THE AMERICAN TARIFF on small imported trucks suddenly leapt up the other day from 4 percent to 25 percent. It's a nice example of the way things work in the sweaty world of trade politics -- press a point with Japan.

The tariff on light trucks is, in a sense, an accident. It also illustrates the truth that in trade policy things are likely to turn out very different from what anyone expects. The United States imposed that 25 percent duty long ago, in the 1960s, in retaliation for irritations of a similar nature committed by the Europeans. The truck duty was aimed at the Germans; in those days, Volkswagon was shipping some small trucks here. The shipments ended, but the tariff stayed on the books.

A decade later, Japanese manufacturers were shipping small trucks here in parts -- two parts of chassis and cab, to be bolted together on this side of the water. The duty on truck parts is only 4 percent. As the volume of those imports rose, the predictable squawks from labor unions and some manufacturers here rose in proportion. The Customs Service looked into the matter, pondered it and decided -- in a process, all parties agree, that was untainted by any political consideration whatever -- that a chassis was, in fact, a truck. The duty automatically went up.

The effects are complicated. All three of the American automobile compaines have been importing very small Japanese trucks and selling them here under their own names. Conversely, the only compact trucks actually produced here in the United States are made by -- yes, indeed -- Volkswagon, in its American plant. The company that was originally the targot of the high duty is now its only beneficiary, and the three companies supposedly protected by that duty are now paying it.

As for the Japanese manufacturers, they are deeply distressed. Mr. Carter has three choices. He has the legal authority to reduce that whopping 25 percent duty by two-thirds. If he invookes it, he will please the Japanese and outrage the American unions. If he does nothing, he will please the labor unions -- but that duty is, after all, a heavy tax on the American consumer who wants to buy a light truck. The best choice is the third choice -- to offer to negotiate with the Japanese. If they want the truck duty lightened, what trade concessions are they willing in return to American manufactured goods? Mr. Carter would be well justified in using this case to nudge the Japanese into opening their own market a little wider to their foreign competitors .