Any automatic transmission made within the last 15 or 20 years is going to be a computer controlled. And, just like a modern engine, the computer now knows when there’s something wrong with the transmission. If the computer thinks that there’s some sort of critical problem is going to destroy the transmission, it will put the transmission in something called limp mode. Limp mode can also be referred to as fail safe mode.
What is Limp Mode?
Limp mode is a condition where the computer will make the transmission operate in a manner that will save it from its own destruction. Limp mode’s job is to minimize the potential harm from a major mechanical failure. As the name itself implies, it is designed to get you and your vehicle safely in for a repair without leaving you stranded on the side of the road.
The computer generally does this by reducing the number of gears the transmission can use. Although which gears are eliminated generally changes depending on which transmission make and model it is. It’s generally a rule that the overdrive gear will always be deactivated by this limp mode condition. Typically, the torque converter will stop locking as well under limp mode conditions.
An automatic transmission has a range of signals readings from the sensors that the computer sees as normal. When one of these sensors sends a signal outside of the normal operating parameters limp mode is triggered. The transmission will often shift rougher, and the torque converter lock will stop engaging. Most people realize this happened when they get on the highway and their vehicle screams down it at a high RPM. Besides the limited and firm shifts coming from the transmission, many vehicles have a noticeable lack of power coming from the engine as the computer does everything that it can to save the transmission.
There are several different conditions that can cause limp mode. The common theme of the issues listed below are a faulty reading from a sensor that is critical to the operation of the vehicle. Once you understand that, than the only question left to ask is it a false reading or is there something really wrong?
- Computer Problems– A problem with the vehicles ECM can cause limp mode, although this is definitely not where you should start looking, but
- Damaged Electrical Connection– Over the life of a vehicle the wiring harness that connects the transmission to the ECM can become damaged. This can alter the signal that the computer receives, triggering limp mode. This is much more likely than a computer problem.
- Loose Connection– Older vehicles have had over twenty years to get a loose connection on one of it’s wiring harness plugs. All wiring harnesses utilize plastic keepers to secure them to the sensors. Plastic melts and breaks just like everything else. Has your vehicle been in the shop in the last few months? Maybe the mechanic damaged a connection or didn’t quite get a harness snapped in all of the way. You would be shocked just how much this can and does occur.
- Water Damage– The thing about modern vehicles is that they are virtually bulletproof compared to the old non computer controlled predecessors. But, their Achilles heal is the complex wiring that they use to operate. It can be confusing to deal with. If you’ve gotten the harness wet or suspect your vehicle has been in a flood, this very well could be causing your limp mode situation.
- Faulty Sensor– Your vehicle relies on many sensors. These include the MAP sensor, throttle position, crank position, and vehicle speed.
What Limp Mode Does to Protect Your Vehicle
If your vehicles ECM receives signal that there may be a potentially catastrophic issue, it’ll trigger the limp mode and trigger your check engine light. It than switches all of the vehicles timing and shift tables over to a less aggressive (and therefore less damaging) performance table. Think about it like starting your computer in safe mode.
In a transmission the computer will increase the pressure in the lines in order to protect the clutches. This makes shifts way more rough (if your vehicles limp mode gives you more than one gear) Although it seems counterintuitive, a hard shifting transmission is better on the clutches and bands because it allows for less slipping.
All vehicles built after 1996 have OBD2 built into them. Head to your nearest AutoZone and have it tested. Make sure that you head the limp mode warning and actually take the vehicle in for repair and don’t drive it like that for long. There is a good chance that you could be causing more damage to the vehicle every second that you continue to drive it. Hopefully you won’t end up finding out that you are looking at a major repair. Good luck!