Consumers took on $1.09 billion in new debt in September, a government report said yesterday, but the increase in installment credit "definitely should not be interpreted as a sign that consumers are coming out of the doldrums," economist Allen Sinai of Data Resources said.

In October consumer confidence fell sharply, the Conference Board said yesterday. Its index of consumer confidence dropped to 49.2 from 54.5 in September, compared with a base of 100 in 1969, suggesting that "the notion of the consumer leading the economy to recovery is not real at this point," director of the board's consumer research, Fabian Linden, said yesterday.

President Reagan hoped that a consumer boom, sparked by the income tax cuts that came into effect in July, would pull the economy out of recession this year. Instead, consumers have so far been very reluctant to increase spending. Retail trade and auto sales have been disappointing and many businessmen now fear sales will stay sluggish for the rest of this year.

Although consumers boosted their outstanding debts by much more in September than the $66 million in August, "you will have just the opposite in October," Sinai said. Much of the September increase was because of strong auto sales, as the last the 1982 model years were sold off cheaply. There was a rise of $505 million in net automobile credit outstanding in September, compared with a drop of $402 million in August when car sales were weak. October auto sales have been weak so far.

Economist Alan Greenspan, an outside adviser to the White House, added "there is no evidence of a change in attitude" among consumers that would suggest an early pick up in sales.

From one standpoint, consumers are now in a good position to start spending and borrowing more. The amount of consumer income going to pay off debt was only 14.8 percent in the third quarter of the year, according to yesterday's consumer installment release from the Federal Reserve. This is the lowest figure that has been recorded in the past 10 years, Commerce Department Chief Economist Robert Ortner said yesterday. A year earlier the figure was 15.4 percent

In addition, with recent sharp declines in interest rates, consumers should feel like saving less and spending more.

But there are two factors working against a strong pickup in spending. First, high and rising unemployment has scared many away from making large purchases or committing themselves to high payments. There are now about 15 1/2 million would-be workers either looking for jobs, too discouraged to go on looking, or working part time when they want to work full time. All of these people are less able to afford to spend and borrow than they would be if the economy were healthy.

Moreover, the plight of the unemployed scares others still at work. "It's pretty hard to be confident when the person at the next bench has been laid off," Linden said yesterday. With layoffs continuing, even those still working are understandably nervous about taking on new commitments.

A second strong influence holding down spending is very weak income growth in recent months. The July tax cut did help raise disposable income by 2 percent. But since then, after tax incomes have risen by only 0.1 percent in August and 0.3 percent in September.

Depressed wages and high unemployment have made people wary of spending what they have, while also cutting into the incomes that they have to spend.