I have a ST 40 deopth sounder which has 12 volts going to the unit but there is no display, all conections appear clean and tight, is there some checks I can do for voltage coming out of the unit going to the paddle wheel and the depth transducer?

There are no worries about battery size if you are using a motorcycle horn, because a mobility scooter battery is 6 times larger than a motorcycle battery. A motorcycle battery just uses a simple button, but larger car twin horns use a relay. A 12 volt sounder is electronic, so no worries there. You could use a truck reversing sounder. Don't forget that they are not omnidirectional, so they need to be pointing forward.

https://www.google.com/search?q=horn+relay+wiring

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Check all the fuses first . Clean up all the earth points on the body ,and grease them .Move all the wire conections under the bonnet make sure they are all on tight. Check the battery voltage .Anything under 12 volts is not fully charged .Sounds like a bad earth ,Hard to find these, if i am not able to look at it . .

My Raymarine E80 depth display kept freezing up (display then says "DSM connection lost"). Of course, this seems to happen more often when entering a tight shallow anchorage in Alaska's inside passage. Raymarine tech support recommended that I update the display software and then update the DSM300 sounder software. I did this to no avail. Then Raymarine told me to install a dashboard mounted toggle switch inline with the positive power input of the DSM300 module so I could do hard re-sets (turning off the system power or powering down the displays does not reset the sounder module). This I did. Now when entering a tight anchorage, the display still freezes, but at least I can do a hard re-set with the new toggle switch and have the unit back up in about 40 seconds (a life time in a tight channel with the current running, etc.). Now, Raymarine says that if I send in the DSM module to them, they will replace it with their new circuit boards that solve this chronic problem that they have known about for a long time (hello, this is a significant safety issue - why no recall?).
Don't mess with this - send your DSM300 in to Raymarine!
Bob S.

clean the battery conections and make sure they are tight
when it is running make sure its getting over 12 volts at an idel

My suggestion is check the regularor after you make sure all connections are clean and tight.

If a cobra 29 as indicated here what are you using for power for it? If a base radio as indicated it also has a 12 volt connection--if no problem via the 12 volt connection the fault is in the built in power supply the radio has.

All "Base" radios have 12 volt connection and their own internal power supply to operate off wall voltage.

install a computer "Amplified" speaker to EXT SPKR. The computer speakers run off 12 volts. Most do--then crank it up loud as you want. If output is not loud...go to Defcom radio mods.

thats how they are listed when they put them in the system.. if you look at ebay they are set up the same way everything is listed as a base Go figure.

now there is a chance that the transducer is bad ,, but I would check the battery voltage and make sure you have a soild 12 volts to the unit. also in my many years of working with and installing these units on boats I have see the needle pins ( where the plug inserts in the back of the display unit ) bend and/or break off completely. look in the plug area of the display unit and make sure all pins are intact. one more place to look for the problem is in the plug itself, take the plug apart and inspect the wire connection and make sure they are all good. if after all the above has been checked then I would assume that it is the transducer itself.
if you have anymore questions with this problem, please feel free to write back and ask,

c...

the 12 volt motor has a problem---possible needs brush replacement