Ignition Diagnostics 1987 70HP

I lost fire on middle cyl at idle as well as top end. Found by pulling plug wires while motor running at idle one at a time until I pulled one that made no change in the way motor was running. Swapped plug with bottom cyl still no fire on middle cyl. Swapped coil with bottom cyl still no fire on middle cyl, swapped plug wire with bottom cyl still no fire on middle cyl, swapped wires from switch box to ign coils between two cyl just to make sure the wires were not broken and still no spark at middle cyl. I can pull top and bottom plug wires and see a good blue spark between pulg wire and plug but no spark at all on middle cyl. plug wire. Well all this redneck testing can be done with a little know how and a pair of pliers and I have traced it back from the cyl to the switch box. Now comes the fun part trying to figure out if its the switch box or stator or maybe even a retifier ? I have a DVM but dont quite understand the DVA testing of the switch box as well as the stator. Have found both online for about 100 bucks each. My guess would be the switch box but my question is if the stator is bad will it burn out my new switchbox if I replace it first. My S/N- is 0B175091 and I hope the year model is 1987 not positive about that. Could anyone tell me if the stator will burn out the switchbox or power pack as some call it ?

Re: Ignition Diagnostics 1987 70HP

If the stator was bad you would not lose fire to just the middle cylinder. I also lost #2 on the switchbox and used an old switchbox terminal after cutting off the top dead capacitor to replace the bad sector and got it to fire for testing. I,m waiting on my new switchbox as we speak. So if the trigger wires where swaped and the ciol wires where swaped and all worked on the good sector of the switchbox, then the #2 sector is shot. pulling plug wires of the spark plugs to test can short out the switchbox. Being it,s bad already I would do it for testing as well.View attachment 174183

Re: Ignition Diagnostics 1987 70HP

As Max has stated you have a bad switchbox and the stator should be fine. Look where the stator wires go under the flywheel to see if they have been rubing against the flywheel and shorting out as sometimes will happen. Any testing as has been done has to have ALL the plug wires grounded or there is a good chance of blowing the diodes in the switchbox. JMO
Oldman570

Re: Ignition Diagnostics 1987 70HP

Happy to report my new switchbox has arrived today and I have assuredly checked all wiring for shorting and i,m clear except my tac is now not funtioning. I hope he takes you advise and finds the reason for the bad sectors before installing the new switchbox. Rain expected tommorow but I,ll get time to install and test it. Whoo hoo!!

Re: Ignition Diagnostics 1987 70HP

pulling plug wires of the spark plugs to test can short out the switchbox.

Click to expand...

Any testing as has been done has to have ALL the plug wires grounded or there is a good chance of blowing the diodes in the switchbox.

Click to expand...

Dont know where this rumor/tale started but CDI and Mercury will tell you this will not damage ignition components, You ground the plugs for safety so you wont get shocked and it wont ignite a fuel/air charge blown from a cylinder...

Re: Ignition Diagnostics 1987 70HP

Because there is NO RETURN on electrical parts you do not want to assume anything and order any parts until tested and proven bad.

Your 'no spark' on #2 could be cause be a bad switchbox OR a bad trigger.

Test again by switching the trigger inputs to the switchbox, swap #1 and #2 trigger then retest again for spark.

If still no spark on 2 and 1 is still sparking that pretty much proves the triggers are good and the switchbox is bad.

If #2 now sparks but #1 does not, then there is a problem with the trigger or trigger wiring.

Either way, test to prove the fault before ordering any electrical part.

Re: Ignition Diagnostics 1987 70HP

Replaced switchbox and found white/black wire burnt all the way to trigger. Do we have to replace trigger/stator at the same time ?

Re: Ignition Diagnostics 1987 70HP

When I search for a trigger the stator and trigger keep coming up as a assembly. Does anyone know where to find the OHM readings for the stator and which wirese should read what ? Seems someone told me to check thoes wires but they run behind the starter. Better to pull the starter than gamble $$ on parts. One good point is I now have a spare switch box and still 600 below dealer rpair cost. Found stator/trigger assembly on amazon for 112.00

Re: Ignition Diagnostics 1987 70HP

No one can be replaced without the other.

Re: Ignition Diagnostics 1987 70HP

112.00 for both New or Used Either way that not to shabby of a price. So your saying the switchbox was not the issue or whatever was grounding shorted the new one out before you looked for a bare spot. Lets try this again With the plugs removed take the middle trigger wire off the switchbox put the lower triger wire on the middle pole where the wire was removed and see if you get spark on #2 plug. If you have spark the trigger has a bad lead. follow it back and see if it is broken inside the insulation, otherwise get anothe trigger. Replace the wires to there original position when finnished. If the stator was bad you would get NO SPARK

Re: Ignition Diagnostics 1987 70HP

The wires looked OK until I pulled them off the switchbox and the white/black wire just turned to powder in my hand. At this point Im not sure if the switch box was bad or not. After I replace the trigger I will reinstall the old one and see if it fires the middle cly. Best thing I can tell anyone is to pull the starter and inspect the wires well before ordering parts. I only paid 93 bucks for the switch box so I wont complain. Main thing is I think I found the problem. Diagnostics can sometimes be a pita. Wires can really be a pita. The wires from the trigger and stator have a cover around them and the only way to inspect the wires is to cut the cover off all the way up to the trigger and stator under the flywheel. I pulled the flywheel to inspect them up close to the trigger and stator. It looked like the black/white wire was compleatly burnt into about a inch from the trigger. Im sure a ohm meter check would have found this the easy and correct way.

Re: Ignition Diagnostics 1987 70HP

Triggers only generate 1 1/2 to 4 volts depending on RPM, it is hard to imagine a trigger wire burnt. I may be possible that a trigger wire grounded and something went wrong inside the switchbox, backfeeding voltage thru the trigger circuit.

Testing every terminal with a DVA meter may tell a story, but if you found a trigger and switchbox for that price then it may be good to just replacel both just to be sure.

Re: Ignition Diagnostics 1987 70HP

I agree as I need a trigger (all wires spliced and connectors added) with a bonus stator would be great

Re: Ignition Diagnostics 1987 70HP

I searched amazon for the trigger and came up with a trigger assembly that had a pic of the stator. Could not tell if I was getting a trigger a stator or both. I checked iboats first but they are out of stock as well as marine parts plus and then found a trigger at ********** with a good description of what I was getting. I opted for **********. Amazon can be a little bland on descriptions of parts. I did not want to chance getting a stator and having to send it back while my boat was out of service. Striper fishing is in full swing in the Carolina/Georgia area right now. Will post results when I get the old merc running smoothly and thank you greatly for all the replys.

Re: Ignition Diagnostics 1987 70HP

Got trigger replaced and now have good spark on all three cyl. Was going to reinstall old switchbox until I resaeched a little and found that a bad switchbox could burn up the trigger. Dont have the nerve to retest the old one. I think amazon made a booboo and put a pic of a stator for a trigger assembly. I think all you would get is a trigger for 112.00. Anyway all is good with the firing issue and now she runs at wot with no issues. Now if I could get the rough running out at 1/4 throttle I would be good.Thanks for the replys.