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Mercury 70HP 2 Stroke NO SPARK!

J

johnbatman520

New member
Hi Guys.
I'm only new here and have a problem with my 1987 mercury. My name is John by the way!
Ive had this engine for over two years and it's a good clean, well running engine, up to now! For the last few weeks I noticed it didnt idle as good as usual and sometimes after driving it and shutting it off it struggled to start. Then last Sunday after a day on the water, I shut it of to let some friends climb out, when I went to restart, nothing!! I took it home and tried for ages with the muffs on to get it going but with no luck.
I did a compression test on the 3 cylinders with the engine cold and I got 100psi at each one.
I tested for spark and there is none at either of the 3 plugs.
I'm mechanically minded and understand engines but not so much outboard and especially electrics.
the engine was winterized last year and got new plugs only 3 months ago. It's run about 3-4 hours a week for the summer months and only on fresh water although is was on salt water up to 2 years ago.
I'd really appreciate any advise of ideas ye may have and thanks in advance!
John.
 
The first thing I would check on this model since you have lost spark on all three cylinders is the kill circuit.

This motor does have a mercury (the liquid metal) tilt "kill circuit that kills the ignition if the motor is tilted too far up (a safety thing) plus the normal kill (stop) circuit that runs up to the keyswitch. It would not be uncommon for an issue in either of those circuits (bad mercury switch, bad keyswitch, frayed wire).

My first line of trouble shooting would be to disconnect these circuits. Both of them piggyback on the same terminal of the switchbox. If you are unfamiliar with the switchbox it is a rectangular unit located on the starboard side of the motor (right side if standing behind the motor - facing the prop) - it will have wires running to the coils etc but along the bottom of it (might be hidden a bit) there are 4 terminals (if my memory is still good) :)

The third terminal from the left should have two (2) black/yellow wires connected. Remove both and recheck for spark. If spark comes back then you can reconnect them one at a time to determine which "switch" is bad. If it turns out to be the mercury switch (will be on the short blk/yel wire and the unit itself (looks kinda like a condenser on old point/condenser motors) I would simply leave it disconnected. The second wire runs into the main harness - if that's the problem it could be the wire itself, the plug for the harness or the keyswitch itself (plus possibly a neutral safety lock in the control), so it would require troubleshooting from there.

If you still have no spark with the blk/yel wires disconnected, then at least you know the stop circuit is ok - and you curse because usually that is the cheap fix.

But try that first and then we can troubleshoot from there....
 
Graham really knows his stuff, I would agree 100%. I am betting he is right where all three are not firing. Look for loose connections while you are in there too. Something could have rattled loose.
 
Hi Graham.
Ok, I'm just after getting to have a look at the engine now. I did what you suggested and removed the 2 Blk/yellow wires from the third terminal on the switch box and tried for a spark but still nothing, I did however notice something strange, the red wire which should be connected to the first terminal(it's labelled) is also connected to the third terminal with the blk/yellow wires. Now I tried it with the two wires removed and the red on the third terminal as it was and also with it on the first terminal where it should be and still no joy. Do you think that there was a fault on the switch box before and someone swapped the red to the third terminal to bypass it or what do you think?
I had a look around all the terminals and wired on the engine and most of them seem fine and tight. The were two yellow wires coming from under the flywheel and the connectors seemed to be Kind of burnt or melted and rusty so I cut them out and connected with new connectors but still nothing.
what do you think my next step should be? I really appreciate the help by the way and I'm delighted with the detail ;)
John
 
Ok, the two "burnt" looking yellow wires from under the flywheel should be going to the rectifier (square looking thing with three posts). The fact that the wires looked melted/burnt could indicate that the motor was run WITHOUT a battery hooked up - that's a bad thing. On rectifier systems the power has to go somewhere - if it can't go to a battery it can (back up) into the charge coils of the stator and cook them - often taking out the ignition coils (bobbins) in the stator as well.

Now before going too far here you should try and get the switchbox at least wired back up properly. Besides the two yellow wires, that red wire you note and a blue wire should also come from under the flywheel (the blue and red are ignition, the two yellow are charge).

Those four terminals at the bottom of the switch box should be wired as follows - left to right.

far left - red wire from under the flywheel
2nd terminal - blue wire from under the flywheel
3rd terminal - those two blk/yel (I would still leave them disconnected at this point)
far right terminal - white/black wire that comes from the trigger (sensor), also under the flywheel but with it's own 4 wire harness/bundle.

Looking at the main body of the switchbox you should have three terminals on each side of the middle portion of the box - the top left one is higher than the top right.

The right side is the hook-ups for the trigger.

From top to bottom you should have -

top - brown wire from the trigger
middle - white wire from the trigger
bottom - purple wire from the trigger

The left hand side terminals are for the coils - again from top to bottom they should be -

top - green going to the #1 coil
middle - green/white - #2 coil
bottom - green/red - #3 coil

At the very top left of the switch box (which is also one of the mounting screws for the box itself) should be the three black (return) wires from the three cylinder coils (all on the same screw - this is the common ground to the engine block from the cylinder coils).

So first, I would wire the switchbox as described above, because if the red wire was moved perhaps someone got at the whole thing and "monkey pawed" - so who knows what you have there.

With it wired up properly, if you still have no spark (and I do suspect that will be the case, but start with the easy fix first), then I would look for a small engine shop in your area. It doesn't have to be "marine". The ignition on these is almost identical to dirt bikes, atv's etc. Any of these shops can check your stator to see it's throwing any power.

The switchbox itself can not be tested. It is ruled in/out by testing all other ignition components. If everything else is ok, you replace the switchbox. In this case some of your observations do lead to the stator (possibly caused by a fried rectifier). But like I say, a simple test at a small engine shop will let you know if the stator is ok.
 
Ok well firstly there's another strange one... The red wire we spoke about seems to be coming, along with a black wire from the kill switch on the throttle control. The red is connected along with the blk/yellows on the third terminal and the black to a terminal to the top right of it. The only wires coming down from under the fly wheel with those two yellows are one green with a white stripe and a white with a green stripe and the two of these go into a little long plastic type tube/box and a blue wire comes out the other side which is connected to terminal two on the bottom of the switch box.
I cant seem to locate the red wire which you talk about which should be terminal 1.
 
Ok, that "red" makes sense (sort of) - it's an added kill switch just piggy backed on the existing kill circuit.

However, you are still minus 1 wire coming from the stator - should be two stator wires (typically 1 blue and 1 red) feeding the switchbox on this particular model.

Most Merc stators (including yours) has both high speed and low sped windings. At idle speed most of the ignition power comes from the slow speed windings (the blue wire) as the rpms increase and the ignition needs more power (more "sparks" per minute) the (secondary) high speed windings (red wire) "kick in" and also supply power.

At say 1000 rpms you get somewhere around 300 volts AC from the blue wire and maybe 20 or 30 volts AC from the red wire.

At maybe 2500 rpms and above you are getting 300'ish volts AC from "both" wires - or double the ignition power which is then stored/converted to DC and distributed by the switchbox to the coils which up convert to many thousands of volts (approaching 100,000) to fire the plugs. If you are missing one of these wires the motor will never perform at it's potential.

Now unless someone has (at some point) installed an aftermarket stator (or something else weird here) that red stator wire should be there somewhere. Perhaps it's time to pull the flywheel and get a look underneath...
 
Ok I see, that makes sense. From having a good look around the engine I can see two cables coming from under the flywheel. One has 2 yellow wires, one green/wht, and one wht/green. Both of these greens together form the blue wire and thats all the wires in that cable. Now the second cable has one wht/blk, one white, one violet and one Brown. Thats all the wires in that cable so I Still cant locate this elusive red!! I cant see any other wires coming from under the flywheel so I think I'll take your advice and remove to have a better look.
Do you think that there is maybe only a one speed stator fitted? Is that possible? And also do you think there would be any use in maybe swapping that blue to the first terminal(red) to see if there is a spark?
 
It wouldn't hurt to try moving the blue.

It's possible that an aftermarket stator was installed at some point. None with a single wire come to mind at the moment but I do recall some differences in triggers for other models that have different numbers of wires from OEM - so it is possible.

It's the green, green/white wires that I find concerning if they are coming from the stator - those are usually only present from the switchbox to the first two cylinders coils. It has me wondering if someone got in there and "replaced" the red and blue wires with these and either tied everything together or "forgot" to splice the red back in.

In any case, they really shouldn't be coming from under the flywheel - so take it off and have a look. If the stator has been "messed" with that would be a pretty good indicator that it could be the current problem.

If you do find a couple of wires coming from the stator (other than the pair of yellows) and you have a ohm meter or multi-meter you can do a "quick test".

There should be continuity (resistance of about 4000 ohms give or take a bit) between the two ignition wires. If you do not have a detectable circuit there then the stator is definately "toast".
 
Its all a bit strange! Ok well I'm going to remove the flywheel today and have a good look about!
Is there anything I have to keep in mind when removing the flywheel? Such as do I have to mark any positions or watch out for something?
 
The flywheel is "keyed" in, so it only goes on one way. The nut is on their "tight" (torqued in), so if you can get use of a impact wrench it will make the job easier. Then a flywheel puller or even a steering wheel puller will do the job - don't use a jaw type puller that grabs the rim of the flywheel, it can do some damage. If the flywheel is stubborn you can crank down the puller tight and then tap the top of the crank (but don't whack the snot out of it)...
 
Hi Graham.
Sorry I have been away for a week!
My next move is to take the flywheel off and I'll hopefully get doing that in the next couple of days.
I'll report what I find!!
thanks again.
John
 
With your puller, use two of the 8 bolts holding the 'wheel on. You might have to smack the puller a few times with a hammer to get it to spring loose. I do these all the time that way without a problem.

Jeff

PS: Use only grade 8 bolts with the puller!
 
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