MANETs (Mobile Ad-hoc networks) are a group of self-forming and self-healing mobile hosts communicating with each other, which do not require central authority or fixed infrastructure. MANETs are mobile and heterogeneous networks. Due to their mobility and heterogeneity, they are more vulnerable to attacks. Grey hole attack is an attack, in which all the packets in a network are forwarded to the attacker node by neighbour nodes and are intentionally dropped. In this work, we proposed a secure data protocol which intends to identify and avert the grey hole attacks by considering the causes of packet drop in promiscuous mode. We improved an existing AODV based routing protocol for detection and prevention of grey hole attacks. The results of this experiments show that MANETs are secure against grey hole attacks with our proposed algorithm with AODV based routing protocol.