Why Won’t My Car Shift Gears?

Like a manual transmission with a bad clutch, a driver with an automatic transmission may have trouble getting their car out of "Park" or into gear.

Wondering why your car is having a hard time shifting gears? Here’s a little history behind automatic transmissions (AT) as well as some possible reasons behind why your car is not shifting properly.

History Behind Automatic Transmissions

The first true fully automatic transmissions made their debut in Oldsmobiles in 1948. An AT performs the same functions as a manual transmission, but shifts gears by applying and releasing clutch packs and band assemblies automatically. That allows drivers to keep both hands on the wheel, instead of one on the wheel and one on the stick shift.

Today, few drivers rely on a stick anymore. According to a leading American online automotive resource information company, in 2016 more than 96 percent of all cars sold in the U.S. came with automatic transmissions.

How Does a Car Shift Into Gear?

When shifting an AT into gear, a valve body routes pressurized automatic transmission fluid (ATF) to internal bands and clutch packs that lock and unlock ingenious planetary gear sets. Transmission gear and gear ratios are determined by which parts of the gear sets are free-wheeling or being held stationary by the bands or clutch packs. An output shaft transfers power from the gear sets to the driveshaft and ultimately to the wheels.

Why Won’t My Car Shift Into Gear?

For an AT to shift into gear, several hydraulic electronic and mechanical systems need to work together seamlessly. When one or more of these systems fails, your car will not go into gear. To help determine whether your vehicle requires attention from a specialist, let’s look at some reasons why your car refuses to go into gear.

Transmission fluid

An AT with dirty or low fluid will not shift into gear. ATF should be a light red/pinkish color. Dirty ATF (deep red to dark brown) loses its ability to properly lubricate and cool internal parts, causing excess wear and premature part failure.

Check your owner’s manual to locate the transmission dipstick and the specific ATF for your make, model and year. With the AT in Park, engine running and warmed up, the AFT level should be between the “full” and “add” mark on the dipstick. Add new ATF using a clean funnel if it’s low.

If your car goes into gear after adding ATF, then your transmission was low on fluid, most likely due to a leak. Your next stop should be your repair shop so your mechanic can check for leaks. This would also be a good time to flush the ATF and replace the filter.

Electronic sensor

A failed electronic AT selector switch cannot signal the computer (ECM) that you placed your car in gear, keeping shift solenoids that move the valves in the valve body from operating. Moving the shifter slowly from Park to Low several times may help remove corrosion from the switch’s contacts and get you on your way. Leave transmission electronic diagnostics to transmission specialists.

Gears

Gear sets that are misaligned, damaged or stripped-out will prevent gear sets from smoothly engaging and will keep a vehicle from going into gear. Caused by worn gears, failed or failing bearings or internal seal or O-ring leaks, gears grinding against each other when putting your car into gear will be noisy, while also creating metal dust and chips that will damage other transmission parts.

Leave internal transmission repairs to the pros. To prevent future transmission problems, insist that your mechanic thoroughly flushes the transmission case, valve body and radiator cooler with solvent as part of any transmission repair.

Linkage

Misadjusted, damaged or stretched shift linkage or cables can keep an AT from going into gear when moving the shifter. Check your owner’s manual, because some vehicles have shift cables that are adjustable. Adjusting a cable is a one-time fix you can make before taking your car in for service.

BE AWARE: AT vehicles have a Park-Neutral safety switch that prevents a car from starting while in gear. A cable too far out of adjustment will permit a car to start while in gear, causing it to lunge forward or backward.

Other causes

Is the engine running? Is your foot on the brake? Is the battery fully charged? Is the Check Engine Light (CEL) on? Sorry, I have to ask.

A faulty charging system can lead to a weak or dead battery and can keep the shift interlock from disengaging or shift solenoids from engaging. Depending on what is causing the CEL to illuminate, that can keep the ECM from operating the shift solenoids and keep an AT from shifting gears. Have your mechanic check if any of these are preventing your car from starting or shift into gear.

Also, parking on a steep incline can jam the lock pin against the gears that keep your car from rolling away when in Park, making it difficult to move the shifter. Rocking back and forth in the front seat should release pressure off the pin. In cold temperatures, let the car warm up before shifting into gear. Automatic transmission fluid may need to heat up before it flows properly (or warm up a hardened seal) especially when it’s cold outside.

What To Do if the Shifter Won’t Move or Your Car Won’t Start

Malfunctioning shift interlock

Modern cars with ATs incorporate a shift interlock safety system. Working in conjunction with the brake light and Park-Neutral safety switches, an electrically controlled solenoid locks the shifter in Park. Stepping on the brake pedal and placing the ignition in the Run or On position unlocks the shifter.

While a failed shift interlock solenoid or out of adjustment/defective brake light switch can keep your car from shifting out of Park, a defective Park-Neutral safety switch can keep your car from starting. You can try disengaging the interlock solenoid by releasing the shift lock release switch. Usually found near the shifter, check your owner’s manual for shift lock release instructions. If this doesn’t work, you’ll need to call for a tow.

Automatic Transmission Jerks When Shifting Into Gears

Low or dirty ATF, slipping clutch packs and bands, or weak or broken springs that cushion/dampen clutch pack and band engagement will cause an AT to jerk when put into or shifting gears. An engine running rough due to an ignition problem can also cause jerking when shifting into gear. An AT specialist has the tools to diagnose these problems.

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Robert Lacivita
Bob Lacivita is an award-winning auto technician and career and technical educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics, as well as writing state, federal and organizational foundation grants, and helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into technical/vocational training, for more than 20 years. His work has been featured in Family Handyman, a Reader's Digest book and Classic Bike Rider magazine, among others. Bob and his wife lived through 20 years' worth of DIY home remodeling while parenting two (now grown) boys and now relax by watching their three fabulous granddaughters.