Last Updated on May 11, 2021
The transmission control module (TCM) is a vital component of any vehicle with an automatic transmission system. Rather than relying on hydraulic or mechanical control over a transmission system, newer vehicles use a transmission control module instead.
A car’s TCM will likely never need replacing, but if you have a bad transmission control module, immediate replacement (or repair) will be necessary. This module is basically a computer which communicates information to other computer systems within the vehicle.
The purpose of the transmission control module is to help the automatic transmission choose the best gear to be in at any given time. That way, the driving performance and fuel economy will be at the optimal level.
Signs of a Bad Transmission Control Module
The transmission control module can often be found below the cover at the back of the transmission case. It should be just below the engine control module’s position. Sometimes it’s found under the center console in the interior or even under the hood near the battery or inner fender panel.
If you ever have a problem with your transmission control module, you’ll need to have it inspected right away. A bad TCM will spell trouble for your vehicle and your ability to drive it. Fortunately, there are a few basic symptoms that you’ll most likely notice before this happens.
#1 – Check Engine Light
If the Check Engine warning light illuminates on your dashboard, it could mean several things. But if you notice any problems with your shifting in addition to the Check Engine light being on, then it is most likely a problem with your transmission control module or some other component of your transmission system.
In any event, get your vehicle checked out immediately by a mechanic or use your own scan tool to check for any Diagnostic Trouble Codes (DTC). If code P0700 shows up, you may have a problem with your transmission control module.
#2 – Unpredictable Shifting
Since an automatic transmission system relies on the transmission control module to do the shifting, a bad transmission module will cause incorrect or unintended shifting. This can prove to be anything from an annoyance to causing a serious accident.
For instance, if your car shifts to neutral or any other gear that it normally wouldn’t shift into without warning, then your control module will likely need to be replaced. Otherwise, it will be very dangerous to continue driving.
Unless you have a manual override option in your vehicle (example: paddle shifters), you will need to get your car towed to a repair shop and have the module replaced.
#3 – Problems Shifting Into Higher Gears
As you gradually increase speed, your transmission should correctly shift into higher gears at the correct time. When this doesn’t happen, you’ll notice that your engine’s RPM simply keeps increasing instead of slightly dropping an upshift occurs.
Because each gear has a maximum speed it can go to, you may never reach your intended speed if the faulty TCM refuses to shift to a higher gear.
#4 – Problems Downshifting
This is similar as above but in reverse order. Maybe you need to pass a slow moving vehicle and your transmission fails to shift into a lower gear when you mash the gas pedal to help you accelerate.
Or you’re cruising down the freeway and take an off-ramp, your transmission should gradually be going into lower gears as your speed decreases. At a stop, an automatic transmission should be in first gear so it’s ready to go when you need to start moving again.
If your transmission control module is bad, you may still be in a high gear as you’re coming to a stop. When it’s time to start moving again, you’re in a too high a gear for proper acceleration which may even prevent you from moving at all.
#5 – Stuck in the Same Gear
This usually presents itself as being either stuck in neutral or first gear. The transmission simply will not shift and you either won’t be going anywhere (stuck in neutral) or you’ll be limited by the maximum speed of first gear.
#6 – Delayed Shifting
The speed of your vehicle relies on cycling to various gears at the appropriate timing. If you have a bad transmission control module, then it will cycle to next gear too slowly.
This will impact your acceleration, causing you to lose speed rather than picking up speed. When you approach a hill, the shifting performance will be even worse.
Worst case is when you’re attempting to pass a slower vehicle on a 2-lane road and your vehicle refuses to downshift immediately so you have more torque available to make a quick pass.
#7 – Poor Fuel Economy
Problems with your transmission system usually result in bad fuel economy. An automatic transmission is designed for optimal gear selection and timing to provide you with the best possible fuel economy.
If you have a transmission control module that’s bad and affects this timing, then your engine will end up working harder than usual. This means it will consume more fuel than usual, causing you to spend more money at the gas pump.
The replacement cost of a transmission control module will be anywhere from $500 to $900. You can expect the parts costs to be around $450 to $700 while the labor costs will be around $50 to $200.
Of course, you can order a new TCM online and ask a mechanic what their hourly labor rates are. However, most of the cost for this replacement job goes toward the parts cost itself and it can vary greatly by car manufacturer. You can expect taxes and fees to be added to the total cost as well.
Another option is to repair your TCM. The transmission control module repair cost will in almost all cases be lower than replacing with a new module. On average, expect to pay around $300 to repair your transmission control module.
While most TCM repair businesses will offer a warranty of some sort, it does pose a bit more risk if the repair is not done right.
Also, you may need to ship your bad TCM to a non-local location so if you can’t afford not having your vehicle for a few days, replacement would be the fastest method.