the shield is tied to the tm housing internally. the trolling motor ground is tied to the tm housing internally thru a brown 18ga wire fused 3a to the tm ground wire inside the tm pwm control module. If the us2 is connected to the head unit, this ties the tm battery to the crank battery.
Some folks run an 18ga wire with a 3 amp fuse between the tm battery ground and the start battery ground. This wire accomplishes the same thing as the 3a fuse and wiring as I previously described in normal operation. This 3a in the tm housing has caused many headaches, especially tin boats.
when you operate the foot pedal power switch, a relay in the pwm module places power ( +12 or +24) on the positive brush and the pwm circuit pulses ground to the - brush. Brushes are isolated from the TM housing.
Did you check the fuse in the tm top head cover?
This is stuff to easy check. May not fix anything.
Bad connections could cause similar problems, so could a bad transducer. Too many to list as a possible cause.
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