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1978 Mercury 50HP 2 stroke electric start

C

compton1963

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I have a 1978 Monarch Aluminum Bass Boat, The wiring harness that goes from the motor to the remote has dry rotted. I have replaced the harness and was in the middle of changing the ignition switch when I realized on the drawing I have the 7 pin plug. I have a white wire that goes to the igntion switch. However the engine side of the harness has no pin and now where for the white wire to go. The drawings show the same thing. I am stumped on what to do next. I am thinking I am going to have to bypass the plug/ harness in some way and route the white wire elsewhere. But I definitely need some help. PLease advise
 
please see attached diagram. This is as close to what I actually have that I can find. The white wire goes to pin "F" on the ignition and then to one side of the choke. and then yes to the tachometer. But again I do not understand why the pin is missing on the engine harness side . Thanks for your help
 

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  • MERC 500 DIAGRAM.pdf
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I have the 1976 version of your motor and have the same wiring diagram. I think the white is tied to the red in the run and start-run positions and becomes the ignition wire to power the choke switch and the tach. It is not needed on the motor side although I don't know why it runs all the way to the motor harness.
 
After looking through some wiring diagrams for different motors from this era, some of them use the white wire on the motor side. I think the same harness was used on many different motors and not all wires are used on every motor, like ours.
 
ocboat said:
After looking through some wiring diagrams for different motors from this era, some of them use the white wire on the motor side. I think the same harness was used on many different motors and not all wires are used on every motor, like ours.
Click to expand...
Thank You for your help.
 
compton1963 said:
please see attached diagram. This is as close to what I actually have that I can find. The white wire goes to pin "F" on the ignition and then to one side of the choke. and then yes to the tachometer. But again I do not understand why the pin is missing on the engine harness side . Thanks for your help
Click to expand...
I know your post is over 2 years old, but thanks for posting the engine wiring diagram. My 1978 50hp Merc "500" harness wire insulation is rotted, bare corroded wires exposed. Surprised it didn't burn up! Looking for replacement harness, so far no luck. Thanks again.
 
fitz73222 said:
Are you talking about the engine internal wire harness or external harness running to the control box?
Click to expand...
It's the internal engine side of the harness. The external "boat" wiring from the controls and the battery plug into a connector on outside of the cowling. It's the wires inside the cowling that are bad; rotted insulation and oxidized copper wire. This harness seems to be called a lot of things but it has two (red and black) battery cables to the starter and solenoid. Also has wires to the choke solenoid, mercury tilt switch, rectifier and one wire to the switch box. My harness connector is like yours, 6 pin on the engine side, and 7 pin on the "remote side" as you stated. Perhaps that's the way it is supposed to be? On the engine side, there is no white wire as far as I can see from my wiring and also your diagram. Going to examine closer tomorrow, map it out to see if I can make sense out of it.
 
compton1963 said:
I have a 1978 Monarch Aluminum Bass Boat, The wiring harness that goes from the motor to the remote has dry rotted. I have replaced the harness and was in the middle of changing the ignition switch when I realized on the drawing I have the 7 pin plug. I have a white wire that goes to the igntion switch. However the engine side of the harness has no pin and now where for the white wire to go. The drawings show the same thing. I am stumped on what to do next. I am thinking I am going to have to bypass the plug/ harness in some way and route the white wire elsewhere. But I definitely need some help. PLease advise
Click to expand...
How did your project go? I have not even got into the boat side of the wiring to examine, except to snip off 15 inches of positive battery cable due to insulation failure and light corrosion on the 4 ga. wire inside. What about the mysterious white wire?
 
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