What is The Reason for Limp Mode?
Computers in automobiles have been responsible for controlling and monitoring different since the 1980s. At first, they were used mainly for fuel economy and emissions control, but within a couple of years, many more parts were operated by the computer such as handling, climate control, airbags, and anti-lock braking.
These days, almost every system in a car is managed by the computer, including the shift timing, line pressure of the transmission, feel and sequence. The speed sensor gives input to control transmission operation, fuel injection, fuel mixture, and ABS. The TPS (throttle position sensor) and MAP sensor (manifold absolute pressure) offer the engine with lots of information that is applied to managing the shift in the transmission.
Limp Mode is a condition that happens when there are any errors with the usual logic of the computer in your car. When a signal value provided by the sensor to a computer is not in default range given by the producer, it would change to emergency programming. In general, these processes are made to keep the transmission from facing further damage which would be the result of a signal error.
As long as the car’s computer could receive a signal from the vehicle speed, TPS, MAP, and other sensors, which is within the normal ranges, the transmission would work normally. Nevertheless, as mentioned earlier, if it gets a signal which is not in the expected range, it would change to emergency or secondary operation.
The precise measure taken in the secondary operation is decided by the logic of the computer and depends on the gap with acceptable range.
Protective Actions in Limp Mode
If the value of the signal is not far enough and outside of the suggested range to show a mechanical error, the first thing a vehicle’s computer would do is to turn on the check engine light.
This is to warn that you should get the car checked out by applying a diagnostic scanner or code reader to know whether there are any listed soft codes, which could mean that a low-priority sensor is beginning to break down or has malfunctioned.
If the light disappears after starting your vehicle again, it would indicate that the sensor just failed once because of a loose connection.
Essential features are often not affected by this type of issue. But if you do not solve the problem, it can have negative effect on the fuel efficiency or overall performance of your car.
Now, when the signal value of a high-priority sensor, which is important for critical features) is far out of the normal range of operation, the car’s computer would switch over to a secondary mode, also known as the hard code.
During this mode, the electronic shift solenoid would be shut off to disable the transmission’s capability to shift gears. In addition, the pressure in the fluid lines of the transmission is changed to a high level to keep the clutches and bands from further damage.
The signal that controls the line pressure is set to full-on to keep the clutch pack from slipping dangerously.
All of those changes mentioned above lead to a Limp Mode, which enables you to bring your car home or to a service center for examination rather than leaving a broken-down vehicle on the road. In addition, this mode helps to reduce the risks of further damage.
How The Computer Find Out an Error in The Sensor?
For example, when the computer in your car gets a signal from the TPS sensor indicating that the pedal is moving to the metal while the throttle is closed actually, it could find out this problem by comparing the status of the car speed sensor, which could show no or low speed.
As soon as the computer recognizes the difference, it will make the transmission to change to a limp mode as well as turn on the check engine light. In most situations, a signal value could not be seen as an error, but the computer could find out an issue if checked with other outputs of the sensor.
The computer will record the fault code of a sensor, which causes the problem. Therefore, a mechanic could use a code reader or a diagnostic scanner to see the recorded codes, and then look up in the table of the manufacturer to know which sensor or system is the cause of this malfunction. Many types of scanner are programmed with those tables and offer more data than only the codes.
The diagnostic scanner, for instance, would show the code 35, which means that the fluid in the transmission temperature sensor would the main cause. We highly recommend you to get a car’s computer scanned frequently for fault code, particularly when the check engine light is turned on. This would be performed by most service centers in the routine tune-ups.
What Should You Do?
If your car changes to a limp mode, there would be a problem happening to the transmission. Therefore, you should have it checked as soon as possible by a professional mechanic. The following steps are what you should do:
- Don’t panic. A limp mode is particularly designed to prevent further damage so that you would to get your vehicle to a service center.
- Directly drive your car to the service center if possible.
- Otherwise, drive it home and call a mechanic to have your car towed.
- If you don’t feel comfortable when driving your car at a limited speed, immediately leave the road and then call for a tow.
- It is recommended that you should not keep driving your car in limp mode because it is not safe and could lead to further damage.