Transmission Shifts Hard When Cold

Transmission shifts hard when cold, especially from gear 1 to 2, is common problem car owners face until their car is warmed up. In fact, it’s so bad, many drivers stop driving and just wait for the car to warm up, which means they miss their next appointment or run the risk of getting late for an important event.

The hard shifting of transmission when cold is normal as the transmission oil is thicker when cold. The engine takes some time to warm up and reach the operating temperature so that the transmission oil gets desired viscosity and operates the transmission system efficiently. When your transmission oil is thicker when cold, its viscosity is high which makes it difficult for the transmission to transmit power to gears. However, if the transmission continues to shift hard, you might have to update transmission module software or completely replace the transmission.

The intermittent hard shifting transmission problem can be caused by a number of factors, the most common being that your vehicle has been driven through the winter months without any oil changes.

A bad transmission oil could be causing your transmission to shift incorrectly. Also, when your transmission is cold, the fluid’s viscosity is high which makes it difficult for the transmission to transmit power. This causes the car to be sluggish and oftentimes results in poor shift quality.

Can Cold Weather Make Car Hard Shift?

Yes, cold weather can make the car hard shift. This is because the viscosity of transmission fluid increases at lower temperatures, due to which it becomes thicker.

Another misconception people have in mind is that transmission oil isn’t pumped when the engine is cold or in park mode. This isn’t true. Transmission oil is pumped by a transmission gear pump run that is mounted on the shaft of the torque converter and is run by the engine all the time.

As the engine’s RPM increases, the flow of transmission oil through the pump also increases. However, at too colder temperatures, increasing the engine’s RPM for the sake of increasing the transmission oil flow through the pump can cause cavitation and it can also result in hard gear shifting.

Moreover, at colder temperatures, parts of the transmission system start shrinking. As a result, clearances between transmission parts increase, which can cause leakage of transmission oil from seals in the transmission valve body.

An automatic transmission system consists of the following parts that require optimum viscosity of transmission oil to function properly:

• Torque converter
• Transmission solenoid
• Valve body
• Clutches and bands

To learn about torque converters, you can read my guide on what low gear means in the car.

The viscosity of transmission fluid has a major role in shifting gears smoothly as the transmission fluid exerts pressure on the clutches and bands to engage and disengage the gears in the automatic transmission system.

Apart from providing hydraulic pressure to the clutches and bands to work, transmission oil also lubricates moving parts, such as gears, of the transmission system.

When a temperature is cold, the transmission fluid will have high pressure and take more time to flow through the channels in the valve body and exert pressure on the clutches and bands. As a result, the transmission gets delayed, which leads to hard shifting of the gears. Due to this reason, your car feels sluggish during gear shifting.

Quick Summary

If the transmission shifts hard when cold, follow these steps

• First, check the condition of the transmission fluid. If it is dirty and dark black, perform a transmission flush procedure. Use only genuine transmission fluid.
• If transmission fluid is fine, check its level.
• Also, check TFT and ECT sensors.
• Check solenoid valves in the transmission valve body.
• You may have to reprogram the control module if the transmission is still shifting hard.

Why Does Transmission Shift Hard When Cold?

Transmission shifts hard when cold due to the following reasons:

1. Malfunctioning Transmission Fluid Temperature Sensor

The hard shifting of transmission from gear 1 to 2 when it is cold is normal if it is because of the transmission fluid temperature (TFT) sensor. When the engine is cold, transmission oil is too thick to flow through the orifice and valve body passages easily. However, a bad TFT sensor can cause hard shifting of transmission if it relays incorrect temperature readings. You can use this OBD2 scan tool to find the trouble codes stored in the engine’s memory. OBD2 codes from P0710 to P0715 usually represent a faulty TFT sensor/

“Note that Transmission fluid temperature is correlated to line pressure”

• Low Line Pressure = Soft Shifting/Slipping Clutches and bands
• High Line Pressure = Harsh Shifting

The transmission oil temperature sensor is used to detect the temperature of the transmission oil. This sensor sends a signal to the transmission control module (TCM) indicating that the transmission oil has reached a certain temperature.

The TCM then sends a signal to the engine control module (ECM) telling it to set the optimum engine RPM.

When the engine is cold and the transmission oil temperature is cold, the engine computer will delay the gear shifting to allow higher engine RPM (the engine reaches higher RPM quickly when the vehicle is in low gear) and help the engine warm up to the normal operating temperature faster. The faster the engine reaches operating temperature, the more efficient it is and the lesser emissions it will produce.

However, if the transmission fluid temperature sensor malfunctions, the PCM will not receive the correct input signal, and will continue to think the transmission is cold. As a result, your transmission will experience delayed shifting, and your engine may sluggish.

The temperature reading of the transmission oil temperature sensor can typically be viewed using a scan tool. If the vehicle has completely cooled down overnight, the TFT sensor reading should be fairly close to ambient temperature, engine coolant temperature, and intake air temperature. If the temperature of the air around the vehicle was 70°F for most of the night, the TFT, and other temperature sensors, should be reading close to 70°F.

If the TFT reading is below 20°C (68°F) after running the vehicle for over 20 minutes, the PCM will detect a fault in the engine. Also, when the transmission oil temperature is 110°C or more after 20 minutes of engine cold start, the PCM will consider it faulty.

2. Malfunctioning Engine Coolant Temperature

The PCM uses engine coolant temperature sensor (ECT) reading not only for optimal combustion but also for transmission operation. ECT sensor is installed in the coolant passage so that it measures the temperature coolant and tells the PCM at what temperature the engine is running.

Similar to the TFT sensor, the PCM also senses engine temperature through ECT and decides the gear shift timing based on the engine’s temperature. If the engine is cold, the PCM will delay the shift to allow the engine to warm up faster and allow for more consistent engine and transmission operation.

You can use this scan tool to read the coolant temperature sensor reading. If the coolant temperature is extremely low i.e. less than -40°F during cold starting, it means either a coolant temperature sensor is bad or its wiring is faulty. In my guide on P0128 code, I have explained the process of diagnosing a faulty ECT sensor.

Follow these steps to test the ECT sensor:

• Immerse the tip of the sensor in the water.
• Connect a digital ohmmeter to the two terminals of the sensor.
• Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance
  illustration.
• Repeat the resistance at other temperatures by heating or cooling the water.
• If the sensor does not meet the specification shown in the temperature versus resistance chart, it must be replaced.

You can watch this video to learn more:

Note 1: Some engines have two ECT sensors. One sensor is for the temperature gauge on the instrument cluster, and another one is probably to control the radiator fan. So, make sure to check the correct one. Correct ECT sensor is usually installed closer to the thermostat.

Note: 2: An ECT sensor cannot accurately read engine coolant temperature when the coolant level is low or the coolant is dirty. Before replacing the ECT sensor, make sure the coolant quality is good and its level is up to the proper level. Moreover, the thermostat is operating properly on the vehicle. If the thermostat is stuck open, the engine will take a lot of time to warm up.

3. Malfunctioning Transmission Line Pressure Sensor

You will find a transmission line pressure sensor on the valve body, next to the pressure control solenoid. Transmission line pressure sensors (TLP) are used in automatic transmissions to measure the pressure of transmission fluid in the valve body and send signals to the engine control module so that it can decide when to open the pressure solenoids in the valve body and maintain a desired transmission oil pressure for gear shifting. So, you also have to check the transmission line pressure sensor if your transmission is shifting hard when cold.

4. Bad Electronic Control Solenoid

The control solenoid of transmission can become stuck opened or closed due to dirt particles in the transmission fluid and can cause hard shifting of transmission when cold. This has also been included in the TSB of 2015-16 Santa Fe (NC) models for hard shifting of transmission when cold. If the condition of the transmission solenoid is already bad, the thicker transmission fluid at extremely colder temperatures will worsen the condition of solenoids, due to which they will not operate under the action of voltage pulses sent by the ECU.

A solenoid is a hydro electronic valve in the valve body of the automatic transmission that causes the fluid to flow into the respective circuits at the correct time is a solenoid. The opening and closing of the pressure control solenoid are controlled by voltage signals sent by the ECU. When the solenoid is opened, transmission oil under pressure passes through it to engage or disengage clutches and bands and shift gears.

There are different types of control solenoids in transmission. These include pressure control solenoid and shift control solenoid. The pressure control solenoid controls the line pressure and the shift control solenoid controls gear shifting. Many four-speed automatic transmissions use 2 shift control solenoids to control the transmission. With the addition of more gear ratios, more shift solenoids have to be added to the transmission.

Electronic solenoids are controlled with the pulse width modulation process. The frequency of the electrical current applied to the solenoid is varied by opening and closing the solenoid at a fast rate to create different voltage pulses. The frequency is varied according to the desired amount of time that the valve is open. The longer the solenoid is opened, the higher the voltage is supplied.

The duty cycle of a transmission solenoid is a percentage of the total time for which a solenoid is kept open. For instance, if the solenoid is opened for 20 seconds and closed for another 20 seconds in 40 seconds cycle time, the duty cycle of solenoid will be 50%.

So, if line pressure is 200 psi, a 50% duty cycle pressure control solenoid will reduce the pressure to 100psi, and the remaining 100psi pressure will be exhausted back to the transmission pan.

I found below Youtube video below quite helpful to test transmission solenoid:

5. Dirty Transmission Fluid

The transmission fluid (lubricant) is used in an automobile to transfer power from the engine to the drive shaft, lubricate gears and engage clutches and bands by passing through the solenoid. A dirty transmission fluid causes a problem called “clunk” in the transmission. Your transmission fluid might be dirty because your transmission filter is clogged. It is recommended to flush transmission oil from your engine every 30,000 miles. 

When transmission fluid ages, it loses its flow characteristics and lubrication abilities due to impurities. If the transmission fluid degrades, it couldn’t apply consistent pressure to clutches and bands, which causes the gears hard to shift.

To check the health of the transmission fluid, you should first check its color. Fresh transmission fluid is usually light red in color. Fresh transmission fluid can even be greenish blue or golden in color. As you can see below image, ATF for Audi is available in two different colors, but their properties are the same.

If the transmission fluid is dark in color, the transmission flush needs to be performed. Do note that the torque converter also holds transmission fluid, So, for a transmission flush, make sure to remove the torque converter and transmission pan.

There are magnets on the transmission pan to collect the metal debris. If there is excessive metal debris, there is extreme wear and tear in the transmission system. Make sure to also replace the transmission filter when flushing the transmission. For a complete transmission flush, an engine may need up to 14 quarts of transmission oil. Make sure to only use transmission oil recommended by the manufacturer.

6. Level Of Transmission Oil

Low transmission oil level will prevent building up an optimum pressure inside the transmission, due to which your transmission will shift hard when cold. If the transmission fluid is fresh, it is important to check its level. Keep in mind that dipsticks of motor oil and transmission oil are different. So, make sure you check the dipstick of the transmission oil.

To check the transmission oil level, follow these steps:

• Park the car on level ground.
• Start an engine for a couple of minutes so that transmission fluid heats up and expands. When the engine is cold, the fluid level is low, due to which you may get incorrect readings.
• Locate the dipstick of transmission oil. You can take help from the owner’s manual.
• Remove the dipstick and wipe it off.
• Insert the dipstick, remove it again, and read the fluid level. The transmission fluid level should be between the upper and lower marks

7. PCM Needs To Be Reprogrammed

Sometimes PCM needs to be reprogrammed if your transmission is shifting hard when cold. This was a problem in some Toyota Sienna models after they introduce the 8-speed transmission.

Automatic transmissions are tied to different sensors and control solenoids for gear shifting. So, after reprogramming the PCM, the hard-shifting problem of transmission might get resolved as the reprogramming of the PCM resets it to the factory default settings. But, this should be your last step after checking all possible causes as reprogramming of PCM is a bit costly.

Some Precautions To Prevent Hard Shifting of Transmission When Cold

When your vehicle is in park mode and the engine is too cold, transmission oil has very little flow momentum to pump through the transmission system components from the transmission pan. As you start the engine, don’t start pushing the gas pedal straight away. If you have a modern engine, give it around 30 seconds to warm up while idling so that engine could reach its maximum performance. After around 30 seconds, you start driving slowly as driving helps modern engines warm up faster rather than running them at idle for long periods.