Anton Weinmann, a member of the firm's board, said he was stepping down to help MAN start over with new leadership. CEO Hakan Samuelsson announced his surprise resignation last week and he was soon followed CFO Karlheinz Hornung.
"The well-being of the company is my primary focus. This has led me to the conviction that I will back MAN's fresh start on the management level," Weinmann said in a statement.
The maker of heavy trucks and other transportation machinery could face up to €300 million ($450 million) in fines, costs and fiscal sanctions stemming from a corruption affair, according to a report by daily Süddeutsche Zeitung on Monday.
A MAN spokesman told AFP the amount was just "speculation," and added: "The only firm figure is €50 million in internal investigation costs that have already been booked."
But according to the paper, MAN expected to also pay a fine of between €200 million - €250 million and a tax charge.
The group is the subject of a German judicial probe into alleged corruption involving around €10 million in foreign payments and around €1 million in Germany.
The money was allegedly paid as bribes to obtain contracts and sales of MAN busses and heavy trucks.
The head of MAN's supervisory board, Ferdinand Piech, is now said to be placing aides in the management board with a view to merging MAN with Swedish truck maker Scania, which belongs to Volkswagen.
Piech is also head of VW's supervisory board and Europe's largest automaker owns almost 30 percent of MAN.