Perplexing electrical issue

[Ferson206]

Under way last week, thought we smelled something like burning plastic, then voltmeter started jumping all around. Killed motor to look around, and it would not restart. Not even the gauges moved. No sign of any electrical power. Never pinpointed the smell, but it seemed to be coming from under the dash, maybe the ignition switch. Back ashore, later discovered one of the 50A fuses by the big wiring plug on the engine was blown. Replaced it, and now I have voltage on the gauges but just a click when I turn the ignition. Very very strangely, my blower now works only with the ignition switch on, whereas previously it operated whether switch was on or off.
Testing under the dash, I am getting something like 5.5 volts from the big red wire coming into the the ignition switch.
A side story (I think it's a side story) to all this is I just replaced the starter. Before the instance of smelling burning plastic and apparently blowing that big fuse, we thought the starter was wearing out. It would just click, and then couple taps on the starter would get it to start on the next attempt. When I got power back after replacing the 50A fuse, I still got just clicks, even after tapping the starter, so I figured I had solved the electrical issue but the starter had ultimately died. Now I've just replaced the starter and am getting all those symptoms I just described. I'm hoping the starter was actually failing and this electrical issue was happening concurrently, but I suppose maybe they are connected.
Confused yet? I sure am. Any suggestions welcome. Voltage test at battery shows around 12.3 volts (tired from all the clicking I think), and I have that same level at the big wire to the starter.

 

[Ferson206]

Forgot to mention this is a 1980 Glasply with an OMC 120hp sterndrive.

 

[dingbat]

Under way last week, thought we smelled something like burning plastic, then voltmeter started jumping all around. Killed motor to look around, and it would not restart. Not even the gauges moved. No sign of any electrical power. Never pinpointed the smell, but it seemed to be coming from under the dash, maybe the ignition switch. Back ashore, later discovered one of the 50A fuses by the big wiring plug on the engine was blown. Replaced it, and now I have voltage on the gauges but just a click when I turn the ignition. Very very strangely, my blower now works only with the ignition switch on, whereas previously it operated whether switch was on or off.
Testing under the dash, I am getting something like 5.5 volts from the big red wire coming into the the ignition switch.
A side story (I think it's a side story) to all this is I just replaced the starter. Before the instance of smelling burning plastic and apparently blowing that big fuse, we thought the starter was wearing out. It would just click, and then couple taps on the starter would get it to start on the next attempt. When I got power back after replacing the 50A fuse, I still got just clicks, even after tapping the starter, so I figured I had solved the electrical issue but the starter had ultimately died. Now I've just replaced the starter and am getting all those symptoms I just described. I'm hoping the starter was actually failing and this electrical issue was happening concurrently, but I suppose maybe they are connected.
Confused yet? I sure am. Any suggestions welcome. Voltage test at battery shows around 12.3 volts (tired from all the clicking I think), and I have that same level at the big wire to the starter.

Makes sense up until the point where you try to tie the starter into it.

Burning plastic, 40 yro wiring and a blown 50 amp fuse is a short to ground.

Being DC power, a hot short to ground can back feed power into the circuit through the ground connection. Miraculously switched circuit that are off become energizes, etc. curiosity of the ground loops in the system

Be easy to troubleshoot on paper if you had a good wiring diagram of the boat. What circuit is common with the ignition switch, big red wire and
blower?

Pull the big red wire from its termination point at the helm and measure the voltage again.
If the voltage on the red wire returns to normal the short is in front of where your pull the wire. If the voltage isn’t normal, follow the red wire back to its source of power and check the voltage there.

 

[Ferson206]

Makes sense up until the point where you try to tie the starter into it.

Burning plastic, 40 yro wiring and a blown 50 amp fuse is a short to ground.

Being DC power, a hot short to ground can back feed power into the circuit through the ground connection. Miraculously switched circuit that are off become energizes, etc. curiosity of the ground loops in the system

Be easy to troubleshoot on paper if you had a good wiring diagram of the boat. What circuit is common with the ignition switch, big red wire and
blower?

Pull the big red wire from its termination point at the helm and measure the voltage again.
If the voltage on the red wire returns to normal the short is in front of where your pull the wire. If the voltage isn’t normal, follow the red wire back to its source of power and check the voltage there.

Super helpful! I think I have wiring diagrams in my manual, will attach photos. Not seeing any reference to blower. I’ll proceed with those suggested tests and see where that leads me. Thank you dingbat!

 

[Ferson206]

 

[dingbat]

This is only the motor. You need wiring for the boat proper

You didn’t swap power and ground on the starter did you?

 

[Ferson206]

This is only the motor. You need wiring for the boat proper

You didn’t swap power and ground on the starter did you?

Right. I’ll try to find appropriate diagram. Problem presented before I had done anything to the starter, and I am pretty sure the new one is wired correctly. Same model starter and I took photos so I could replicate how it was wired.
I did just pull the big red wire from the ignition and I’m getting 8+ bolts there (at end of wire) now instead of 5. Now following that red wire back to see what else it connects to between ignition and battery.

 

[hugh g]

What caused that 50A fuse to blow in the first place?

 

[dingbat]

Now following that red wire back to see what else it connects to between ignition and battery.

 

[kenny nunez]

Be sure to check for 12 volts at the starter solenoid.

 

[Lou C]

What you have is the engine harness, you need the boats wiring harness as well. The fuse box should be marked is it not?

 

[Ferson206]

Thanks for the many helpful suggestions. Fuse box not marked as far as I can tell. There is some old masking tape on some of the wires indicating what they are, but some are faded or have fallen off. I do have 12v at the starter solenoid.
Other stuff I discovered:
1. The big red wire is getting about 9v at the ignition rather than 12. Still trying to find its next connection at the back of the boat.
2. There is a big terminal block in the back, and some of the connections look pretty corroded. Picture attached. I'm wondering if this is source of some voltage drop, and possibly a ground short?
3. There are some other small ground wires that look really bad, like this small one pictured running to a bolt inside the fuel filler.
Had to head home, so I'm no longer where the boat is. I'll keep looking online and ask at Glasply forum to see if I can find a wiring diagram.

 

[Ferson206]

Got this back from the Glasply forum: "No wiring diagram exists. If you think there’s a bad wire, abandon it and running a new one to see if that fixes your issue. Not sure what condition your GP is in but after 40 years the wires certainly become brittle and do not work so well." lol

 

[Lou C]

Well then the only way is to start pulling fuses and see what doesn't work and label the fuse box so at least you know.