Wiring Schematic?

I'm sure this has been aked a thousand times and someone will get upset by my asking again,but what the heck. I bought a different boat (18 foot Four Winns-1993)during the winter,90 horse Johnson runs great. BUT I'm showing a charge rate of 15 amps on the gauge,but my fish finder shows low voltage after about an hour. I figure I'll have to clean the contacts on all the gauges as my tach is on again-off again. the boat has had the fish finder and an electric trolling moto added to it and now I'm not sure if they were wired right.
My question is,is there anyplace I can get some sort of schematic that would show how to wire up a boat? Also,how do you check the charging output for the 90 horse Johnson?
I figure it may be somewhat the same as my Harley,but just thought I could get some ideas.
15 ampa at full throttle seems a little low to me.

are you talking 15volts or 15 amps?15volts is correct for regulated outboard charging system..

i assume you have the standard voltmeter on your instrument panel...thats the reason for the above question...the reading on that is important...if its a regulated system then it will be regulated to about 14.8 volts...if it is a rectifier only system it will creep higher...if it creeps into the 16v and higher range that could give tach problems and depending on brand and make etc of your fish finder it could give an over voltage alert...
wiring on a boat is fairly simple...the problem on a lot of boats is that its hard to get under the console to really spook the wiring....1.you want your trolling motor to have heavt enough wire going all the way back to the battery to handle it... it draws a lot of current and you cannot ''tap'' off anything to run it....the standard wiring under the console is for hot to go through a contact on the ignition switch to your tach and the voltmeter and gas gauge etc if you have a gas gauge...the ground side of the gauges will be looped together and go to ground...the volt meter is just two wires and will read battery output when the key is turned on..the tach will go to zero if its not already there..the gas gauge will read the level of the tank at that point also...the tach signal is fed with a grey wire going to the output of the voltage regulator and will read when the motor is running and you have voltage output from the regulator.(or rectifier with a rectifier only system)...you fish finder should be wired according to the manufactures specs...this should be on the web or possibly they have a support number...

the charging voltage to your battery is not a straight line dc...it has pulses riding on top of your battery output and these pulses are what give you your tach reading.if these pulses are too high with a non regulated system then your tach will go bonkers...it will read low and the high and just go all over the place..you cannot run a sealed or a gel battery with an unregulated system....sooner or later you will lose the rectifier..
most hard to find wiring problems under the instrument panel are cause by crimps...especially if the boat if left in the elements or used in salt water...the only real way to fix that is to remove the crimps and solder the wires together...boat people should be shot for using them in the first place....same with wires in the terminal connections...they slide the wire in and then crimp it...
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Let me add a few things to the very good advice from Papyson. Having never bought a new boat in my lifetime, I've spent more than a few hours unraveling old boat electrical systems. Here are a few things I've learned...

* Whatever you find or do to that electrical system, document it and draw diagrams of what is connected to what. Get a box of tags-on-a-string-and tag each wire once you know what it feeds. In a pinch, masking tape will work, but eventually fades or comes loose.
* Invest in a good voltmeter/ampmeter that includes a resistance measurement function, which can also be used to test continuity. A diode tester is a plus.
* Spend the bucks to get different colored wire spools so that you can duplicate the original ones as best as possible. If you are re-wiring, which you will eventually do, you will need a lot of red, black, and green.
* Most devices use only 16 gauge wire, some use 14 gauge. However, I find that on a boat, especially in saltwater, all my new wires are 12 gauge or larger.
* I've always found that it is a good idea to run #4 gauge red and black wires directly from the battery to a distribution block under or near the console. From that block, you can feed everything you may need - voltmeter, console lights, running lights, fishfinder, GPS, radio, windless, spotlight, horn, etc. either directly or through a fused control panel. The 4 gauge wire will minimize voltage drop and can carry all the amps you will likely need. That also minimizes the number of wires you have in a long run from your console to the engine or battery.
* If possible, build a dedicated conduit between the motor, battery, and console in which to run your wires. Have holes or access points about every three feet or so to facilitate running new wires. The conduit protects the wiring, prevents kinking or pinching, and makes for a neat looking wiring system. I've used PVC, flexible conduit, and large split wiring covers at different times and places.
* Fuse everything, and document and mark which fuse is connected to which device. Pay attention to the power (amperage) requirements of each device, and use the appropriate fuse for that device.
* Whenever you make an electrical connection, make liberal use of liquid electrical tape. It minimizes corrosion at the connection.
* As for the trolling motor, it needs to be isolated on its own battery that can be charged by the motor through a switchable main battery switch.

Yep,the old man said 15 amps and meant 15 volts. I had thought 15 volts should keep the battery charged,but just doesn't seem to be doing it. Put in a brand new battery and still run out of juice. Luckily,I have 2 extra batteries to carry with me,but that still doesn't cure the root problem.
Guess I'll just have to do as suggested and run each and every wire and clean and check each connection. Hey,the basic way seems to always pay off. Wiring is a little more abudant than on my harley,but not as hidden as my 50 Chevy. As always,thanks for all the help and info.

I run 2 batteries..they are completely isolated...no switches between them or anything...one is dedicated to the trolling motor..the other has the instrument panel and lights and ignition...both batteries are fully charged when i put the boat in the water...the charging system keeps the ignition battery topped off....the trolling motor is charged by shore power and a charger if it needs it at night when i am staying at my camp..if i was running a trolling motor enough to run it down on one days fishing i would add another battery....some people on here dont agree with me but i look at the charging system on most motors 100 hp and down as a way to ''maintain'' a charge...not a a tool to recharge a run down battery...that depends on the amp rating of your charging system of course but i can tell you that most under the cowl fires on the motor you are running are because the regulator is going up in smoke and flames..

I'll take issue with the crimps not being any good and add the caveat that they need to be made with the correct prep, parts and tools to be worthwhile....if you attempt to use the bigbox cheapo parts and a regular "plier-like" crimp tool, then yes - end result won't last long...

To make sure the charging system is working, you need to verify the max voltage it produces and verify it will provided the rated amperage...and check without both measurements is not sufficient. Common causes are bad (dirty/corroded) connections as well as damaged components.

Final "additions" to the list: 1) make sure every wire you add is marine grade & 2) use double-walled heatshrink on your connections...

When it comes to wiring components, you usually get exactly what you pay for (material wise) and it is best to pay for premium components.