THIS YEAR MORE and more people are faced with a terrible decision: whether to prolong the lives of their used cars, knowing they're terminal, or pull the plug and let them die in dignity and peace.

I was faced with this dilemma last week when the AAA towed my car to the nearest emergency repair station.

The Head Mechanic, a member of the American Mechanics Assn. (AMA), came out and opened the hood as I stood nervously by. He hit the engine a few times with a wrench. Then he took me aside, out of hearing range of the car.

"I'll be honest with you. I don't know how long I can keep it alive. It's been through a lot. My mechanics will do everything in their power to keep the motor running, but it will cost you."

I bit my knuckles. "Maybe it would be better if you just took it out in back and shot it," I said.

"I can't do that. When I became a mechanic I took an oath that no matter what condition an automobile was in, I would repair it. We can't decide which cars will live and which will die. That would be playing God."

"But it seems to be in such pain," I said. "The battery is half dead, the engine sputters and the doors keep falling off. Keeping it alive would wipe me out financially. I'm sure it would be happier in a junkyard."

"You're talking about me getting involved in a mercy killing, and that is forbidden by the AMA," he said.

"If mercy killings of used cars were permitted, many people would take advantages of them just so they could buy a new one. Our profession demands that if there is the slightest sign of life in a vehicle we have to keep it on the road even if it's just for a few hours."

"But surely you have seen many automobiles that would be better off if they were put away forever. Why give hope to the owner when you know in your heart there is no way to save it?" I asked.

"There's always hope," he said, putting his arm around me. "I've seen cars in even worse shape than yours, and their owners have pleaded with me to dump them in the Potomac River. But hours later the drivers were sitting behind the wheel whistling 'Dixie' and tooting their horns. When you're dealing with the life and death of a car you have to believe in miracles."

"But there are some unscrupulous mechanics who will pretend they can save your car when in fact they know the engine will die as soon as it leaves the repair shop," I said.

"Admittedly, there are few. But a majority of those in our profession are dedicated to prolonging the life of an automobile no matter what it costs in time and spare parts. Of course, it's your decision. You have to tell us whether you're are willing to allow us to keep your car alive until the bitter end."

I looked over at my automobile. The oil was leaking tears, anda the tires were sinking into the pavement. The rear-view mirror was cracked and the seat was torn in three places. Its headlights started pitifully at me.

The chief mechanic said "I'll bet you had some wonderful times with her in the old days."

"I did," I admitted. I showed him a picture of the car when I first bought it.

"She was beautiful," he agreed. "Well, what have you decided?"

I finally blurted out, "Do everything you can to save her. Maybe I'm just a sentimental fool, but even if it's only for a few more days I never want anyone in my family to say I didn't give her every chance."

The chief mechanic smiled and started to write on his clipboard. "I was hoping you would say that. Now I think the first thing we ought to do is put in a new transmission and then . . ."