Employees are participating in their 401(k) plans at a record rate, with record-high balances. But that doesn't mean they're taking an active role.

That's according to a survey from Aon Hewitt, which analyzed data from 138 defined contribution plans and found that participation rates hit 79 percent in 2014. That's the highest they've been since the company began tracking such data in 2002. Average plan balances are also at a record high, rising from $91,060 at the end of 2013 to $100,320.

But Aon found that only 24 percent of workers boosted their contribution rates in 2014, and even fewer — just 15 percent — rebalanced the investments in their accounts. When workers whose accounts are fully invested in target-date funds or other premixed portfolio options that do not need to be rebalanced were eliminated from the mix, that figure rose to 19 percent.