Patricia Montgomery, the Cadillac public relations person, punches one of the 12 extra buttons on the dashboards of the silver Seville with the $14,000 sticker on the window.

Next to the air conditioning outlet, a green number flashes.219. That's the engine temperature.

Another button clicks, 660, 660, 680, 660, the numbers flash, showing the engine is idling at 660 revolutions per minute.

Out on the road, another punch of a button shows the car is getting 14 miles a gallon as it rolls along at highway speed.

Punch again; 70 miles from now, the gas tank will be empty, the green digits warn. Punch again; only 45 miles from home, the computer assures the driver, no need to worry about gas this trip. Another button, at this speed, you'll be home by 5:30, the flashing light promises.

This is Cadillac's ultimate optional accessory. A computer for the car that has everything. (Well, almost everything, but we'll get to that later.)

Other autos may measure engine temperatures with a simple thermometer, or monitor engine speeds with an ordinary tachometer, or trust the driver to figure his mileage with a pencil on the back of a credit card slip, but not the computerized Cadillac.

A black box under the dashboard fills three digital displays with 11 facts at the press of the proper button. And all for only $875.

Between 3,000 and 4,000 of the 50,000 Cadillac Sevilles sold this year will come with this costly option, but General Motors engineers are predicting the on-board computer will one day be as commonplace as carburetors.

Once the computer is in the car, it can be wired and programmed to check the tires and oil, control the air conditioning, or manipulate half a dozen engine operations so the car produces better gas mileage and less pollution, explained Henry Seckel of the local Cadillac zone office.

The first computerized cars are now going to local Cadillac dealers, along with a 4 37-page wiring diagram, operating manual and trouble shooting guide - just in case.

Simultaneously - but not in the same car - Cadillac dealers are getting models in diesel engines. Like the computer, the $150 diesel option is available only in the top-of-the-line Seville.

The diesels don't come with computers, however, because the computer figures gas mileage using electronic information that can't be sensed from the diesel engine. The engineers will fix that soon, Seckel says.

The diesel V-8 in the Seville is the same engine made by Oldsmobile and installed in some of that division's cars and in some Chevrolet trucks.

Only 1,500 Diesel Cadillacs will be made this year, but production is expected to be greater for the 1979 model year.

The diesel's big advantage is economy. The engine is good for 21 miles per gallon on the Environmental Protection Agency city test, and 30 mpg on the highway cycle, for a composite rating of 24.

That's 50 percent better than the gasoline-engine version of the Seville, and within one mile per gallon of the Mercedes diesel, the Cadillac spokesmen point out.

The stress that the diesel Seville, at $14,949, is considerably cheaper than the $19,000 Mercedes 300-D, but otherwise they go to great pains to avoid mentioning the German competition.

"The diesel Seville is made for the customer who wants a diesel," they say, without a hint that Mercedes' success with diesels has taken that engine out of trucks and into the luxury-car market.

The introduction of the Calillac diesel coincides with the debut of another Mercedes model, and the beginning of a campaign by the German auto maker to increase sales of diesels, which now power more than half the cars Mercedes sells in this country.

Unlike the computerized car, which flashes its identity all over the dash-board, the diesel Seville is almost indistinguisable from the standard version. A little noisy when its two batteries first crank up the engine, the Cadillac quickly idles down to a sound that is unnoticable at curbside.

Only a small chrome badge on the back identifies the engine. PR-person Montgomery, who's driven the demonstrator stops go wild when she pulls up to the diesel pump.