My buddy's got a Garmin--I think 160--that's older.  When he first turns it on in deeper water--over 20 ft--it will read something outrageous like 750 ft. and won't read the bottom contour.  As he moves to shallower water it starts flashing and switches to the correct depth.  When he moves back towards deeper water it will change to reading incorrectly again.  Any tips?  

I used to have a Hummin Bird that acted like this.  It worked great while sitting still, but when the trolling motor was on, it would act up.  I was told it had something to do with interference comming off of the battery.  I was too dumb to figure it out, so I sold the boat.

Go to the adjustment menu & change the range to auto.

i don't think he knows how to work the settings!  That would be the logical first step though. Thanks.

He is probably picking up interference from something else electrical within the boat. Or his transducer might be out of whack. Unless he has the money to buy a new one, it may be worth sending back to the factory to be tested. I have two Garmin 250's on my boat and the one at the console took a little tweaking at the transducer from the factory to get it to work at speed. Something to consider...  

Is the transducer in the hull or on the trolling motor?

The transducer is in the transom.  Haven't got a chance to talk to him yet so we haven't tested it.  I will try adjusting the range first then consider interference.  What can you do if there is interference?  

If the transducer is shooting through the hull it could just be a problem with the way it was installed. When you silicone or epoxy the transducer to the hull, make sure the silicone or epoxy is evenly distributed. if there is a gap it could cause it to act up. First thing you should try is hooking another depth finder to it. that will tell if its the unit or the transducer that has a problem..

If it were a bad installation or Interference it would most likely do it all the time. It's working in shallow water.

I have the same problem, 15 metres becomes 150, but only at speed, so I'll leave that be for now

Basstronaut- Where is your x-ducer located? It's sounds like when you running your x-ducer is in the turbulence from your boat.

There is an article on interference at http://bassresource.com/fishing/electronic_interference.html maybe it will help