New guy here with minor question

[CarlCVX20]

Hey everyone. My first post and hoping someone can point me in the right direction.

Im going to start investigating a no spark on one cylinder issue on my 1978 merc V2000. I’m going to test the resistance and voltage on the suspect switch box but I’m wondering if I should first disconnect the battery or not. Common sense tells me I should but will I still get accurate readings with the battery disconnected?

thanks in advance.

[Smalltownbassin]

Welcome! Hopefully one of the seasoned guys with knowledge of that motor will help. Be patient and they will come.

[racervboat]

Hey everyone. My first post and hoping someone can point me in the right direction.

Im going to start investigating a no spark on one cylinder issue on my 1978 merc V2000. I’m going to test the resistance and voltage on the suspect switch box but I’m wondering if I should first disconnect the battery or not. Common sense tells me I should but will I still get accurate readings with the battery disconnected?

thanks in advance.

You need the cables hooked up to crank it over to test it.More then likely a switch box, replace them in pairs.Thats a old motor look for crumbling insulation on all the wires very common.

[CarlCVX20]

Thanks for the replies, and advice. I’m hoping the resistance tests will tell the story as I don’t have a DVA adapter for my multimeter yet. I’ll go over all the wiring as well when the rain stops.

[CarlCVX20]

Here are my findings so far..

with the black/white lead between both packs disconnected, and with my meter set at 20k ohms, the inner pack 1-3-5 reads 14.5. The outer pack 2-4-6 doesn’t read at all. #2 cylinder is the suspect not firing. If I change my meter to 200k the second pack reads 66.5 which is strange to me.

I also did readings on brown to white, white to purple, and purple to brown according to the CDI troubleshooting guide. Each of those read 1.4.

So my question now is, since this seems to be a bias circuit issue should I go ahead and replace both packs or are there other tests I should be doing to confirm?

thanks in advance.

[25two.stroke]

Here are my findings so far..

with the black/white lead between both packs disconnected, and with my meter set at 20k ohms, the inner pack 1-3-5 reads 14.5. The outer pack 2-4-6 doesn’t read at all. #2 cylinder is the suspect not firing. If I change my meter to 200k the second pack reads 66.5 which is strange to me.

I also did readings on brown to white, white to purple, and purple to brown according to the CDI troubleshooting guide. Each of those read 1.4.

So my question now is, since this seems to be a bias circuit issue should I go ahead and replace both packs or are there other tests I should be doing to confirm?

thanks in advance.

Yeah something is funny there...I would swap out both boxes for a new set. Especially if they are the original boxes. No need to disconnect the battery to test resistance in the switch boxes. Just follow the steps in the CDI Guide. You need the battery to test the trigger output via DVA...cause you do that while cranking. I would verify trigger output and if its there then swap out the switchboxes.

I assume you have already moved the coil lead from #2 to any other coil to make sure that the coil itself is not bad?

[CarlCVX20]

Hey 25, thanks for the reply. When you say coil lead are you referring to the green wire that’s connected to the coils primary circuit? No I haven’t done that but I have tested resistance on all the coils. I did find one that wasn’t measuring anything on the #1 cylinder so it’s been replaced thinking that was causing the issue. All the coils ( except for the new one) read in the high 500s from the coils tower.

They are are the original boxes, there’s an inspection stamp dated 1977.

Motor or has new plugs, wires, fuel filter/water sep, water pump kit, this year.

[25two.stroke]

Oh yeah I would be swapping those boxes even if there was no problem with them. Make sure you get genuine merc ones, preferably the clear resin boxes (and not the ones with a red dot/resistor/thing, those are supposedly look-alike foreign knock-offs). Pay the money, like $110 ea rather than 60, to get the real ones. Those or CDI electronics blue boxes.

Coil leads, yes, take the one going to #2 and send it to #4 and see if #4 looses spark (not while running, just doing a cranking spark test) If so then you know the switchbox is not firing the #2 coil lead. You can also switch them on the box without switching them on the coils themselves.

[CarlCVX20]

Thanks for the advice on which boxes to buy. will definitely look into it getting good ones.

I don’t actually have any spark test test equipment so I won’t be able to perform the lead swap you mentioned. My evidence for no spark on #2 is that that spark plug is super clean, like it just came out of the packaging with no burned fuel on it whereas the other five look “normal” for a motor that’s been running a few hours on new plugs.

[25two.stroke]

Well shoot...just pull the plugs out and ground them and see if they spark! Disconnect your fuel so you don't blow up...I seen it before.

[racervboat]

Well shoot...just pull the plugs out and ground them and see if they spark! Disconnect your fuel so you don't blow up...I seen it before.

Well shoot... just hold the plug wire and give it a crank you will know for sure.

[25two.stroke]

Well shoot... just hold the plug wire and give it a crank you will know for sure.

grab the plug wire in one hand and put a finger from your other hand over the empty spark plug hole with the other..compression test, spark test, and defibrillator all in one!
I had a coworker who would hold the plug wires to test spark...he liked getting shocked...not for me. I sometimes you cant help it though when you touch a plug wire and you are standing in a puddle of water. Or metal test tanks...many, many times those have gotten me.

[CarlCVX20]

Are you suggesting this because my switch boxes might still be ok?

whats the best/safest way to ground spark plugs?

thanks for your help, I’m really eager to learn and maybe I can help others at some point.

[25two.stroke]

Are you suggesting this because my switch boxes might still be ok?

whats the best/safest way to ground spark plugs?

thanks for your help, I’m really eager to learn and maybe I can help others at some point.

Yes, I was, but I would still change them now or in the near future simply because of their age. They are known to fail, not all the time, but sometimes.
Its always best to diagnose the problem first though, even if you plan on replacing other parts. It would be a shame to swap the boxes and assume the problem will be fixed but then find a bad trigger, wire or coil...

[CarlCVX20]

if I do the spark test and actually get spark from #2, what would cause the plug to still be in such new condition?