Wiring harness interchangeable? '73 Merc 500 to '76 Merc 850

[BR]

I?ve got a 1973 Merc 500 with original control box and engine wiring harness. I would like to use this same control box and harness to get a 1976 Merc 850 started and running. My question is can I use the same control and wiring harness to run both motors by plugging into one or the other? I have studied the wiring diagrams for each and I am concerned because the polarity of the main terminal pins from control box to engine harness appear to be opposite for each motor. (See attached diagrams) The 500 shows a Red (pos.) connection on top pin and Black (neg.) on bottom. The 850 diagram is opposite. Is this correct? Should I swap the cable connections at the battery for the 850? (seems like a bad idea) I have looked at lots of other Merc wiring diagrams and this is the only one I can find with a neg. pin on top. All others seem to be like my 500. I really don?t want to fry any expensive parts with a trail and error method to see if it works!

 

[emckelvy]

Re: Wiring harness interchangeable? '73 Merc 500 to '76 Merc 850

A control box that'll plug into a '73 50 HP Merc will plug in and operate a '76 Merc 850 as well. I think what may be confusing you is that in the diagram they drew the connector upside down.

About the only thing that control box won't run (for motors with an external plug) are Dockbusters and perhaps some of the early Full Gearshift Inlines (if I recall the pins had a different orientation).

But, if it'll plug in, it'll work.

HTH........ed

 

[BR]

Re: Wiring harness interchangeable? '73 Merc 500 to '76 Merc 850

Ed
You are exactly right about my confusion. It never occurred to me that the diagram could be drawn upside down. That's what I get for relying on convention! Glad to know my contol box is good for the 850. Thanks for your help.