Electronic transmissions are kind of like sharks; only those who have seen the inside of one really understand how they work, and they usually wish they didn't know. But knowing what you can get away with requires at least a functional understanding of the thing; how the mechanism works, where the hydraulics come in and how the solenoids orchestrate the whole thing.
A transmission is three different systems all working together. The mechanical system -- composed of the input/output shafts, clutches and gears -- routes power from the engine to the driveshaft. A hydraulic system -- the pump and dozens of little fluid passages and valves -- controls the clutches, which determine what part of the gearset receives power. The electrical system -- the computer, sensors and solenoids -- controls the valves in the hydraulic system, and so dictate the transmission's shift points and shift firmness.
There are two basic types of electronic transmission systems: purpose-built electronic transmissions and those retrofitted to use electronic controls. Retrofit transmissions typically use a specialized valve body -- a series of fluid control passages -- that connects to solenoid-controlled valves. Transmissions like this may still have provisions for limited hydraulic control over the shift points and firmness, but odds are that yours doesn't. More sophisticated, purpose-built electronic transmissions will function no better without its solenoids than you would function without a central nervous system.
Transmissions typically have one less solenoid than they have gears. For instance, a four-speed transmission will generally have three solenoids; one for the first-to-second gear shift, another for the second-to-third gear shift and a third for the third-to-fourth gear shift. Failure of any one of these solenoids will result in a loss of fluid control during that shift event. So, a transmission with a dead two-to-three solenoid will shift into second gear, but not to third or fourth. It will work fine in those first two gears, but only in the first two gears.
Can You Drive It?
The short answer is that, yes, you can usually drive a car with a bad shift solenoid. Granted, it might not shift past a particular gear, but you should be able to drive it for a short period of time without causing any serious damage. Fluid pressure control should continue to function in the gear with the working solenoid, but you should avoid putting any serious stress on the transmission -- towing or drag racing -- just in case. Of course, all of this assumes that your particular transmission doesn't use a solenoid to engage first gear, and that the first gear solenoid didn't go bad. If it did, then you'll know right away because the car won't move.
- Today's Technician: Automatic Transmissions and Transaxles; Jack Erjavec