1990 40hp 4 cylinder no spark at 2 cylinders. Stator or switchbox?

Normally the loss of 2 cylinders (on the 4 cyl model) means a Trigger issue. Your ohms tested ok but here's a follow-up trigger test (since you don't have a DVA) which I would do before moving on.

There are 4 wires coming from the trigger - White, Brown, Purple and Black - they all feed to your switchbox.

Mark the switchbox (or take a pic) and then swap the wires as follows.

Swap the white and the brown and the purple and the black (so white where brown was and brown where white was etc).

Then test again for spark - it shouldn't start but be careful anyhow, but it should still send power to the coils.

If your problem moves, then your trigger is bad.

If you lose spark on all 4 then your switchbox is probably bad and your trigger is bad also (would tend to indicate a bad SCR or capacitor, plus one of your trigger sensors is fried, BUT without doing a stator voltage test with a DVA you could never say 100%)

If the problem stays the same, the trigger is good and you are back to a bad switchbox OR stator...

You can do an ohms test on the stator (check for continuity between the stator leads) - that will quickly tell you whether or not you have toasted bobbins.

Check between the blue and blue/white, then red and red/white (doesn't matter which lead from the meter is on which wire) - you should get 5700-8000 ohms between the blues and 56-70 ohms between the reds.

If that's ok, then check between the blue (solid blue) and a good engine ground point and then with the Red (solid) and a ground point - you should have NO continuity (if you get some reading your stator is grounding out).

If that looks ok, then you can do one more "wire swap" to test the stator.

At the switchbox you can swap the Red for the Blue and the Red/White for the Blue/White and check for spark.

If the problem moves then your stator is toast. If it stays the same then your switchbox is probably shot.

(I have edited this once when I realized I gave some wrong info - after this sinks in a bit I may have to change it again, but it sounds correct at this moment )

Graham:

How do they get 4 timing signals out of a trigger coil with only 4 wires in it, and no ground wire?

Jeff

The trigger is essentially two coils that produce alternating current when the magnet passes them.

A trigger signal is a "pulse" of power that triggers (trips) the switch (SCR or Silicon Controlled Relay) which in turn releases the power stored in the switchbox's capacitor (which came as AC from the stator, was rectified (converted to DC and stored))and sends it to the coil and then on to the sparkplug for whichever cylinder the "trigger" told it to go to.

(sorry - needed to add some background/context here for other readers)

Alternating current doesn't have/need a ground like Direct current does to complete it's circuit. AC is a positive/negative waveform. When the waveform is (going positive) it's pushing power down the line. When negative it's (sucking) it back.

So with the trigger when it's "pushing" power it trips the SCR (that is set to fire cylinder 1 or 2), the SCR switches to cylinder 1 and releases power. When it sucks the power back (or reverses direction if you like), the SCR is (thrown) the other way (cylinder 2).

The trigger bobbin and the second SCR in the switchbox (with it's other assorted parts - capacitor, rectifier, blocking diodes etc) does the same thing for cylinder 3 and 4 (they could be paired in any combination - 1&3, 2&4 or whatever).

(wandering a bit here)

So, the 4 wires from the trigger work as follows - with no ground, because it's "alternating current".

If the power travels from the red wire to the switchbox and returns on the red/white back to the trigger that would be a "positive" pulse and cylinder 1 would fire. When the "wave" starts it's negative crossing, the current flow would switch direction and travel from the red/white through the switchbox and back to the trigger on the red wire - a "negative" pulse which switches the SCR to fire cylinder 2 (or 3 or whatever the "paired" cylinder is).

The firing order is dependent on however they have engineered (chopped up) the trigger coils and wired them. The two positive pulses (waves) will be 180 degrees apart and the two negative waves will be 180 degrees apart with the two complete waves separated from each other by 90 degrees such that you would get something like this - (in the example below the "degrees" would represent rotation of the flywheel utilizing a trigger being energized by two magnets - as the first magnet passes the trigger coil it produces the positive portion of the AC wave, the second magnet produces the negative portion - for each of the two trigger/sensor coils)

0 degrees - positive pulse from trigger coil 1 to say #1 cylinder

90 degrees - positive pulse from trigger coil 2 to say #3 cylinder

180 degrees - negative pulse from trigger coil 1 to #2 cylinder

270 degrees - negative pulse from trigger coil 2 to #4 cylinder

Sorry Jeff, long winded, but that's why there is no "ground"

(now my brain hurts - been over 11 since I used to teach this theory in the Airforce

Outstanding explanation! I got it soon as you mentioned "AC current".

Thanks again.

Jeff

PS: Into my Keeper File it goes.

Your welcome Jeff.

It is not 100% "technically" correct, but best way I could think to explain what is happening without writing about 20 pages of "gibberish"

Dang! Thought I was back at the US Army Missile, Munitions and Radar School at the Redstone Arsenal, Huntsville, AL. That was 1969/70...a long time ago...8 hrs./day, 5 days/week, 26 weeks then off to repair Hawk radar for 18 months at Okinawa, Japan. Wish I was back doing it all over again.

Darn, just barely missed you . I was in Hanza, Okinawa in 1993 doing some training at the HFDF site for the US Navy - and I miss it too, but have to admit, being retired does have it's benefits...

I would have re-enlisted for a $10K bonus but my wife said she'd leave me if I did that; would have had a great career. Oh well...dreams of yesterday.

Graham:

I thought you were in the RCAF?

Jeff

I was but did exchange duty with the US military on a few of occasions - sometimes working on personnel exchange programs and sometimes going to US Navy "C" schools (go figure, the comparible trade to my Canadian Airforce trade was in the US Navy).

My longest stint was with the US Navy during the first Gulf War. Spent a couple months in the gulf itself and then spend about 20'ish months working out of EastPac in Hawaii, doing mostly training at NSG station around the Pacific rim.