The City of Brotherly Love has brought loan repayment backsliders to heel by taking away their wheels. Federal marshals in the Philadelphia area have impounded the automobiles of 17 student-loan defaulters. To get their cars back, the debtors had to pay off their loans in full or, if they could prove financial difficulty, make a substantial down payment and set up a repayment schedule. All 17 complied. About 20 others whose names hadn't yet appeared on the defaulters' list have come in and made payments on their loans.