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Wastegate solenoid fully explained

I've encountered quite a bit of confusion from several people about their results after doing the "Knock LED" mod. I've done some digging and encountered some info that should clear up the situation. However, I'd better define a few things first...

I'm going to assume you already know the point and purpose of a wastegate in a turbocharger system, and how it works.

The boost solenoid acts as a bleeder on the control pressure sent to the wastegate system. When this solenoid is open, the wastegate system "sees" less pressure in the system and doesn't open the wastegate as much as it would if the solenoid was closed. This causes more air to flow past the turbine wheel, which in turn compresses air at a faster rate, and thusly raises the boost pressure in the system.

Mitsubishi designed the wastegate actuator system so that on a totally stock car, boost pressure is about 7 psi. They added the bleeder to the system so that the boost pressure would be raised to 11 psi.

The normal operation of the ECU is to allow the car to run at the full 11 psi. However, when the ECU encounters severe knock for a long time (a possible indication of low-grade fuel), it will close the boost solenoid, chopping boost down to 7psi. This will give the engine a little bit of protection against severe knocking, as high boost simply aggravates that problem.

Knowing this, some people have added an LED that indicates when the solenoid is closed. They believe that this LED will give them an indication that the ECU is detecting a lot of knock. However, some people are reporting some funny results. For example, the LED blinks during startup, and can also always blink or be on solid whenever the car reaches a certain RPM range.

Q - Why does the LED blink? Why is it not just solid on or solid off?
If the ECU were to quickly chop the boost level from 11 psi to 7 psi rapidly, you would feel the car lurch a bit. So instead of shocking the engine, the ECU uses a technique called PWM to gently transition the valve from 100% open to 90% to 80% to... to 10% to 0%. PWM relies on the "averaging" of air to make it seem like a two-position valve that is open 50% of the time is the same as an infinite position valve (very expensive) open halfway. This rapid on-off is what shows up as blinking on the LED.

Q - Why does the LED blink at startup?
The solenoid defaults to closed when the ECU is off. When you turn it on, the solenoid transitions softly from closed to open.

Q - Why is the LED on, even at idle (when, supposedly, there is no knocking going on)?
There is a "long-term" knock term that tracks the long-term average of knock. When this term rises above a certain point, the ECU closes the solenoid. There is a specific set of conditions under which this term will get updated. Under high airflow (I.e., WOT) or cold engine conditions, this term is not updated. Therefore, the LED will not change state during these times. Thus, if you trigger the LED by driving around on a tank of bad gas, then shut off the engine, wait, and start the engine again after it is cold, the LED will reflect the state of knock left over by the previous bad-tank trip, and will be on.

Q - You just said the LED will not change state under WOT. However, my LED seems to blink all the time above 5000. How so?
Well, I've seen this complaint a lot, so I did a bit more digging in the code. Turns out, if the air flow (not boost level) exceeds a certain amount, the ECU will treat the situation the same as it does excessive knock - reduce the boost pressure. This reduces the air flow, so the ECU then opens the solenoid once again, increasing the boost pressure. It does this fast enough so that there is no oscillation, but a steady state is reached. This is the blinking you will see sometimes at higher RPM at WOT. If the LED stays steadily on above some RPM, you most likely removed the solenoid completely from pressure control system. The ECU desperately closes the solenoid permanently in vain - a steady state is never reached.

Todd Day today@dsm.org
Thu, 12 Jun 1997

This document was originally posted on the Talon Digest. It has been posted here with Todd Day's permission