The German-U.S. automaker said it found the problems at the unit, the world's largest bus manufacturer, during a "routine investigation of its own business."
"This has led to the suspension of some executives, effective immediately," it said in a brief statement.
Diez, chief executive of the company's EvoBus unit since it was set up in 1995 and head of DaimlerChrysler Buses, resigned "for personal reasons," it said.
It did not give details of the irregularities or identify any of the suspended executives.
Ursula Mertzig-Stein, a spokeswoman at DaimlerChrysler's headquarters in Stuttgart, said EvoBus production manager Harald Landmann would replace Diez.
She declined to elaborate on the internal investigation. However, she said it had nothing to do with an ongoing probe of DaimlerChrysler in the United States.
The U.S. Securities and Exchange Commission and Department of Justice are investigating whether the company violated U.S. anti-bribery laws.
German prosecutors are also investigating former employees of DaimlerChrysler for suspected corruption in its business in Poland and Ghana.
In a March filing to the SEC, DaimlerChrysler said an internal investigation of bribery claims found evidence of "improper payments" in Africa, Asia and Eastern Europe and that several employees had been dismissed or suspended.
DaimlerChrysler sold 36,200 Mercedes-Benz, Setra and Orion buses in 2005, and says its market share of about 17 percent makes it the world's largest bus manufacturer.
In the third quarter, its bus and van businesses together contributed 3.2 billion euros ($4.1 billion) in revenue, or about 9 percent of the overall 35.2 billion euros ($45.2 billion).
DaimlerChrysler shares rose 0.9 percent to close at 46.39 euros ($59.55) on the Frankfurt exchange.