Mercruiser 502MPI - No Ignition power.
 

Was heading home last week and one of my motors was running rough. Could only do 1800rpm, when I gave it more throttle it would die out, then slowly the motor was dropping rpm and what I gave it more throttle it would bog down like it wanted to die. I kept it at like 1600 to keep it going without pushing it. but then when I slowed for the no wake to a idle it died.
This is where it gets worst. Tried starting it and nothing. No beep from the ignition, no power, no start, nothing.
Finally got to the mooring and checked the following-
ignition panel breaker - wasn’t popped.
Big red breaker on the motor - wasn’t popped
changed the ignition relays on the top of the motor - no change.
I didn't have many tools and that’s all I checked. Pulled the boat and will be working on it this week. Any suggestions on things to check? Top of my list is to get a test light and try the ignition, starter. Anything else? Any thoughts on what to check when I go to the boat?
motor is 99 502mag MPI. Serial # OL352192
any help is really appreciated.

also to note - a couple weeks prior I was adding drive fluid and someone sped by me swamping the boat and the motor did get wet. Ran a little ****ting at first but then fine for the ride home.

So, starter will not crank ?

I would test battery first. If that checks good i would get your volt meter out and start looking for your voltage drop. I assume Positive power goes from battery to starter to box on top of engine.

not only will the starter not start I’m getting no power to the gauges or to the ignition that creates the beeping when you put it on

check and see if it has the 50 amp fuse on the starter that wire takes all the power to the top of the engine
https://cimg9.ibsrv.net/gimg/www.off...1c45338251.jpg

All good advice above.
Start right at battery then each connection from there.
bsttery
battery switch
connection at fuse attached to starter.
fuse at starter connection
don’t forget ground from battery to back of engine block

If all good, then big bullet connector for main harness. Loose ? Attached ? Clean ?

If all good, then main harness connector up at dash.

if all good ck wires at ignition switch.

Anyway, quite easy with multimeter. Just have to systematically ck for power and ground from battery posts to each connection after….1 by 1 until you find where loss of power is. Skipping areas just makes for frustrating guessing.

First thing check for a good ground by taking a set of jumper cables and ground both leads to the engine and and to both neg posts on a battery. May not be it but I chased gremlins for two weeks only to find it all Led back to a bad ground

so today tested this. There is 1 wire from battery getting 12v
then two on the other size. One is 12v and other is .80 volts.
top of the motor some areas have 12 and some have .8. Assuming this fuse isn’t broken but it looks rough and melted. Going to replace and check wires for corrosion.

, can you tell us the color /color combo of the .8volt wires ?

red and orange is giving me that low voltage (today it was .4)

on the same lug as the power coming in, there is a red wire (before fuse) that is getting 12. But after the fuse it is just low. Going to replace that fuse. Start there.
not sure how this fully happened. Water did get onto the motor a couple weeks prior. After that this happened.