What I mean is you can still select park, reverse, neutral and drive with the shift lever, but the computer will not switch to first, third, or overdrive when it has detected a problem. Staying in second gear is called "limp mode" that allows you to drive slowly to a repair shop without needing a tow truck.
All automatic transmissions were hydraulically controlled in the past. Two main pressures in the system varied to cause valves to move to different positions. One pressure was related to gas pedal position. The other one was related to road speed. When one overcame the other, a valve moved and opened a port to apply a clutch pack for a different gear. THAT'S what is meant by hydraulically-controlled.
Chrysler was the first company to develop a computer-controlled transmission. It was used in many 1989 models. There were some advantages that I won't go into, but to me they did not outweigh the disadvantages, most notably, that intermittent defaulting to limp mode. Anytime you add a computer to anything in a car, you add problems and high repair costs. That's why I refuse to give up my rusty trusty '88 Grand Caravan.
Instead of those two pressures determining when a shift should occur, the computer looks at two speed sensors, engine load, and throttle position to determine when a shift is needed, then it does that by electrically turning on or off a series of solenoid-controlled valves. That's computer-controlled, or electrically-controlled vs. Hydraulically-controlled.
When the system goes to limp mode, those solenoids are not activated by the computer. Instead, they are spring-loaded to default to the positions necessary to put it in second gear. You can still switch to park, reverse, and neutral with the shift lever. If the vehicle doesn't move when you shift to reverse or drive, besides a broken cv joint, the most likely cause is something came apart in the differential. A small bolt breaks and lets a very large pin slide out of place. Normally that pin catches on the housing and cracks it. There's lots of noise associated with that.
Sunday, April 1st, 2012 AT 12:52 AM