Transmission Range Sensor
The Neutral Safety Switch and the TR sensor are protection devices used to prevent starter operation in all gears other than Park and Neutral. The TR sensor is a member of the starter control circuit and informs the TCM of the current gear selection. Use a high impedance 10 megohms digital multimeter to check voltage and resistance on the TR sensor. 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’s 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’s 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 sensor can be diagnosed with an ohmmeter. An 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 using an ohmmeter 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.