Why is My Automatic Car Not Shifting Gears?

Is your automatic transmission shifting poorly or not shifting at all?  Modern cars utilize sophisticated automatic transmissions to provide a smooth and efficient driving experience.  When something malfunctions within the transmission the result can be very frustrating.  However, when your car struggles to shift gears, it is more than just an inconvenience, it is a potential warning sign of serious mechanical issues that could leave you stranded or with costly repairs.

Understanding Transmission Shifting Problems

Automatic transmissions are a complex combination of mechanical, electrical, and hydraulic systems that work together to seamlessly change gears without the driver doing anything beyond putting the car in “Drive” and mashing the gas pedal.  When something breaks or malfunctions you will notice several symptoms:

• Delayed engagement when shifting between park to drive
• Rough or jerky gear changes
• Unexpected RPM surges
• Warning lights illuminating your dashboard
• Grinding or whining noises while driving or during shifts

Common Causes of Transmission Shifting Failures

Low Transmission Fluid

Transmission fluid is one of the most important things within your transmission.  Transmission fluid functions as a coolant, lubricant and hydraulic fluid to ensure everything within the transmission functions correctly.  If the fluid level is low it can impact the quality of the shifts and potentially damage other internal components.  

If there is not enough fluid in the transmission it cannot cool components as efficiently as there will not be enough fluid to absorb the heat generated from the internal components.  This will result in accelerated wear clutch packs and internal bearings which can result in slippage and poor shifting due to overheating.

Lack of fluid will also damage internally lubricated parts like bearings and gears.  If there is not enough fluid to protect metal-to-metal contact of components you will see accelerated wear of components which can lead to catastrophic failure in a short amount of time.

Low fluid levels will affect shift quality because there will not be enough fluid to build the necessary pressure within the lines and valve body to engage the next shift.  Typically, you will feel this by the car seemingly wanting to slam into gears as you drive.

Some transmissions still have dipsticks, but they are becoming rarer and rarer.  If you do have a dipstick, the best way to check it is to get the car up to operating temperature and while leaving the car running (with some exceptions) pull the dipstick and check that it is between the hash marks.  If not, top up to ensure the level is correct.  If you do not have a dipstick, you will likely need to go to a service shop like Borst Automotive to check it for you as it requires special tools to determine the exact temperature of the fluid to verify the correct level.  If you have any doubt on how to check your transmission fluid level reference your Owner’s Manual or give us a call.  

If you find that you have low transmission fluid it is a good idea to get your vehicle looked at as soon as possible.  Transmissions are a sealed unit and should not “consume” fluid.  A low fluid level is likely due to a leak somewhere in the system or an issue with filling the transmission to correct level during a service.

Transmission Fluid Quality

Not all transmission fluids are created equal.  The incorrect transmission fluid can cause catastrophic damage.  Every transmission is designed for a specific type of transmission fluid that has the correct combination of detergents, friction modifiers, and lubricants to ensure your transmission functions correctly.  If you have incorrect transmission fluid you are very likely to have poor shifting quality.

In addition to the type of fluid, over time the fluid degrades due to heat exposure, mechanical wear, and contamination from internal components.  While transmission fluid doesn’t require servicing as frequently as engine oil it still has a regular service interval.  Depending on the type of transmission and use of your vehicle this can range from 40,000-60,000 miles.

Mechanical Component Failures

Several mechanical issues can impact how your vehicle shifts.

• Worn clutch plates or clutch packs – either from age or overheating the clutch packs can wear out causing shifting issues.  The tell-tale sign of worn clutch packs is when you accelerate and notice the engine RPMs surge up without the car accelerating.  This is often referred to as slip.
• Failed bearings – bearings can fail from age or lack of lubrication.  When bearings fail you may hear a whining or a grinding sound.
• Faulty torque converter – the torque converter creates the hydraulic pressure your transmission needs to function correctly.  If you have a failing torque converter you will feel loss of acceleration, slipping between gears, or the vehicle won’t shift at all.
• Faulty or clogged valve body – if you have a faulty valve body you will have a poorly shifting or non-shifting vehicle.  If the passages of the hydraulic circuit within the valve body are clogged from debris then the correct pressure won’t make it’s way to the solenoid valves to shift the transmission correctly.
• Worn gears – over time the gears within the transmission can wear.  When they wear to a point they are not engaging correctly your transmission will not shift correctly
• Broken bands – if any of the bands holding the gear sets together break your transmission will not shift properly.

Many of these mechanical issues will require an entire transmission overhaul or replacement.  However, some components can be replaced individually and won’t require the expense of a new transmission.  It is best to get your transmission properly diagnosed to determine what is needed to ensure you are not spending more than you need to get your car back on the road.

Electrical Faults

Modern transmissions utilize a number of electronic components to monitor and shift your transmission.  Any fault in these can cause warning lights on your dash, poor shifting, or no shifting at all.

• Electrical wiring issues – any broken wire or bad ground can adversely affect your transmission shifts.
• Faulty transmission shift solenoids – a shift solenoid controls the flow of transmission fluid.  It does this by a module sending an electrical signal telling the shift solenoid when to transfer fluid in and out of the transmission.  Faulty solenoids can cause hard, delayed or no shifting into specific gears.
• Faulty sensors – transmissions have sensors that provide signals to the transmission control module that help it determine when to shift.  Faulty transmission speed sensors, engine speed sensors, input sensors, output sensors and temperature sensors can all cause issues with how your transmission operates.
• Computer software calibration issues – software within the transmission control how your vehicle shifts under different conditions.  If there is an issue with the calibration this can cause all sorts of headaches.  Verifying the correct calibration before any repairs is a good idea before replacing any components
• Faulty valve body – the valve body in your transmission controls how hydraulic pressure is distributed.  Modern valve bodies are electro-hydraulic units.  If anything malfunctions electrically within this unit you can experience transmission troubles.
• Faulty control module – the transmission control unit is the brain of your transmission.  If anything in the control board or processor fails it will require replacement and reprogramming to get your transmission back to proper operation.

Electronics play a vital role in the operation of your transmission. Any issues in this electrical system will prevent your transmission from functioning as designed.

Transmission Services at Borst Automotive

Modern automatic transmissions are very complex and require specialized training to diagnose properly.  The last thing you want is to replace an entire transmission unnecessarily.  The experts at Borst Automotive in Tucson, Phoenix, and Mesa AZ are here to help you figure out what is wrong with your transmission and provide you with the best options to get your vehicle back on the road.  If you notice any transmission shifting issues give us a call at (866) 594-2415 or schedule an appointment online and our transmission experts will be happy to help!