NEW YORK (CNNMoney.com) - The financial challenges for General Motors' GMAC finance unit may not go away, even if a majority stake in the operation is sold, as is widely expected.

Embattled GM (Research) announced last fall that it was looking at a possible sale of its most profitable unit in order to restore its credit rating to investment grade. The parent automaker's junk bond status is what caused GMAC to also fall into junk bond status last year. That in turn has limited GMAC's access to the low-cost capital that it's used to make loans in the past.

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The Wall Street Journal reported Thursday that GM is close to selling a majority stake in GMAC to an investor group led by Cerberus Capital Management. The paper reported that GM recently settled upon the broad structure of an $11 billion deal for 51 percent of GMAC with Cerberus, a New York hedge fund and private-equity group.

But Bob Schulz, the top auto credit analyst for rating agency Standard & Poor's, said on Thursday that he couldn't assure such a transaction would be enough to return GMAC to the investment-grade status it hopes to gain with this sale.

"If they sell a controlling stake to one highly rated financial institution, that will likely get them to investment grade," he said. "If it's a consortium, we wouldn't view it as favorably, but we haven't tried to estimate what the rating will be."

GMAC spokeswoman Joanne Krell said she couldn't comment on Schulz' comment other than to say, "We are working to get a strengthened credit rating. That is the purpose of this -- to have access to lower cost of funds. That is critical to the strength of the business."

Certainly GM does not want to sell GMAC if it does not have to. GMAC contributed $2.5 billion to GM in 2005, when the parent company lost $10.6 billion -- its worst performance since 1992.

GM has said that it has other alternatives to raise the funds it needs to make auto loans, even without a sale and a credit upgrade for the unit.

Last year it raised $15 billion through the sale of auto loans to Bank of America (Research) and the Bank of Nova Scotia (Research), even as it continued to service those loans. It has agreements to sell another $60 billion in loans over the next four years to those banks and is eyeing additional sales.

GM competitor Ford Motor Co. (Research) is also suffering under junk bond status and has had its Ford Credit unit downgraded into junk bond status as well. But it is not looking at a possible sale of its credit unit, as a statement from the company said that "Ford Motor Credit is a core element of our business and an important piece of Ford Motor Co."

Ford Credit is primarily only an auto finance unit, while GMAC has significant mortgage and insurance business. In 2005 GMAC had 62 percent of its income before a special charges come from its mortgage and insurance, and auto finance was only a majority, not all of the remaining business.

For more a look at GM's recent restatement of results, click here.

For why GM CEO Rick Wagoner can't catch a break, click here.