Limp home mode defined, computer, sensor diagnosis, driveability, check engine, service engine soon

Auto Computer Controls

As you may know, modern automobiles (from the 1980's to present) have at least one computer controlling them. This computer (or computers) control the fuel to air mixture via either electronic fuel injection or a feedback carburetor. On the newer vehicles the computer also controls the ignition timing (That's when the spark plugs fire).

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!!!

Disadvantages of Computer Controls

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!

Limp Home Mode

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.

Here are some of the many MIL (malfunction indicator light) designs

COMPUTER CONTROLLED TRANSMISSION LIMP HOME MODE

Most of the time a computer in limp home mode will just turn on the CHECK ENGINE light and give you bad performance and fuel economy. A big exception to this is on computer shifted transmissions. With these cars (Chryslers especially) if the shift control computer gets a bad sensor value, or if it detects a problem with the transmission, it will put the transmission into a VERY obvious limp home mode!!! You will have a normal reverse gear, but in any forward range you will have second gear only. No 1st, no 3rd or 4th. Just second gear. This, of course, limits your top speed to around 45 to 50 MPH. Quite annoying, but it beats walking! It allows you to "limp home" and get the car to a shop to find out what is wrong.

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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.

Automotive Limp Home Mode Defined

Auto Computer Controls

Disadvantages of Computer Controls

Limp Home Mode

Here are some of the many MIL (malfunction indicator light) designs

COMPUTER CONTROLLED TRANSMISSION LIMP HOME MODE