No spark on plug #3

S

stunmun

Cadet
Joined
Mar 6, 2011
Messages
22
Hey all I have a 1978 Mercury Black Max 1750 and It runs but no spark on plug #3. I tested the wires all the way to the inner switch box. They work. Tested the plugs, swapped the orange coils. I believe it is in my inner switch box but, I would hate to buy a pair of new ones and still have the problem. Is there anyway to test the switch boxes? I created a spreadsheet to check my stator, trigger and coils for the proper resistance. The stator is used to charge the battery isn't it? I read where the Trigger advances the voltage during rev up. I am thinking it maybe in my inner switch assembly but want to make sure it is that. Also, anyone know where I can get some cheap switch boxes if I need to buy them? The cheapest I found was 1 for $116.00. The switchbox part number is 332-7778 A1
 
M

Maxz695

Guest
Re: No spark on plug #3

check compression in the non firing cylinder before movng on. Just to be sure before buying anything electrical
 
M

Maxz695

Guest
Re: No spark on plug #3

actually thats great news. I don,t know of a way to check the switchbox. but when mine went only 1 cyl would fire. i have 2 cyl. I changed it out at a great price 115 brnd new and worked like a charm
 
M

Maxz695

Guest
Re: No spark on plug #3

I sent you notification check your inbox
 
C

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: No spark on plug #3

Stator has low speed windings, high speed windings, and charge coils.

Low speed windings charge ignition switchboxes from minimum cranking speed of 350 up to the transision to the high speed set around 2500 RPM.

Charge coils handle battery charging.

3 Triggers tell the both switchboxes when to fire the appropriate cyls.

Generally losing spark on a single cyl is a switchbox problem, two cyls may be a trigger, all 3 cyl on one side of the motor may be either a stator or the Bias gone bad in the other switchbox.

Disconnect the Bias (White) wires from both boxes and retest for spark. Do NOT start the motor. If spark is restored with the Bias disconnected the fault is a bad Bias in the 'other' switchbox.

Prove the switchbox by switching the position of the two switchboxes, if the 'no spark' moves to the other side with the box, then order a new box from CDI Electronnics.

Factory boxes are recommended replacement in pairs, CDI boxes may be replaced individually.

Your 'sparking' switchbox may still be useful on a 3 cyl motor, (75 - 90 HP), (the Bias is not used).
 
Last edited:
j_martin

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: No spark on plug #3

One plug missing, swap coils with a good one.
>>> misfire moves = bad coil
>>> Misfire doesn't move
>>>>>Remove the bias jumper between the switchboxes
>>>>>Plug now fires >> The "good" switchbox is bad.
>>>>>No change
>>>>>>>Put the jumper back, but swap the switchboxes with each other.
>>>>>>>Misfire moves >> bad switchbox
>>>>>>>Misfire doesn't move >>> Bad coil primary wire (assuming you follow all steps.)
Trigger takes out two opposing cylinders at a time
Stator takes out one or both banks.

hope it helps
John
 
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