Dead #3 cyl...

[PropWash]

I have no spark on #3cyl on 2.0 Merc V6. Other cyls are all good. Once the motor is warm it runs on all 6, makes this fault hard to find.

Swapped coils and the problem remained on #3. Moved down to the switchboxes and found a loose wire (BLK/WHT) connecting the front and back boxes. Tightened it up but the problem remains.

My next step is to swap swithboxes over and see if problem changes sides but there are many wires to swap so...

First question...

Is just one cyl out of spark consistent with a switchbox problem or should I look elsewhere? Did the loose wire kill it - both other cyls on this box are OK.

Second question...

Can I swap just a few wires from trigger, and then swap sides at the coils, to get the functiong of the switchboxes to swap sides - or do I need to change all the wires?

If anyone has a wiring diagram that would help immensely.

Thanks in advance

[PropWash]

Update

Swapped the switchboxes over and the fault moved to the other side, so it looks like I found the problem.

I can get a secondhand switchbox to replace it but just wondered if there is anything I should check that could have caused it to go out - don't want it to happen again.

Or is switchbox death just one of those things?

[baja200merk]

oh man dead ur luckey when you said dead i thought u ment no compression like mine!! that really sucks

[Maniac]

I've got a good (used) switch box for you. $50 delivered in the US.
Let me know if you want it.
Thanks!

[Superdave]

To replace BOTH switchboxes at the same time. Reason being the bias circuit. That is the black/white wire going from switchbox to switchbox. I have replaced one at a time before, but since you specifically had a problem with a loose wire for the bias circuit I would replace both. The bias circuit could be bad and make your new/replacement switchbox go out again. Or even make the good one left go bad.
Dave ;)

[PropWash]

Maniac:
Thanks for the offer, that price sounds very nice from where I'm sitting, but I am in New Zealand, so I will source one here.

Superdave:
About the bias cct - what happens in this cct and what can go bad - I see the boxes are linked through this wire but that is about it. The only other wire here is from the idle stabiliser, which I have read is unnecessary (and can be trouble) anyway.

I'm not questioning your experience, I am just trying to understand why I might need to replace both boxes and if so, do I need to source these as a matched pair off the same engine?

My original plan was to replace just the dead inner box and ditch the idle stabiliser at the same time.

[Maniac]

Although SuperDave is right about replacing them both, it's more inportant if you are getting a newer version or unpated part number, or a box from the aftermarket such as CDI or Rapair.

[Superdave]

It controls spark timing between boxes. Some "experienced" guys have used resistors between the boxes in place of the wire to effect timing characteristics. Just my $0.02, but since your bias jumper wire was the specific wire that was loose it probably changed resistance as the wire made and broke contact with the terminals. If the bias circuit does go out it will probably burn a piston. I would replace both. Do a search for "bias circuit". Include the quotation marks. Read the different articles and pay special attention to comments from US1.
DAve

[PropWash]

Superdave - I agree that the cost of an extra switchboxes is much cheaper than a melted piston. If I can find a pair I may replace both and keep my good one as a spare.

I found some stuff on the bias circuit from sosmerc. The post related to an inline but described a test for the bias circuit function....

you may have a bias voltage problem between the two switchboxes. There's a white wire with black tracer that connects the two boxes together. Negative voltage is applied to that circuit to keep the timing stable between the two boxes. This negative voltage should increase smoothly as you bring up the rpm's. This check should be made with an analog meter, not a digital peak reading voltmeter that is required for all other voltage output tests. Any sudden swings in the meter would indicate a bad bias circuit....replace BOTH switchboxes if this happens. I recommend CDI boxes...2 year warranty and double grounded. 114-7778 is the CDI part number.

The numbers are the same as my box numbers so I did this test tonight on my motor. Negative voltage increase was smooth and steady as I increased from idle to 2500rpm. I didn't take it any higher.

I did this with the boxes jumpered together - the engine also ran on all six after it warmed up - is it false hope to think this steady voltage test indicates that the bias circuit looks OK?