Foreign Service

Motor, Oct 2006 by Marinucci, Dan

A picture may be worth a thousand words, but it also can provoke a surprising number of questions. This month, Dan explains what's inside the box depicted in the August Foreign Service.

I showed you a homemade signal substitution box in the August 2006 column. This resistor box, which creates substitute cold and hot values for both the ECT and IAT sensors, enables you to run a Toyota evap monitor in the bay. Reader response to that story has been very gratifying. Because every respondent requested a resistor box schematic, I'm devoting this months column specifically to those inquiries. Let's go!

First, I did not include the actual wiring or the resistor box enclosure in the drawing below due to space limitations. Second, I divided the illustration into three simple sections-resistors for the ECT signal, resistors for the IAT signal and the double-pole, double-throw toggle switch. The resistor values were chosen because besides being correct, they're readily available at Radio Shack or other electronics stores.

Third, the illustration shows the pins of a common double-pole, double-throw toggle switch (also known as an ON-ON DPDT switch). The various brands of toggle switches usually don't share a common pin numbering system. Just remember that when you flip the toggle switch to the left (HOT coolant position), pin #1 is connected to the ECT pin while pin #3 is connected to the IAT pin. Flip the toggle switch to the right and it connects pin #2 to the ECT pin and pin #4 to the IAT pin.

Last but not least, both the ECT and IAT sensors have a two-pin wiring harness connector (refer to the photos on page 14 of the August 2006 issue). Unplug those connectors from the sensors. The two pins here labeled "TO ECT" go to the ECT harness connector; the pins labeled "TO IAT" go to the IAT harness connector.

Dan Marinucci

dmarinucci@motor.com