A House-Senate conference committee yesterday agreed on legislation repealing a law Congress passed less than a year ago that required the use of daily logs to substantiate business use of autos and personal computers.

The committee met for only an hour, and a final compromise left the law little stronger than it was before the original provision was passed as a deficit-reduction measure last summer. Users of cars and other business equipment that might also be put to personal use will be encouraged, but not strictly required, to keep written records of their business use. The House version of repeal had called for written records, but the final version will permit oral evidence of business use under most circumstances.

If it is approved by the House and Senate, the compromise would kick in next year. For 1985, taxpayers will be covered by the old law. The old law required that taxpayers show use of the auto or equipment was necesary, without setting specific standards. The new law sets up several questions taxpayers must answer to prove use of the property was for business.

Thousands of letters had poured into congressional offices protesting the rules, which the Internal Revenue Service first made effective last January. The service weakened them almost immediately, and more than 20 members of Congress testified at a hearing in March that the rules would be arduous to comply with and represented excessive government interference.

Rep. Sam Gibbons (D-Fla.) reflected that view during the conference, saying that if the rules were permitted to remain, "pretty soon we are going to get tax withholding on the second cup of coffee" a worker gets for free on the job.

Conferees also agreed to exempt certain vehicles from the remaining record-keeping requirements. Among them: school buses, marked and unmarked police and fire vehicles, cement trucks, hearses, moving vans and heavy trucks. House Ways and Means Committee Chairman Dan Rostenkowski specifically forbade the exemption of unmarked IRS vehicles.

The conferees also let the Treasury Department resolve another sticky issue, the valuation for taxation purposes of the use of company planes by executives for personal travel. The Senate had tacked onto the House bill an amendment weakening the provisions of the 1984 law.

Ronald A. Pearlman, assistant Treasury secretary for tax policy, said the department had agreed to propose rules that would value a flight on the fanciest corporate jet at twice the price of the equivalent first-class commercial fare. The Senate had called for a value equal to the first-class fare, while current law requires valuation equivalent to the price of a comparable charter flight.

The compromise also limited the tax benefits from corporate ownership of luxury cars, making them more strict than current law. The The repeal of the provision shows the increasing difficulty of raising revenue by cracking down on tax benefits enjoyed by millions of small businesses and individuals. maximum investment tax credit that can be taken will be $675 rather than $1,000; the maximum anount that can be depreciated the first year will be $3,200 instead of $4,000.

Partly because of that provision, the legislation will have little, if any, effect on federal revenue. The agreement is expected to come to a vote in the full House next week, with the Senate following soon after.

The repeal of the record-keeping provision shows, observers believe, the increasing difficulty of raising revenue by cracking down on tax benefits enjoyed by millions of small businesses and individuals. Like the enactment and subsequent repeal of withholding of taxes on interest and dividend income two years ago, the record-keeping rules seemed to hit a raw nerve across the country.