April 2007 Articles & Features

Launch Tech


adjust text size: Foreign Service

By Dan Marinucci | April 2007

In the on-again, off-again digital world in which we live, you might be surprised at just how many problems a misadjusted closed-throttle switch can cause.

Many popular Nissans have a distinctive, two-connector throttle position sensor (TPS). Compared to a typical three-pin TPS, this design is unusual because it houses both a throttle sensor and throttle switches. Servicing it isn't very difficult, provided you learn how and allow ample time for the task. This month, I'll explain the key details of this multipurpose sensor.

The concept of a combination TPS and throttle switch may sound vaguely familiar. Indeed, Nissan used a combo TPS/throttle switch years ago. I covered it way back in the June 1996 Foreign Service column. The newer version, shown in photo 1 below, appeared from the mid-1990s until the early 2000s on a wide range of Nissan engines. Basically, both versions perform the same chores but are adjusted differently. The later version also carries extra diagnostic responsibilities.

The brown wiring harness connector usually identifies Nissan's traditional three-pin TPS. Typically, the middle terminal is the “throttle valve,” or sensor output signal going to the ECM. If the combo TPS/throttle switch is properly adjusted, the TPS signal at closed throttle is usually about .50 volt. But going by the book, the normal range is .30 to .70 volt at closed throttle.

A gray harness connector identifies the throttle switch. Typically, when the ignition switch is in the START or RUN position, it applies battery voltage to the middle pin of the gray connector. The closed-throttle and wide-open throttle switches are the other two pins in the gray connector. Floor it and the wide-open throttle switch should close, applying battery voltage to that terminal. So when you mash the “loud” pedal, the wide-open throttle signal should change from 0 to battery voltage.

The closed-throttle switch is the more important of the two. As its name suggests, this switch is closed at closed throttle and opens soon after the throttle blade does. Another way to look at it is that the closed-throttle switch closes during deceleration when the throttle is very nearly closed. So, voltage at the closed-throttle switch terminal is battery voltage at idle and 0 when you crack open the throttle blade slightly.

You may need to look in the automatic transmission section of your shop manual to find a detailed schematic of the closed-throttle switch.
Also, note that the DTC for the closed-throttle switch is P0510.

The ECM monitors the minimum TPS voltage signal and then bases a calculated idle value called “soft idle switch” on it. Predictably, the soft idle switch value influences all of the ECM's decisions on anything related to idle position. Because soft idle switch is a learned value, the ECM has to relearn it anytime the TPS position is changed.

Traditionally, the closed-throttle position switch provided a backup closed-throttle signal to the ECM if the TPS failed. But after OBD II came on board, the closed-throttle switch became responsible for activating the self-diagnostic process for some trouble codes. So, a faulty, worn or misadjusted closed-throttle switch can trigger such erroneous trouble codes as transmission gear function (P0731-P0734), closed-throttle switch (P0510) and/or TPS (P0120-P0123)! Furthermore, it can cause a high idle speed or prevent OBD II monitors from running.

Therefore, I see lots of incentive to adjust the combo TPS/throttle switch carefully. First, be sure the engine is at operating temperature and the throttle blade is closed. Then identify the closed-throttle terminal in the gray harness connector. As I explained earlier, this switch should be closed when the throttle is closed and then open almost immediately when you crack the throttle blade.

Over the years, Nissan adopted the same technique Toyota did (refer back to the March 1997 column) to adjust a combo TPS/throttle switch�a pair of feeler gauges. That is, first slide a specified feeler gauge between the throttle lever and throttle stop screw. In photo 2, I substituted a piece of white cardboard for the feeler gauge. The throttle switch should be closed. Then remove the first feeler gauge and slide the specified thicker feeler gauge in there and the closed-throttle switch should open. Patiently tweak the combo TPS/throttle switch adjustment accordingly.

Here's an example of how touchy this adjustment is. I shot the photos for this month's topic on a Maxima. The specs for this car state that the closed-throttle switch must be closed when you slide a .012-in. feeler gauge into the throttle stop. The switch should open when you slide in a .016-in. feeler gauge. That's only a .004-in. spread between switch-open and switch-closed states! Always repeat the feeler-gauge test at least two or three times to make sure the adjustment is correct before you cinch down those screws.

Remember, many technicians have caused themselves untold grief by assuming that setting the combo assembly to a TPS signal of .50 volt at closed throttle is adequate. Unfortunately, experience shows that shooting for this common TPS value at closed throttle will not ensure that the throttle switch adjustment is accurate. Don't shortcut this procedure.

Some techs monitor closed-throttle switch operation during this adjustment with a scan tool. Depending on the vehicle model year, some scan tools may not show the closed-throttle switch value. Other techs backprobe the switch terminal in the gray harness connector and do the job with a common digital voltmeter. Still others perform a continuity test between the closed-throttle terminal and the middle terminal.

If you've replaced the combo TPS/throttle switch assembly or simply adjusted it, play it safe by relearning or resetting the TPS idle position memory. To do this, warm the engine to normal operating temperature, then shut off the ignition switch. Next, turn the ignition switch on, engine off and pause for at least five seconds. Finally, turn the ignition switch off again and wait at least 10 seconds. Now your scan tool display for the closed-throttle switch should read ON or CLOSED.

Repeat the ignition switch sequence of five seconds on, 10 seconds off until the scanner display confirms that the ECM believes the closed-throttle switch is really closed.