Transmission Range Sensor
The Neutral Safety Switch is used to prevent starter operation in all gears other than Park and Neutral. Voltage should be present at this switch when the ignition switch is turned to the start position. With any other gear position other than park or neutral (R, D, D1, D2) the starter circuit is open and voltage is prevented from engaging the starter motor.
If there is a no start condition this switch can be tested using a voltage meter. A technician would back probe the “voltage out” wire while someone turns the ignition. Check to see if battery voltage is coming out of the wires leading to the rest of the starter circuit in the Park and Neutral positions. If there is voltage coming in to this switch and no voltage coming out replace the neutral safety switch. The neutral safety switch often has the backup lamp incorporated in its design.
The Transmission Range Sensor is part of the transmission computer control system found on late model vehicles. It can be analog or digital. Both of these are best diagnosed with a scan tool. If one is unavailable the analog type can be diagnosed with an ohmmeter. The analog TR sensor is constructed of a series of resisters that reduce the voltage returned to the PCM according to the desired shift lever position.
A technician would measure the resistance between the appropriate connections and compare these with manufacturer specifications. Both the older neutral safety switch and the newer transmission range sensor will cause a no start condition as well as no back up lamp operation.
The TR sensor can affect several different systems and will set a transmission code if it is defective or misadjusted, example: "P0705 Transmission Range Sensor Circuit Malfunction". A misadjusted TR sensor can cause harsh shifting or confuse the PCM if the resistance is out of specifications. Both the neutral safety switch and the transmission range sensor will prevent starter operation.
