Remember GM's bright idea for a complicated digital lock that would prevent drunks from starting their cars? Well, Ford has come up with something like it, only this one is designed to thwart thieves.

The device is a computerized keyboard display for the car dash that will prevent the car from being started until the driver enters a personal four-digit code into the system.

Called Theft-Guard, the system will retail for $70 plus installation, Ford says.

The keyboard is about the size of a telephone push button system. The computer hardware is wired through the car's electrical system to the battery. When the driver enters the car, he pushes four preprogrammed numbers to make the ignition work. If incorrect numbers are pushed, the car won't start.

"Theft-Guard interrupts the ignition wiring in such a way to keep the car from starting even if the keys are in the ignition," said Martin Jones, marketing manager for Ford's parts and service division.

"A thief is concerned about the amount of time it takes to steal a car," Jones said. "If he cannot get the car running in a few minutes, he generally won't persist. Theft-Guard was designed to frustrate the would-be thief."

For years General Motors Corp. had been working on a similar system, but the intent was to prevent drunken drivers from being able to start the car. Under GM's system, the driver would have to enter a long sequence of numbers into a computer terminal on the dash before the car could be started.

The Ford system concentrates on thwarting thieves but would provide a similar obstacle for drunks.

John Emmert, a spokesman for Ford's parts and service division, said Theft-Guard will fit any car, including domestic competition. It will not work on some foreign cars with a positive ground electrical system, however.

Ford is emphasizing that the system is aimed at amateur car thieves and joy riders."We aren't making any claims that a car can't be stolen if the thief is given enough time," Emmert said. "And nothing will stop the pro who might even back up a tow truck to the car he wants."

If the keyboard is smashed or ripped out or the wires cut and crossed, the thief still can't start the car, Ford claims.

There is an override button to deactivate the system so that the car can be loaned to a friend or parked by a valet without having to give out the code.