To see how to hook it all up, take a look at this circuit diagram.

How it works

As with most turbo-charged engines, the 2GT's boost level is controlled by a turbo wastegate.  As we all know, for wastegate-controlled turbocharging systems we can easily raise the boost level by using a small bleeder valve to "bleed off" some of the boost as seen by the wastegate actuator, thereby delaying the opening of the wastegate and allowing the turbo to generate more boost.

Believe it or not, the 2GT actually comes from the factory with a bleeder!  This bleeder is actually an electrically-controlled solenoid (called a boost-control solenoid, or BCS) and is controlled by the ECU.  Under normal conditions, the ECU allows the BCS to stay open.  With the BCS open, the maximum stock boost level is about 14psi.  By removing the little restrictor in the BCS (as most people do), we can increase the amount of air that is bled off by the BCS and gain about 1 or 2 psi of maximum boost.

As I mentioned earlier, the BCS is kept open by the ECU under normal conditions.  However, when the ECU detects excessive knock, it will close the BCS, thereby reducing the maximum boost level and protecting the engine.  A "knock LED" is simply an LED that lights up when the BCS is being closed by the ECU, thus giving us an indication of when the ECU is sensing too much knock since it's trying to limit the boost level!

The BCS is a ground-activated solenoid.  One end of it is connected to ignition (+12V as long as your key is in the "on" position), while the other end of it is switched by the ECU between 12V or 0V (ground).  When the ECU switches its BCS signal to ground, the BCS is open and higher boost is allowed.  When the ECU switches the BCS signal to 12V, the BCS is closed and boost level is reduced.  We want to build the knock LED such that it lights up when the ECU is closing the BCS (BCS signal is 12V), and the knock LED should stay off under normal conditions when the BCS signal is 0V.

How to build the knock LED

It's easy!  To build the knock LED you need:

A. any LED (you can get those from any Radio Shack)
B. a 600-ohm or greater resistor (you can also get this from any Radio Shack)

You'll need to be able to solder the resistor to the LED and then access the ECU to tap its BCS signal.  I won't waste space here to discuss how to access a 2GT ECU, but you can easily find this information by searching Tom Stangl's VFAQ page or the Club DSM website.  Refer to the circuit diagram mentioned at the beginning of this page if any of the following description doesn't seem clear to you.

1. Solder one end of the resistor to the anode (positive end) of the LED.  I recommend choosing a resistor that's between 600ohm to 1200ohm.  Keep in mind that the higher the resistance, the dimmer the LED will be because less current flows through the LED.

If you don't know which end is anode, do this simple test: solder the resistor to either lead of the LED.  Then, connect the other end of the LED to ground while connecting the unsoldered end of the resistor to battery 12V.  If the LED lights up, that means you got it right and you can proceed to the next step.  If the LED doesn't light up, remove the resistor and solder it to the other LED lead and re-do this test.  Don't worry, you won't damage the LED even if you connected it backwards.  Take a look at this diagram if you didn't understand the description above.

2. Now solder the cathode of the LED (the unsoldered, negative end) to ground (any good body ground will do), while tapping the unsoldered end of the resistor into the ECU's BCS output signal.  The BCS signal wire is a yellow-red wire and it should be the 11th pin from the left in the top row when looking into the 26-pin ECU connector (two rows of 13 pins each).  There are four ECU connectors, but only one has 26 pins total so you can't miss it.

That's it!  You can now put everything back together and enjoy your knock LED!

Expected behavior of the knock LED in a 2GT

The knock LED should flicker for a few seconds everytime you start the engine, and should stay on solid for a few seconds after you shut off the engine.  I theorize this is because the ECU is just setting the "default" or "fail-safe" condition of a closed BCS.  Otherwise, the LED should stay off.

However, the 2GT ECU seems to always cycle the BCS on and off rapidly when the engine speed is between 2400 - 3700 rpm regardless of load/throttle position.  You can see this as the knock LED rapidly fickers on and off in the said RPM range.  Don't worry, you're not getting knock just by blipping the throttle in neutral. 8^)  I suspect Mitsubishi engineers put this "feature" in the ECU code to soften the torque curve as boost kicks in, thereby reducing the sensation of that dreaded "turbo lag."  I know this is somewhat stupid for an enthusiast, but I guess for the general public this is a desirable feature.

In the 2-3 years since I've installed the knock LED, I've never seen it light up under any conditions (except in the RPM range mentioned above) until just very recently when I was at the racetrack.  After a few runs my car probably experienced enough knock that the ECU has decided to close the BCS (thus lighting up the knock LED).  This is easily confirmed by the fact that my boost is only limited to 10psi even though I have a manual boost controller (just a bleeder valve) and removed BCS restrictor.  The knock LED stayed on until I reset the ECU.

If your knock LED isn't behaving as described above, you may have made a mistake in one of the steps so please double-check that your resistor-LED combo will light up when connected to the battery, and that you have tapped the correct ECU wire.  You could always reset the ECU if you suspect it's already keeping the BCS closed too.

That's all, folks!

Warren Tsai
Last modified on: October 21, 2000