Return to the Econofix Home Page
The newer a vehicle is,the more things the computer(s) control. The newest vehicles have the computer shifting the automatic transmission or transaxle as well as computer controlled spark and fuel mix. On some vehicles the throttle is controlled by a servo motor driven by the computer. With this "drive by wire" throttle system there is no direct link from the pedal to the throttle! 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!!!
A newer car computer is 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.
Auto Computer Controls
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).Disadvantages of Computer Controls
The downside to all this computer "smarts" on a car is obvious: what happens when something breaks? The good thing is that the onboard computer itself is very reliable. The problem is that the computer relies 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.
Here are some of the many MIL (malfunction indicator light) designs
Click here to see more images of CHECK ENGINE or MIL (malfunction indicator) lights
(Note: some systems have a "bypass" that "creates" a physical link to the throttle if the computer control fails. (Click here to see a system like that on a SAAB)
The manufacturer goes to great lengths to program electronic throttle control to be just like the old style throttle as far as the driver is concerned.
The limp home mode on electronic throttle control WITHOUT a mechanical backup system like the SAAB above, is to hold the engine at a high idle. This idle will be high enough to "limp home" at 30-40 MPH, but obviously can't be driven much before repair.
A high idle on any vehicle less than 30 years old is probably due to an incorrect sensor reading (like throttle position sensor). The computer has gone into a "limp home mode" where it idles the engine up super high. There will be a check engine, or MIL light, unless it's burned out! This means the computer has stored a trouble code to help diagnose the problem.