These wipers begin their refrain when the first rain is on the pane

There is barely a cloud in the sky as the bright red Volkswagen rolls along a Stuttgart side street. Suddenly, a blast of water from an open fire hydrant sprays across the windshield, momentarily blinding the driver. Before he can grope for the controls, however, the windshield wipers swing into action. A couple of cycles later, with the windshield cleared, they shut themselves off - much to the surprise of the driver and two of the car's three passengers.

``What you have just seen is a prototype of our new rain sensor,'' explains Peter Huebner, his confident smile contrasting with the surprised expressions of his guests.

Mr. Huebner is director of the switches and electronics division of the German-based SWF Auto-Electric GmbH, a major European automotive supplier, and subsidiary of the multinational conglomerate ITT Automotive. The rain sensor is one of the company's newest electronic projects - one that Huebner believes will show up on at least some luxury cars in the next few years.

Though it requires some complex programming, the system is relatively easy to understand.

A small sensor box is mounted somewhere on the front of the car - in the case of the VW, near the base of the wipers. The sensor consists of a small piezoelectric ceramic element. Commonly used in scales, outdoor propane gas grills, and other electronic devices, these crystals produce electricity when twisted or squeezed - even by the minute impact of rain drops.

The tiny surge of current is then measured by a microcomputer that starts the car's conventional wipers.

``If you leave the wiper switch on automatic,'' adds SWF president Karl Bleyer, ``the system will be controlled by the computer. The system is much faster than you are in turning the wipers on and off.''

That may not be a problem when there's a steady rainstorm, but it is useful during an on-and-off drizzle or when passing a truck, for example, that is kicking up spray from a wet road. It is handier than the intermittent wipers found on many of today's cars, Mr. Bleyer asserts, since a driver does not have to go fumbling for a switch, and since the wipers will work only as long as the car is getting wet.

The sensors are actually triggered by anything that deforms the piezoelectric crystals: rain, dust, or snow. The wiper controller's tiny computer must be programmed to tell the difference. It can also be connected to other sensors which detect the car's speed, helping to determine how many times the wiper blades cycle once they detect a drop of rain.

SWF is also using microcomputers and miniature sensors as part of another experimental system that can help nervous drivers comfortably squeeze into even the tightest parking spot.

As the red VW Passat is backed into a parking spot along the side of SWF's Stuttgart headquarters, a beep suddenly echoes from a small speaker near the back window. As the driver cautiously edges up to the car parked behind him, the beeping gets faster and faster. When it turns into a steady tone, he stops, cuts the wheels, and shifts into drive, moving forward until another, higher-pitched tone warns him that he is coming up on the bumper of the car in front of him.

Parking sensors are not entirely new; they have been offered for years on a number of American luxury cars. But there is a difference with SWF's system. Older warning devices relied on contact sensors - long feelers that looked like metallic insect antennae - jutting out from the car. They actually had to touch the curb or obstacle in order to warn the driver, and they were unable to measure distance.

The SWF system uses four sensors mounted in each of a car's bumpers. Ultrasonic transceivers, they operate just like radar, sending out and receiving back reflected high-frequency sound pulses that can detect - and measure the distance to - any obstacle between seven inches and five feet away from either bumper. The system is designed only to work at very low parking speeds, so drivers won't hear a beep every time they pass a car on the street or highway.

The device can issue its warning through high-pitched tones, on a visual display, or a combination of both.

Bleyer says the high-tech parking system will appear on a production passenger car sometime in the next year. While he declines to say where, auto industry sources say it will make its debut on the BMW 750, the German automaker's new, $75,000 sedan.

SWF's electronic wiper sensors are a bit farther away from the market, though Bleyer says they are under serious study by several manufacturers - reportedly including Mercedes-Benz.