Hummer H3T Auto Trans Brake Shift Interlock Solenoid Replacement Cost

Automatic transmissions have a failsafe built into the shifting mechanism that prevents the transmission from being shifted into gear without the brake being pressed by the driver. This is called the brake shift interlock system. This system also prevents the shifter from being moved out of park when the key is not inserted, or when the anti theft system has been alerted. This is done by using a solenoid on the range selector (shifter) which, in one position, allows the shifter to move, but will not allow it to move in the other position.

The solenoid is actuated by two things. The brake light switch, which detects when the brakes are pressed, and the ignition switch or anti theft system. As a system, these three components work together to cause the brake shift interlock solenoid to remain in the locking position or move to allow the shifter to move. When the brake pedal is pressed and the key is on, you may hear a little thump or light metallic sound near the gear selector, and this is the solenoid moving so you can take the vehicle out of park. These solenoids are electronically actuated, and move by extending or retracting a push rod in a linear motion.

There are two possible symptoms that the brake shift interlock solenoid can have, but know that these symptoms may be intermittent or occur under different conditions. IF the solenoid fails to extend or retract, the result will be that the gear selector will be able to move freely at all times, or the gear selector will be stuck in park. This failure may be due to a faulty brake shift interlock solenoid, but there are other concerns.

Depending on the year make and model, failure of this solenoid is not an immediate concern except for convenience. It is in the best interest of the vehicle owner to have this repaired immediately as it is part of both the safety and anti theft systems of the vehicle, but there are ways of operating the vehicle regardless of the condition of this solenoid. However, if there is no manual override for the brake shift interlock solenoid it will need to be replaced before the vehicle can function.

These solenoids will fail on a fair amount of vehicles. The majority will never have a problem, and many others will be due to a brake light switch failure. For this reason, it is difficult to estimate how often the brake shift interlock solenoid will fail, but considering the amount of use this solenoid endures, failure around or after 100,000 miles would be no surprise.

When diagnosing a concern of inability to remove the shifter from park, the technician will first ensure there is no issue with installing, turning, or removing the ignition key, and that the brake lights are functioning properly. Then, the technician will listen for the solenoid to make noise upon pressing the brake pedal, signifying that is is either moving or not. If nothing is heard, the technician will check fuses, if any, and check for power and ground at the solenoid. If the solenoid is receiving proper voltage and ground but fails to operate, the technician will remove, bench test, and replace the solenoid as needed.

Removal of the solenoid may involve removing a trim panel and a few fasteners, or it may require that the gear selector surround trim, console trim panel, and gear selector be removed for access. Once access is available, the solenoid will be removed from electrical connections, and linkages that are connected to the solenoid pushrod, and finally removed after disconnecting fasteners. Replacement is the reverse of removal, and the solenoid can be tested through normal operation of the gear selector and brake pedal.

Even if the brake lights work, we recommend testing the brake light switch if the brake shift interlock solenoid is not receiving power. This is because the brake light switch may have a secondary or tertiary circuit for cruise control or the brake shift interlock system. We also recommend replacing any worn bushings in the shifter assembly while it is removed or accessible. These bushings wear out and can cause failure to engage in any drive range, including park.

When no power is present to the brake shift interlock solenoid, this may not be due to a component failure, but the vehicle anti theft system could be locking the shifter to prevent operation. If the security warning light is on, flashing, or otherwise indicating it is active, consulting a technician is the preferred next step.

As this repair can range from simple to intricate, it is of importance to review the steps needed to perform this repair for the specific vehicle with a defective solenoid. This will prevent the DIY mechanic from getting too far into a job before realizing they cannot complete the repair. In most cases, this is a simple repair and an easy diagnosis as long as simple electrical circuits are understood.