I have a 998 in dash and 858 in bow.The boat came with the transducer in hull for the front. I get interference on the 998 (dash) unit but not on the 858 (bow) unit. But that is not an issue with me since I turn the 998 unit off when I stop to fish – less noise going through the water. I push the on/off button once and the window that pops up shows sonar off. Very easy and fast to do. You can probably do that with either of your units. If you turn the dash unit off does the interference on the bow unit stop?I put a transducer on the TM foot and use it for deep water lakes. The interference isnt near as much as the through hull transducer but again, not an issue since I turn the 998 off when fishing.One thought – if you have the transducer wire running along the cable from the TM to the pedal, that can cause interference too. The other thought is to ground the sonar at the trolling motor. Im not sure how to do that properly as I havent needed to – but it is something to look into.I kept the through hull transducer wired up and when I fish shallow lakes I switch to that one. I tape the transducer mounted on the trolling motor up on the TM shaft to get it out of the way….in case I get too shallow and hit bottom. That way I wont damage it. Then when fishing deep water lakes I switch back to the transducer on the TM foot.
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