Merc 850 No spark on cyc 3, switch box?

hi there<br />if any has any ideas on how to fix this problem it would be a great help. Motor info:<br />850 Thunderbolt Blue Band Merc Eng No. 4366928. 4 coil electronic ignition NO DISTRIBUTOR<br />This engine is only running on 3 cyc. I've test all the cyc for spark and found no. 3 has no spark at all. Yesterday after going about 300metres in the boat it lost a lot of power. On the way back the cyc would kick in for a second then drop out again. It now seems to have fully dropped out. I've re-wired all connections from the switch box to the coils because the wires were very old and corroded. I've swapped the coild and leads around the found they are all working fine. I'm assuming this problem is coming from within the switch box because all the other cyc have spark. Is there anything else I can check? If the problem was behind the switch box the other cyc would spark or fire...? But they all do so I'm assuming the problem is associated with the switch box. I've priced a new on on the net and they are very expensive so i was wondering if i could take the old one somewhere for repair or at least test it to make sure that is my problem. Any help anyone can offer would be great. Thanks Nial.<br />PS - I just purchased 4 new coils for this engine about 2 months ago and I noticed when I pulled them out to swap them around the grey magnet looking piece on the outside of some of them was cracked? Shouls this happen so quickly and will it be a problem in the future.

Re: Merc 850 No spark on cyc 3, switch box?

The spark plug wire could be grounding out. To check the switch box, you will need a voltmeter with DVA. But going by what you said, make sure that spark plug wire is good and the plug is good, not cracked. It those are OK and the coils work on the other cylinders after switching them around, then it is most probably the switch box. Boatfix.com has the lowest prices on switch boxes, I believe.

Re: Merc 850 No spark on cyc 3, switch box?

While you're at it, you can check the resistance of the stator, trigger, coils, and rectifier with an OHM meter.

Re: Merc 850 No spark on cyc 3, switch box?

swap the #3 primary lead for the #2 primary lead at the switchbox and retest for spark, if the problem moves to #2 than chase a trigger or switchbox issue as the number 2 coil to plug circuit was working. if the problem does not move test the coil primary,secondary and plug lead on #3.<br /> other than that its break out the DVA and test inputs and outputs at the switchbox.

Re: Merc 850 No spark on cyc 3, switch box?

HAven't tested my stator and that cause I replaced that 2 years ago, along with the trigger and the coils just recently. I'm guessing these should all be working ok cause the other 3 cyclinders fire perfectly. HAven't tested or changed the rectifier so not sure if that could have anything to do with it.

Re: Merc 850 No spark on cyc 3, switch box?

Thanks rodbolt, I'll give that a go tommorrow.

Re: Merc 850 No spark on cyc 3, switch box?

I swapped the leads on the switch box for #3 and #2 and the problem did move to number 2. Its sparked slightly at first but didn't spark again after that. Also can any help me out with the cracked thing around the coil. I'm guessing this isn't supposed to be crack and can I replace that part easy or does it come with the coil?