Merc V-6 not getting spark. Need help with Switch Box.

[achilles555]

I have a 1988 Merc 150 XR4. Half of the cylinders are not getting spark. I have decided that I need to replace one of the switch boxes, but I'm not sure which one to replace - the inner or the outer. According to my manual, one switch box controls cylinders 1,3,and 5...while the other fires 2, 4, and 6. Looking from the back of the motor, does anyone know what spark plug is 1, 2, 3, and so on? This will help to tell me which switch box to replace. Thanks in advance for the help. achilles.

 

[Barnacle_Bill]

Re: Merc V-6 not getting spark. Need help with Switch Box.

Starboard side is 1, 3 and 5. Port 2, 4 and 6. The red and blue wires coming from the stator go to the sitch box controlling the starboard bank. The red/white and blue/white from the stator go to the switch box for the port side. There is also a yellow sleeve around the port side wires.

 

[hkeiner]

Re: Merc V-6 not getting spark. Need help with Switch Box.

On my motor, the 1, 3, and 5 cylinders are on the starboard side (right side looking at the motor from the rear), with the 1 cylinder at the top. I expect that all of the Merc V-6 motors are set up the same. Also, the "inner" switch box on my motor controls the 1, 3, and 5 cylinders. You can always trace the wires from the switch boxes to the coils at each cylinder to confirm if your motor has this same switchbox setup.

 

[j_martin]

Re: Merc V-6 not getting spark. Need help with Switch Box.

Before you do anything, pull off the bias jumper wire between the switchboxs. If you then have fire on all six, you need to replace both switchboxes, or you will have a repeat performance.

The "good" box takes out the other one. Anyway, fuddled up bias can grenade the powerhead.


hope it helps
John

 

[achilles555]

Re: Merc V-6 not getting spark. Need help with Switch Box.

thanks everyone for the info. i now know what i am looking for.

j_martin, could you please explain your post regarding the bias jumper? i don't understand why all 6 cylinders would fire if both switches were bad. and why would a good switch box take out the bad?

is the bias jumper something that commonly needs replacing? as you can tell, i'm pretty new at this stuff...so ant help would be greatly appreciated. kind regards, ac.

 

[j_martin]

Re: Merc V-6 not getting spark. Need help with Switch Box.

The mercury switch boxes fire off an ac signal from the trigger that not only varies timing, but voltage with speed. Internally they have a bias circuit that finds the average voltage so it fires at approximately the same time by varying the voltage at which it fires. That circuit is brought to the outside world on a terminal, and the two switchboxes are wired together with a short wire jumper so that any slight variation in the switchboxes is averaged out. That same circuit can be manipulated from the outside to vary timing, which is how the "advance" and "idle stabilizer" modules work.

Very often, a switchbox will fail to fire because the bias circuit on the other one has failed, and causes the misfire through the bias crossover. Sometimes the failing switchbox will run fine, but kill the other one.

If you disconnect the bias wire, the switchbox bias circuits are isolated from each other, and each can run independently. In the above sceneraio, now all cylinders will fire. It's a diagnostic only. If it goes this way, you need to replace both switchboxes because the "good one" is bad, and the "bad one" has probably been hurt by it, because it's bias overvoltage that will cause the misfire. It's also likely that timing is wild (uncontrollable) on both switchboxes, which is a good way to grenade your motor. If the trouble remains the same, just replace the failing switchbox. In either case, be sure to check the timing.

Remember, you asked.

hope it helps
John

 

[achilles555]

Re: Merc V-6 not getting spark. Need help with Switch Box.

thanks for the detailed response j_martin. i understand now. what i hope to see when i disconnect the bias circuit is that 3 of the cylinders are still not firing. that would be best case scenario...meaning the replacement of one switch box. if all cylindars are firing...both switchboxes need to be replaced.

and another question. how do i check the timing. i have a merc service manual that i could read through and try to figure out, but i thought since you are so knowledgeable, i would ask you first. maybe you have an easy way, or some pointers that i could follow when testing the timing. you've been such a big help to me, i'll have to buy you a beer sometime.

thanks again for your help. achilles.

 

[j_martin]

Re: Merc V-6 not getting spark. Need help with Switch Box.

Achilles,
You really need to read the manual. Other than that, there are a couple of tricks.
1. I punched the guts out of a spark plug, and welded a metal tab on it so it could hold my cheep digital caliper with a couple of rubber bands. The depth gauge then goes into the spark plug hole. I pull the engine slowly over TDC, Zero the gauge, push it in, and come around again to the mark called for (.462, if memory serves me, something else if it doesn't)

2. You need remove all the plugs and ground the wires. I took the top connector nuts off 6 plugs, found screws with a nut on them to fit them, wired them together daisy chain style and put an alligator clip on the end of the chain. Now I can just push on the wires and ground them at once.

3. A remote starter switch will replace a crabby kid (or wife) well, at least for the task of turning over the engine.

There is a link N sync procedure for each engine. It goes in sequence, and it's worth while to go through it correctly, especially if others have been "tweaking" the engine. Generally it involves checking and setting up the carb linkages, then setting an initial timing, then a throttle pickup point, then WOT timing. Final idle timing is done by ear on the water.

Hey, if you ever get in the area, I'll show you some of the prettiest water in the nation, and some pretty decent bass fishing also.

hope it helps
John

 

[CharlieB]

Re: Merc V-6 not getting spark. Need help with Switch Box.

If BOTH switchboxes fire OK with the bias disconnected then MARK them '3 Cyl Only', they will be fine on a 75 - 90 HP three cyl.

Someone else might be able to save a few dollars.

 

[JoeBass1986]

Is it a tell tale sign when you un hook the bias from the outer box (controls 2 4 6 spark) and it seems to have more idle/neutral rpms than when it's hooked back up to both? Is the outer box bad or the inner . maybe both? haven't checked to see if all 6 spark when unhooked but it does seem to (by ear) get a little more power. (on motor flusher) .