First, be certain that you are not running both the units at the same time as this will cause them interfere with each other and show the "---" reading (see note below for more information). Secondly, test the unit first by hanging the transducer overboard directly in the water. Once a correct reading is obtained, go back to the mounting location and test again with either the Petroleum Jelly or Water in Bag Methods (Make sure that the depth is at least 2.5 feet deep, and not over 200 feet from the mounting location to the sea floor). If correct readings are not obtained using these method move to a new location and retest. If you cannot obtain correct readings anywhere, you will need to transom mount the transducer, or upgrade to different style transducer. Once correct readings are obtained operate the vessel under normal conditions and verify the performance is acceptable; you may need to relocate the desired mounting location several times to find the optimal location. Finally glue the transducer in using an adhesive that is suitable for your application (refer to the operators manual for more information).
**NOTE: If the boat is large, say 50 feet or longer, you may get two units to read correctly if the transducers are mounted far away from each other (one in the front, the other in the back) However, as the water gets deeper, the chance of correct readings is reduced.