Burned plug on wiring harness, Cause?

[d*becker*123]

i have a 1985 mariner 75hp outboard and it wouldn't start the other day so found that the 20 amp fuse was blown. Replaced it and tried starting it again but immediately it blew it. Started looking into it and unplugged the wiring harness plug and found one of the plugs looked burned totally up. i can put new fuse in and run trim up and down but when i try starting the motor it blows. question is what burned the plug in the first place? could it just be a bad connection? I ordered a new wiring harness but don't want to install and burn it out to!? Thanks Dan

 

[aussieflash]

Re: Burned plug on wiring harness, Cause?

Start by checking all ground wiring and clean and tighten all connections.....shiny clean.

 

[Texasmark]

Re: Burned plug on wiring harness, Cause?

1. Answering the question: A burned connector occurs because the connection develops a resistance, usually due to corrosion. Heat is generated by current flowing through a resistance, defined by Ohm's Law: P in watts = current squared x resistance. In the case of your plug, the more heat the more destruction and it is a snow ball effect.

2. A burned open or high resistance coupling shouldn't blow your fuse. Resistance in the circuit means less current, not more. What has to happen to burn the fuse is an outside influence.

The fuse is connected to the engine's low current wiring and contained in that is the wiring from the battery through the start switch to the starter solenoid control wire, which when powered by battery voltage, closes the internal high current switch and applies power to the starter....separate circuit and not fused. Since nothing happens to the fuse until you attempt to start there is something in that circuit that has a lower resistance than required under normal circumstances. So, either you have a wire contacting another along the wiring path I mentioned, or shorted to ground, possibly through the insulation melting, or your starting switch (solenoid) energizing coil is shorted internally.

Disconnect the small power feed wire to the solenoid an, with a new fuse, attempt to start the engine. If the fuse blows you have a wiring short. If not your solenoid is internally shorted. Simple as that.

HTH,
Mark

 

[d*becker*123]

Re: Burned plug on wiring harness, Cause?

Thanks for the interest, I have checked all the wiring to and from the motor and am not finding anything shorted out. will keep checking. Is there anyway to check the solenoid to see if it is the prob? Thanks Dan

 

[oldman570]

Re: Burned plug on wiring harness, Cause?

I use dielectric grease on ALL electrical connections to help keep the connection good and clean. Can be bought at most auto supply stores and is cheap insurance againest correded electrical connections. JMO
Oldman570

 

[aussieflash]

Re: Burned plug on wiring harness, Cause?

Disconnect, clean till shiny,reconnect tight....have you done that yet?

 

[Texasmark]

Re: Burned plug on wiring harness, Cause?

Thanks for the interest, I have checked all the wiring to and from the motor and am not finding anything shorted out. will keep checking. Is there anyway to check the solenoid to see if it is the prob? Thanks Dan

Inside the solenoid is a sealed (plastic) coil of magnet wire with a hole down the center like a doughnut. This is your control circuit that you activate when you put the ign key to start. It has a very low current, like MAYBE 1 amp max in a 12v circuit.

There is a steel shaft with a copper plate on the end which is operated by current flowing through the coil. Current through the coil forms an electromagnet which sucks the shaft down and into it's hole.

The two large copper (high current) terminals have a large flat surface on the inside of the bakelite holding them in place and they are positioned such that when the shaft is sucked into the coil, the copper plate on the end of it make connection to the two terminals and in so doing, apply high current to the starter. Take the current off the coil and the internal spring opens the high current circuit.

You could remove the high current wires from their terminals and ohm between all terminals. The primary circuit consisting of the small wires and terminals should be at least 12 ohms and will probably be more like 30 or so (300 milliamperes to energize). There should be no resistance between either small terminal and the high current terminals. The small terminals will show a connection to ground as the low side terminal is in fact grounded.

Then, you might have someone hold the ign. switch in the start position and see if you have any voltage (to ground) on either large terminal with their wires still disconnected. You should have none. If you do, it's your smoking gun.

Mark

 

[d*becker*123]

Re: Burned plug on wiring harness, Cause?

Hey thanks for the response! I changed out my solenoid and everything has worked great, Thanks for the help! Dan