The Treasury Department said the new aid, which comes from a taxpayer-financed bailout fund, is less than the roughly $6 billion the government had earlier thought GMAC would need to stabilize the company.
The fresh infusion is on top of $12.5 billion in taxpayer money Detroit-based GMAC has already received from the government. The new agreement will boost the federal government's ownership in GMAC to 56 percent, from 35 percent.
Even with the government upping its stake, Treasury officials said the government intends to stick to its policy of leaving day-to-day business decisions about financing to GMAC management. Still, with the additional stake, the government will have the right to appoint two additional directors to the company's board, Treasury officials said.
GMAC will continue to be subject to executive pay restrictions imposed by the government's pay czar.
Shoring up GMAC has been a major component of the Obama administration's massive effort to rescue ailing automakers General Motors and Chrysler. The lender provides critical wholesale financing to thousands of GM and Chrysler auto dealers, allowing them to stock their showroom floors with vehicles.
But GMAC also operates a large residential mortgage business, ResCap, which was battered by the recent housing collapse. GMAC was obligated by the Treasury Department to raise $11.5 billion in additional capital earlier this year after failing the government's stress test for banks, largely because of ResCap's big losses.
The stress tests were to see whether banks had enough capital even if the economy worsens next year. However, GMAC had difficulty raising money because of its financial woes, making an extra government infusion necessary.
Michael Carpenter, who succeeded Alvaro De Molina as the company's CEO in November, has said the company would need no more than $5.6 billion in aid. Lawmakers estimated the company would receive between $2 billion and $5 billion in additional aid.
Any additional government money would come from the $700 billion Troubled Asset Relief Program that has been used to bail out troubled financial institutions and automakers.
Even after the latest capital infusion, the government will likely take steps to help GMAC as it tries to ensure the recovery of GM and Chrysler, said Kirk Ludtke, senior vice president at CRT Capital Group LLC. That includes helping GMAC refinance its debt as it comes due, he said.
"The government has come this far, it is not going to destabilize GMAC at this point," he said.
GMAC still remains on shaky financial ground. Last month, it reported a quarterly loss of $767 million, though the results were an improvement over a giant loss a year ago. ResCap lost $747 million during the third quarter as homeowners continued to default on their mortgages in large numbers.
GMAC, which also provides financing to car buyers, has also been hurt by the rapid decline of the U.S. auto industry after sales crumbled due to the recession and financial woes of the big automakers. Sales of cars and trucks were down 24 percent through November compared with the same part of last year. The industry is expected to sell around 10 million cars this year, one of the worst performances for autos sales in decades.
Despite the drop in auto sales, GMAC's auto lending business has shown some signs of revival. The auto financing division earned a profit of $395 million during the third quarter. The company's online consumer banking unit, Ally Bank, has also been a bright spot by bringing in billions of dollars in new deposits by offering relatively high interest rates. It now accounts for about 29 percent of GMAC's assets.
AP Business Writers Candice Choi and Dan Strumpf in New York contributed to this report.