How to Test TCM Duramax

A picture representing ACDelco Professional 24256861 Transmission Control Module

If you have a vehicle with an automatic transmission, one of its most important things is a transmission control module (TCM). It is how the vehicle is able to choose the correct gear at any specific time.

If your vehicle experiences problems shifting gears, you might be looking at a TCM problem. Such a problem can critically affect every aspect of your driving experience. So knowing how to diagnose it can save you some headache. 

This article looks at how to test TCM Duramax. Read along to learn how to test the TCM on your Duramax V8 engine.

Also Read;

What is TCM?

The transmission control module (TCM) is the device responsible for controlling modern automotive transmissions. It receives data from the vehicle’s engine control module (ECM) and input from various sensors and uses this information to choose the correct gear for the car. 

Every car with an automatic transmission has electronic sensors that monitor the vehicle speed, gear position selection, throttle position, and many other parameters. The transmission control module adjusts the positions of various valves and gears at each moment based on this information.

The TCM processes the input to calculate when and how to shift gears in the transmission and produces the signals responsible for driving actuators to complete the shifting of gears.  In simple terms, the TCM is a computer that provides information to other systems telling the automatic transmission what gear to be in and when.

The TCM has software designed to optimize shift quality, fuel efficiency, and overall vehicle performance to accomplish these functions.

So a problem with the TCM can create critical miscommunication to the systems in your car that depend on it for proper function. It might result in shifting at the wrong time and potential accidents.

Thankfully, a TCM malfunction will always send signals that you can read from the get-go and address the problem before it impairs your driving experience or put you in danger. 

Some common signs of a bad transmission control module include:

Does it feel stiffer than normally? On top of all these, if your check engine lights are on, chances are your TCM (Transmission Control Module is doomed.

Signs of a faulty TCM

A faulty TCM on your Duramax engine will usually leave some telltale signs to alert you. Knowing the signs can go a long way in keeping you safe, as you will tell if there is a problem as soon as it occurs. 

You do not want to ignore these signs due to how potentially risky it can be to do so. 

Below, you will learn how to check the TCM once you notice the signs, how to test it, and possibly fix the problem. But first, here are the signs to watch out for. 

Unpredictable gear shifting

When the TCM is not working properly, you will likely notice a hard clanking when you shift into gear. 

Since the transmission control module is the brain that sends shifting signals, a malfunction can cause unintended or incorrect shifting. Relaying incorrect information to the transmission can result in random shifting with no notifications or signs.

Poor fuel economy 

If your car loses its fuel economy and starts to guzzle fuel out of nowhere, you may be staring at a bad TCM.  

An automatic transmission is designed with the best fuel economy in mind. It achieves this through the transmission control module’s optimal gear selection and timing. So if that ceases to be the case all of a sudden, the module may be malfunctioning. 

Slow acceleration 

A vehicle with a bad TCM tends to have difficulty shifting to higher gears. Getting into speed becomes a problem when you cannot engage higher gears. Consequently, the car will be dragging and struggling to get any speed when you try to accelerate.

Usually, the transmission should shift into higher gears when the car accelerates down the freeway. When you have an automatic transmission, the gear change happens at the right time in a seamless fashion. 

However, a TCM malfunction can mean the gear change doesn’t happen even as the car hits the freeway. As a result, you end up with an engine whose RPM is increasing instead of dropping as it should after an upshift.

This situation is what makes it practically impossible to reach your intended speed.

Trouble shifting into a higher or lower gear

Gear shifting can become impossible when the TCM ceases to work properly. Gear shifting should be automatic for a car with automatic transmission. But the car relies on the transmission control module to select and change gears automatically. 

When the module malfunctions, the transmission fails to shift up during acceleration or shift down when decelerating. Sometimes, the gears can get stuck in neutral—which can be extremely dangerous.

It means your transmission does not go into lower gears gradually as your speed decreases. As a result, the car might come to a stop while still in top gear.

Alternatively, you might get stuck in the first gear instead of shifting up as the car accelerates. This situation complicates your driving experience in every possible angle.

Check engine light coming on. 

Check engine lights corresponding to the P0700 range of codes are the other main sign of a TCM malfunction. The check engine light can always pop up on your car’s dash for a number of reasons, one of them being TCM-related. 

You can tell the problem is related to your car’s transmission system if the light pops up while you are experiencing issues with shifting.

Technicians usually use a scan tool to examine the system’s Diagnostic Trouble Codes to determine the source of the problem. 

The video below illustrates the various TCM problems

How to Test TCM Duramax 

A TCM test is usually recommended when your truck’s system sends the P0700 range of codes or when you encounter one or more of the signs discussed. 

Like many truck owners, your first instinct may be to replace the Transmission Control Module (TCM). But that is not always the best course of action. 

You need to diagnose the problem further to determine the exact issue and its root cause. From there, you can know the best way to resolve it. Jumping to conclusions can only result in a hasty decision that may not always be the best. 

So, here is how to test the transmission control module in your truck as soon as you see a sign of malfunction. 

What you will need to test TCM on Duramax

If you are like many people, you are likely to have no clue how to test TCM on Duramax. In which case, it may be best to take the car to a specialized mechanic.

Nonetheless, if you prefer to do things on your own, you came to the right place. This tutorial will walk you through the entire process. 

The supplies you will need to complete the task include:

Safety gear  

Your safety accessories should include glasses and protective hand gloves. Ensure you wear these before you begin the work. 

Scanning tool 

You may use the on-board diagnostics tool OBD II connectors and your car’s PCM module for this part. This is all most vehicles will require, although some vehicles have the details in a different unit. 

In any case, you will require a scanning tool to test the TCM on Duramax. So this would be a good time to contact your manufacturer for that clarification. In many cases, they will just recommend the OBD II. 


A multimeter measures multiple electrical properties, making it handy in this test. You will need it for measuring resistance, reading frequency, and checking voltage. 

You might not use it, but just have it with you in case the need arises. Once you begin the testing, it may become necessary to check the voltage, the solenoid, and a few other parameters.


You should be able to get a pressure sensor in a car parts shop. The scope comes in handy when you want to measure the pressure of the car’s critical fluids, including transmission oil, engine oil, and gearbox fluid.

Since you will be diagnosing transmission problems, the tool may be more important than you know. You may also use it on cooling and exhaust issues. 

The vehicle’s system can typically register any anomalies associated with these critical fluids and air pressure as potential problems and send alerts for diagnostics. 

Any of these areas could be the reason your vehicle’s dash asks you to check the engine. The more equipped you are for it, the better.  

Battery charger 

Suppose it turns out that your current transmission control module is faulty and requires a replacement; you will need a battery charger for the installation. 

TCM software transfer tool 

Installing a new TCM unit on your Duramax may require a TCM software transfer tool, so ensure you get one. 

As you can see, the changes in technology have necessitated the use of a few more tools, unlike in the past when all you needed were various gauges to carry out this kind of test. 

Today, cars are far more computerized, so you need a bit more tech-savviness to get things done correctly. That said, read on for the procedure to follow.

Steps to test TCM Duramax

Once you have put together everything you need for testing your vehicle’s TCM, you can follow these steps to get the work done. 

Step 1: Inspect the TCM unit 

Sometimes the problem is not the module, but the wiring connecting it for wiring connecting it to the system. 

So before you diagnose a bad TCM for your controller, start by disconnecting the harness and manually inspecting the pins or wires. See if there are any pins dislodged. If any, simply reseal them and secure them to see if the problem goes away. 

In many cases, the pins are usually the issue. 

Step 2: Locate the OBD II connection port.  

The on-board diagnostics tool diagnoses just about every issue within your car. The gadget is typically located in different spots for different vehicles, so it may take some looking around to find yours. 

In most cases, you should find it under your vehicle’s dashboard. If you have difficulty locating it, we recommend you consult your car’s instruction manual for some heads-up.

Step 3: Connect your scanning equipment 

Whichever tool you have should come with a user manual to guide you through the process. The devices are generally designed to be intuitive, so you should have no difficulty finding your way around them.

Despite being intuitive, it always helps to follow the steps recommended in the manual. In most cases, this will involve selecting the car’s make and model and moving on from there. 

Once the scanning commences, you will know if there is a problem with your transmission control module. Often, it will send you the corresponding codes to tell you what the problem might be. 

The manual should tell you whether to run these diagnostics with the car turned on or in ignition mode. You want to work in the right mode to get the correct results. This is why we couldn’t emphasize enough how important it is to read the manual. 

Anyways, once you have the code, check the manual to determine what it means. Often, you will not know the meaning of all the codes unless you are a vehicle technician for that specific make and model. 

After knowing the problem, your options can include repairing or replacing the TCM unit altogether. If there is a repair to be done, you may need the help of a mechanic. 

However, if pins are the problem, you can finish the work without having to pay a technician for assistance. 

How to install a new TCM unit—step by step

Suppose it turns out that you must put in a new transmission control module; you will know how to do it correctly. Of course, your instruction manual will help, but still, here is a summary of the steps you will want to follow. 

Step 1: Wear your safety glasses and protective hand gloves.

Step 2: Connect your battery charger to the vehicle. It is essential to power it up before starting to use the TCM unit.

Step 3: Put the TCM software transfer tool in the reading position. Consult the manual that comes with the tool instructions. 

Once that is done, switch the car’s ignition on.

Step 4: Plug the TCM software transfer tool into your vehicle’s OBD II slot. Once that is done, you should hear a couple of beeps (typically five) over a period of about half a minute.

The software transfer tool will immediately start reading the information in your system. You will know this is happening when the LED light goes red and keeps flashing. 

The flashing will stop as soon as the reading is complete, and the light will stay red. You should also hear one beep as the flashing stops. 

Step 5: Turn off the ignition and remove the TCM software transfer tool.

Step 6: Wait for about 30 seconds and then unhook the battery charger.

Step 7: Finally, install your new TCM unit following the same steps—except this time, ensure you put the software transfer tool in the writing position instead of the reading position.


You will notice that diagnosing your transmission control module for shifting-related issues can be pretty straightforward if you know where to look and have the right set of tools. The most important thing is to test the TCM on your Duramax and address the issue without delay.

The TCM on your car is crucial to your safe driving, and delaying with a problem once you see the sign of malfunction can be akin to endangering your life on the road. Anyway, we hope this tutorial helped show you how to test TCM on Duramax.

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