The 9 week shutdown will reduce production by 170,000 cars, trucks and crossovers, and impact 55,000 U.S. workers.

There’s a mystery to the news of the morning.  What is so far known is that General Motors, facing the immediate threat of bankruptcy, and the more extended problems caused by the massive slump in car sales, plans to idle the majority of its U.S. assembly plants, over the summer, for as much as nine weeks.

The downtime would eliminate about 170,000 cars, trucks and crossovers from its already bulging inventory.  What we don’t know, yet, is which plants are to be idled, but that the automaker is expected to reveal that by the end of this week.

The move is the latest in a series of setbacks for the giant automaker, which has barely five weeks left to convince the White House it deserves an additional bailout.  As if to underscore that situation, GM officials are telling bondholders they won’t receive an expected $1 billion debt payment that was due on June 1^st.  Whether this will help convince reluctant lenders to move ahead on a demand to exchange $28 billion in debt for equity is uncertain, but it certainly gets the message across.

GM is not alone, here.  Chrysler is also struggling to reduce its $6.9 billion in debt, but along with Treasury Dept. officials, it rejected a bid from lenders, including JP Morgan Chase and Citibank, which would have given them a major equity stake in Chrysler while giving up only a small portion of the carmaker’s debt.

The Treasury Dept. yesterday countered with an offer of $1.5 billion and a modest 5 percent equity stake.  Chrysler must resolve its debt issue – and overcome other obstacles, including new union concessions – by the end of this month.

GM, which was given 30 more days than its cross-town rival, is struggling to put together a plan that would permit it survive in even the weakest American car market the industry has seen in decades; the current annualized sales rate is barely hitting 10 million, down from a peak of more than 17 million, earlier in the decade.

The giant maker has already announced plans to permanently shutter a number of assembly lines, and that list is expected to grow larger.  But for the near-term, it needs to balance inventory to demand, and the equation has been way out of whack.

Normally, GM takes a two-week summer vacation, using that time, in part, to convert its plants to handle the coming model-year’s products.  This time, some plants will be down for up to nine weeks, though facilities building the company’s strongest sellers, such as the Chevrolet Malibu, may see little or no extended down-time.

In all, about 55,000 workers are expected to be affected by the extended shutdowns.  Though this will have some impact on their pay, they’ll continue to take home about 70 percent of their normal wages, thanks to the SUB, or Supplemental Unemployment Benefit, program, the United Autoworkers Union long ago negotiated for itself.