Moving the gear shift from park to drive or reverse lets you move the car when you’re ready, but what happens if you bump the shifter accidentally and aren’t ready to roll? In older cars, this was a very real problem. However, newer models don't have that problem thanks to the shifter interlock solenoid.
Basically, the shifter interlock solenoid is a switch designed to lock your transmission into park unless the brake pedal is pushed. If you try to shift your car out of park without also holding the brake pedal down, you’ll find that it won’t move. This is a safety precaution and is installed on all automatic transmission cars manufactured in the last few decades (classic cars don’t have this feature).
The shifter interlock solenoid should last for the life of your vehicle, but premature failure is not that uncommon. When it happens, the shifter won’t move out of park, even with your foot on the brake. As a note, the same thing will happen if your brake light switch fails, or if something happens to your brake light circuit. It’s illegal and very unsafe to drive without brake lights, so a safeguard has been built into your car to prevent that from happening (note that this won’t do anything if the brake light bulb burns out – it only applies to the circuit itself). If the brake light switch fails (and it does happen), then the shifter interlock solenoid will prevent you from putting the car in drive.
Given the importance of the shifter interlock solenoid, it’s important that you know the signs to watch for that may indicate it’s about to fail, or has already given up the ghost. These include:
Shifter won’t move out of park (can indicate a failed solenoid, or a failed brake light switch)
Shifter is hard to move out of park (indicates a problem with the solenoid, but not complete failure)
You have to use the interlock bypass to get the transmission out of park
If you suspect that your shifter interlock solenoid is about to fail, or has already failed, the mobile mechanics at YourMechanic can help. A mechanic will come to your location, inspect and replace the shifter interlock solenoid, the brake light switch and other components if needed.
This article originally appeared on YourMechanic.com as How Long Does a Shifter Interlock Solenoid Last? and was authored by Valerie Johnston.