Re: 1978 Mercury 80hp can I use control box from 50hp around same year?
Tony, if your 80hp uses the round externally-connected wiring harness and so does the 50hp set of controls, it'll work without any modifications.
Some later Mercs have a different style of conn plug which is smaller in diameter and connects to a corresponding internal harness, inside the cowlings instead of an externally-connected plug.
If the 50hp wiring harness matches up to the 80hp's internal plug, it could be used as well.
If you find the wiring doesn't match up, you might be able to locate a compatible harness and swap it out. Either harness should fit the box. And the shift & throttle cables will definitely work.
Note that in the case of different style of internal/external connectors, the wire-colors for the various functions are consistent. It is possible to cut & adapt wiring harness either way if you are adventurous to do a cut-&-splice.
All this is quite simple if you have external connectors, just plug-in & enjoy!
Here's some wiring diagram info that may help:
http://www.maxrules.com/MERCwireindex.html
