1988 Merc 115 Inline 6 - Still no Spark on #4

[chuck_boulos@yahoo.com]

First thanks in advanced for any responses. 1988 115 TOP. Originally no spark in even banks. Replaced both switch boxes. Still no spark on #4. DVA read average 1.5V on all other cylinders.Suspected bad trigger. Although it ohmed out correctly no trigger voltage. Replace trigger and still no #4. Even replaced stator. Three hundred and fifty bucks later no #4. Conducted all the usual test. Swapped coils, check all wires, swapped switch box banks, and all the other reading in the CDI troubleshooting guide . Still no #4. Grateful for any suggestions. I'm stumped.

 

[Chris1956]

Well, it sounds like you renewed all the components. That leaves only the wires and grounds to check/replace. Is the #4 spark plug wire arcing to ground? Is the #4 spark plug cracked or otherwise defective?

Are the ground wires from the block to the cowling intact?

 

[chuck_boulos@yahoo.com]

Hi Chris, I'm using a spark tester so no plugs in the equation. All grounds intact. Not getting the 1+ volts trigger to the power pack. All the other 5 are fine. Good signals. New trigger so that's the baffling part.

 

[Faztbullet]

What volt are you getting? Good be a bad switchbox .. when trigger coil filp/flops voltage to SCR in box one leg is the ground for trigger coil and this is also balanced thru bias circuit. Flip pack and see if problem follows it, if so replace/return both packs as bias circuit is damaged. ONLY use Serria/Mallory/CDI or OEM boxes .The Amazon ones are junk.

 

[chuck_boulos@yahoo.com]

Hi Faztbullet. Thanks for the response. I can rule out the switch box for two reasons:

1) #4 is not getting a signal from the trigger assembly. Used both my DVA and oscilloscope. 1.5-1.8 volts on the average. BTW I'm an electronics guy. Well use to be. Ohmed out the trigger and everything was in spec. . Even lifted the input and nothing.

2) swapped trigger inputs to #4 and #6 and problem moved. Meaning switch packs are good. So this is totally baffling as I've already replaced the trigger and stator.

Fired-up the engine thinking maybe it needed more flywheel RPMs. Still #4 input croaked. I highly doubt its the flywheel. Checked it over and all the magnets are in place and secure.

Any crazy thoughts? I'm totally stumped............Thanks

 

[Faztbullet]

Ohm the bias circuits...

 

[Faztbullet]

Mercury Outboard 6 Cylinder with ADI Ignition

No fire on the 1,3,5 or 2,4,6: Swap red and red/white wires, also blue with blue/white wires. If the problem moves to the other set of cylinders, the Stator is at fault. Disconnect rectifier and retest. If the engine fires normally, replace the rectifier. If no change, we recommend replacement of the Stator. If you replace the Stator and the problem remains try another flywheel, if possible.

No fire on one cylinder: Since this condition can be caused by the opposite switch Box (pack), disconnect the white/black jumper between the packs and retest. If the dead cylinder starts firing, replace the pack that was firing all three cylinders. As a verification, swap the Trigger and spark plug Wire to the cylinder closest to the dead cylinder, (i.e., if #1 is dead, swap the brown Trigger Wire with the white one). If the problem moves to #3, replace the opposite power pack (switch Box). If #1 is still dead, swap the green and green/white coil wires. If the problem moves to #3, replace the power pack (switch Box). Please remember to put the Trigger wires in the correct order after you finish.

Always check the bias circuit: Disconnect the jumper between the packs on the white/black posts. Check the resistance from the white/black posts on each pack to No fire on the 1,3,5 or 2,4,6: Swap red and red/white wires, also blue with blue/white wires. If the problem moves to the other set of cylinders, the Stator is at fault. Disconnect rectifier and retest. If the engine fires normally, replace the rectifier. If no change, we recommend replacement of the Stator. If you replace the Stator and the problem remains try another flywheel, if possible. No fire on one cylinder: Since this condition can be caused by the opposite switch Box (pack), disconnect the white/black jumper between the packs and retest. If the dead cylinder starts firing, replace the pack that was firing all three cylinders. As a verification, swap the Trigger and spark plug Wire to the cylinder closest to the dead cylinder, (i.e., if #1 is dead, swap the brown Trigger Wire with the white one). If the problem moves to #3, replace the opposite power pack (switch Box). If #1 is still dead, swap the green and green/white coil wires. If the problem moves to #3, replace the power pack (switch Box). Please remember to put the Trigger wires in the correct order after you finish. Always check the bias circuit: Disconnect the jumper between the packs on the white/black posts. Check the resistance from the white/black posts on each pack to engine ground. Standard packs will read from 13-15,000 ohms. Racing Boxes will read approximately 9,600 ohms. MAKE SURE THAT the BIAS Reading is the Same on BOTH Packs!!! If the bias is out of specification on one pack you must replace both packs to prevent future damage. This circuit effects Ignition timing and could cause a powerhead failure.

 

[chuck_boulos@yahoo.com]

Hi Faztbullet, Did all of the above. Everything is now pointing to a bad flywheel. Last thing I can do is to try another flywheel if I can find one off a parts engine.