Abstract: The present invention provides a portable cylinder contribution tester for an electronic diesel engine, the diesel engine including a plurality of cylinders. The tester comprises a plurality of switches, each of the plurality of switches coupled to and for controlling one of the plurality of cylinders and a case for mounting the plurality of switches. When an abnormal diesel engine operation is detected each of the plurality of switches can be activated. When one of the plurality of switches is activated and the diesel engine does not decrease in performance it is indicated that the associated cylinder is malfunctioning. Existing test equipment for most electronic diesel engines is PC based and available to franchised dealers only, as well as not being portable for drive testing. Through the use of a cylinder contribution tester in accordance with the present invention, these type of repairs could be available to many independently owned non-franchised facilities.