When your automatic transmission slips, it could be due to one or more problems, such as mechanical, electronic, or both. Also, some pre-existing issues can aggravate when a vehicle’s engine and automatic transmission get hot. So, let’s discuss why your automatic transmission is slipping when hot.
Automatic transmission slipping when hot may be due to low, excessive, contaminated, or burnt transmission fluid. The other common problems are a clogged filter, broken shift solenoid, failing torque converter clutch solenoid, and bad transmission control module.
You may check the electronic component before inspecting the mechanical parts or vice versa, and the causes and solutions listed in this guide are not necessarily chronological. Read the causes and use the symptoms you encounter to prioritize one problem over the others in this guide.
1. The Automatic Transmission Fluid Is Not in Optimum Condition
The transmission fluid has two purposes, which are lubrication and cooling. Thus, a significant dip in the volume of transmission fluid in your vehicle can cause issues, especially when the gears and other parts get hot.
Also, this fluid plays a vital role in facilitating the necessary hydraulic pressure for the automatic transmission system to operate all the gears and modes.
Low transmission fluid may lead the oil pump to be dry. In effect, the throttle converter will be dry, and the automatic transmission will lack the hydraulic pressure to operate the shift solenoid. Therefore, your automatic transmission may slip, miss a shift, or respond erratically.
On the flip side, excessive transmission fluid can cause cavitation. The extra volume will foam up, increase the pressure inside the system, and the cavitation bubbles will interfere with your automatic transmission.
Similarly, dirty or contaminated transmission fluid is also a problem.
If your automatic transmission is slipping when hot, you may have burnt transmission fluid with black debris and metal flakes, and the magnet on your transmission fluid pan can grab some of this metallic debris. However, dirty and gunky fluid is still present in the system, and it can cause the automatic transmission to slip when hot, shift erratically, and essentially not work smoothly.
How To Fix?
Your vehicle may have a transmission fluid dipstick, so you’ll want to take it out to check the level and condition of the fluid. You don’t need new fluid if its level and condition are alright. Otherwise, you need to change it, but not before you flush the car’s automatic transmission system.
You may use the Lubegard 95001 Transmission Flush (available on Amazon) to remove varnish and other deposits from the system so that you can pour in new fluid. This product has corrosion and rust inhibitors, but the solvent will not harm your automatic transmission parts, such as seals or gaskets.
Alternatively, you may use the K&W Trans-X Slip-Stop Leak-Fix or the K&W Trans-X High Mileage Automatic Transmission Treatment (both also on Amazon). The former will prevent automatic transmission from slipping when hot, eliminate varnish and gunky deposits from the system and condition the gaskets or seals.
These Trans-X flushing solutions will not interfere with your selected transmission fluid. The Trans-X High Mileage variant is for older vehicles with over 75,000 miles (120,700 km), as the automatic transmission in such cars is usually more vulnerable to slipping when hot.
Finally, use a type of transmission fluid designed to endure high heat, especially if you drive a lot in a hot climate. Fully synthetic fluids and an additive like Lucas Transmission Fix on Amazon can stop slipping, rough shifting, and erratic responses. Also, the solution fixes seal leaks.
2. The Automatic Transmission Fluid Filter Is Clogged
You need a new filter when you change the transmission fluid. While the fluid may be in usable condition in some cases, a clogged filter causes slips, rough shifting, or delayed response.
Thus, you can change the filter and reuse the automatic transmission fluid.
How To Fix?
Replacing a transmission fluid filter is not expensive. However, you must flush the transmission system as already discussed to eliminate all debris and gunk buildup. Otherwise, even a new filter will clog soon. Avoid such issues as often as you can, because working on the transmission system can be tedious and rather annoying.
3. Your Automatic Transmission Shift Solenoids Are Bad
Failing or broken shift solenoids can cause an automatic transmission to slip, not only when hot but anytime. However, defective or worn-out solenoids that are still somewhat working may not endure high heat.
Thus, the problem might be more evident and disrupting than otherwise.
How To Fix?
Modern vehicles with automatic transmissions have multiple shift solenoids. Unfortunately, you cannot replace only one even if you detect the faulty solenoid. The shift solenoids are encased in a metal housing, so you’ll have to check the part number and replace the whole unit.
4. The Torque Converter Clutch Solenoid Is Broken
Like the shift mechanism, the torque converter in your automatic transmission also has a solenoid.
This one works in synergy with the shift solenoids, and whenever you operate the gear shifter and accelerate, the throttle converter solenoid works with the others to put your vehicle in the intended driving mode. These electro-mechanical parts are not immune to heat or damage.
How To Fix?
You must replace the torque converter clutch solenoid, which is usually adjacent to the set of shift solenoids. However, do not replace any solenoid before you test the related components for possible failures. For instance, these parts are perhaps not getting the signal to do their jobs.
5. The Automatic Transmission Control Module Is Failing
Your vehicle has two automatic transmission control systems, one mechanical and the other is electronic. The automatic transmission control module or unit (TCM / TCU) is the mechanical component, so you’ll want to inspect this to find evident problems, such as damage, broken pins, or corrosion.
How To Fix?
The solution depends on what you find during the inspection. It’s possible to clean the pinout of an automatic transmission control module to get rid of corrosion. However, a bad module needs to be replaced. Otherwise, it will not signal and regulate the transmission and shift solenoids.
Read: Car Diagnosis Cost
6. The Electronic Control Unit Is Malfunctioning
Like the mechanical module, your vehicle has an electronic control unit (ECU / ECM). This part is in charge of the circuit of your automatic transmission. The solenoids and all components relying on electronic signals will not function properly if the electronic control module is broken.
You may start your investigation by checking the electronic control module or unit, even before you inspect the level and condition of the transmission fluid in the vehicle. If your ECU or ECM is the problem, addressing mechanical issues will not solve the transmission slipping issue.
How To Fix?
You need an automotive diagnostic tool to test the electronic control unit. Such scanners can read the error codes and tell you if the electronic control unit is malfunctioning.
However, not all scanning and diagnostic tools can test for automatic transmission slipping issues.
For example, the Ancel AD310 OBD II Scanner on Amazon can read fault codes of most car engines, but not the automatic transmission. You need the Ancel FX2000 Automotive ABS OBD2 Scanner (also on Amazon), that can read error codes for automatic transmission, engine, and other vehicle components.
The solution depends on the problem your scanning tool detects. You can solve connection issues, such as wiring harnesses or open circuits in the transmission system. However, you may have to replace the module if the ECU has internal defects or failed components.
7. Your Automatic Transmission Has Other Problems
Some cars have an electronic throttle body that regulates the automatic transmission, the clutch converter, and gears, in particular. If you have this unit in your vehicle, it may be broken or not responding.
Thus, your automatic transmission may slip, miss a shift, or not shift as required.
Last but not least, you may have broken or failing parts in your automatic transmission system. Hence, you need a thorough diagnosis of literally everything, from the gear shifter to the levers, nuts and screws, transmission bands, switches, and more.
How To Fix?
It is impossible to guess significant failures in your vehicle’s automatic transmission. However, a critical issue should have more symptoms other than slipping transmission or gear shifter when the car is hot. Similarly, the appropriate solution depends on the problem you diagnose.
Check the following if your automatic transmission is slipping when hot:
- Transmission control module
- Electronic control unit
- Transmission fluid and filter
- Shift solenoids
- Transmission solenoid
Opt for the appropriate solutions shared in this guide. If none of these common issues is the problem in your case, contact a trusted mechanic for a more comprehensive inspection.