'77 Merc 20hp Ignition Problems (fires one cylinder)

[mtskinne]

I have a '77 Mercury 20 hp that is only firing one cylinder. I checked the coils by switching the wires coming from the switch box and found that they were working fine. I then took the switch box off to get the part number for replacement. When I did, I saw that the brown wire ( one of two that leads to the trigger) was frayed badly, and was almost broken. I was wondering if this could be the whole problem or if I should replace the switch box as well. <br /><br />If I should try replacing the wire first, what type of wire should be used? Copper primary wire of the same guage? Can this be used to rewire all the way back to the ignition coils?<br /><br />Also, how hard is it to replace a switch box? Is it just splicing into existing wires?<br /><br />Thanks in advance for your comments

 

[achris]

Re: '77 Merc 20hp Ignition Problems (fires one cylinder)

If the brown wire was broken you would have no spark on either coil. the switch box fires one cylinder on the positive going pulse from the trigger and the other cylinder on the negative going pulse. If either wire from the trigger (brown or white) is broken, the circuit for both cylinders is open. Switch boxes are not hard to replace. The wires should be just "plug and pray".

 

[mtskinne]

Re: '77 Merc 20hp Ignition Problems (fires one cylinder)

Thanks for the reply, achris, could it be a problem with the wires running to the coil?

 

[achris]

Re: '77 Merc 20hp Ignition Problems (fires one cylinder)

Yes, but unlikely. Have you removed the switch box and visually inspected the wires? If you give the wires a bit of a pull (not too hard), if one is broken it will usualy 'stretch' the isolation.<br /><br />If you do need a switch box the part number is 339-6222A4.<br /><br />Chris...........