Ad-production firms are protesting GM’s onerous payment terms and are calling on Uncle Sam — the car company’s majority owner — to intervene.

The Association of Independent Commercial Producers, a trade group that represents production houses, is up in arms over GM’s policy of withholding payment until well after the shooting of commercials is underway or already completed.

The trade group sent a letter to Treasury Secretary Timothy Geithner yesterday urging him to examine the issue. GM’s payment schedule, which it announced in September, is at odds with most of the ad industry. Typically, production houses get about half of the production budget before shooting begins and the other half upon delivery.

The automaker is telling firms that they’ll get 50 percent of an ad’s production costs 60 days after the first day of shooting and the balance on completion. But most production houses are small shops with limited access to financing. GM’s pay scheme forces production companies to front most of the money for a shoot or try to line up outside financing with high interest rates, the trade group said.