for the next closest ground which is on the other side of the LED - lighting the LED on the way.
In my car, I am using a High Intensity LED on the Orange wire, grounding the other lead from the LED. I have removed my BS and replaced it with a 510 Ohm resistor with a couple of spade-lugs soldered to it so that it plugs into the BS connector. Here are the Radio Shaft P/Ns for these two products.
My LED is bright enough to see during the day without blinding me at night. I have it mounted beneath my top gauge in my Dual A-Pillar gauge mount.
How to tell if things are working properly: When you start the car, the LED flashes quickly for a few seconds and goes out. When you turn the key back off, the LED will stay on solid for about 5 seconds until you hear the ECU click off.
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Page updated March 21, 1997 - Damian Sigman