The newer a vehicle is,the more stuff the computer(s) control. The newest vehicles have the computer shifting the automatic transmission or transaxle as well as controlling spark and fuel mix. Many vehicles also have a climate control computer which controls the A/C and heat. Luxury cars often have vehicle anti-theft controlled by computer: some even will adjust the seat and steering wheel to suit different drivers when they enter the car!!!
The downside to all this "smarts" on a car is obvious: what happens when something breaks? The good thing is that the onboard computers themselves are very reliable. The problem is that the computers rely on a whole flock of sensors and wires to give them the data needed to make your car run. These sensors are not nearly as reliable as the computer itself, in fact they fail quite often!
This is where a "limp home mode" comes into play. Whenever the vehicle computer gets a reading from a sensor that is obviously wrong, it will "assume" a value that it "knows will work". On the earliest computer controlled cars with feedback carburetors, if the computer got bad readings from sensors, it would run the carb at the richest setting. You would get horrible gas mileage, but the car WOULD run. The computer would then turn on an amber "CHECK ENGINE" light. It would also store a trouble code in its memory telling what sensor was giving a "bogus" reading.
The newer computers are much more sophisticated, and just because a "CHECK ENGINE" light comes on doesn't really mean the computer is totally in a "limp home mode". Depending on what sensor reading is out of specs the computer may still be doing a pretty good job of controlling the engine. For example, some cars can have the "CHECK ENGINE" light come on if you don't tighten your gas cap tight enough after filling up!
On most vehicles the "CHECK ENGINE" light will go out if the sensor starts giving normal readings again, although some computers will make you use a scan tool to turn the light off.
COMPUTER CONTROLLED TRANSMISSION LIMP HOME MODE
The Economechanix Cyber Mall
We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry!
Thank you for visiting the ECONOMECHANIX WEB SITE. Please feel free to comment. We also serve the surrounding communities of Alachua, High Springs, Hawthorne, and Newberry! Gainesville has been my home since 1974, and I've loved Gvl and the Gators since I came here in the fall of 1974 to attend the University of Florida. I loved it so much I stayed and opened my car repair business. Originally it was out of the back of a 1963 Chevrolet wagon, but in 1977 a fellow mechanic and I opened an auto repair shop with actual walls, etc. I stayed in the same location for 26 years, and recently moved my operation to property I bought 15 miles east of Gainesville. I am doing most all the repairs myself now, having reduced my overhead from $1500 per month to practically nothing. I do work by appointment only. I mostly work only on my established customers cars, but I will occasionally take on new clients. E-mail me and I will either make arrangements to look at your car, or I will recommend you to someone who will.
George G. Scott, Jr.