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Switch box failure?

K

Korn16ftl3

New member
I have a 78 mercury 900 90 hp that I had replaced the timing belt and re timed due to when I had turned the motor over it had done nothing but backfire, because it had backfired I had assumed it had spark prior to the replacing of the belt (I could be wrong?). After reassembling the the flywheel and aligning the timing marks according to my service manual I had no spark at all. I started to test things with a test light to see where I had and did not no have power at, the stator and everything has power right up to the switch box (red wire has all time power white wire is keyed power and the green wire to the coil) how ever the green wire to the coil is not out Putin any power to the coil. Is this an indicator that the switch box is bad or is there something else in the circuit I should be checking first?
 
No power at the green wire is either a bad switchbox, or the magnetic switching circuit in the distributor.

Remove the brown, black and white wires to the distributor and check for continuity; you should see two circuits with resistance. If any are open the distributor is shot. Otherwise, look for another switch box.

Jeff
 
I really hope it's not the switch box.....apparently the switch box for this motor is made of gold or something....just searching for the oem part # it's about $400+.....unless I'm looking at the wrong part.....which I hope I am

Using your above method u mention both continuity and resistance, should I check with the ohms setting or continuity setting on the multimeter?
 
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If either of the circuits is open, it's defective. That said, if they both show okay it can still be defective--had that happen to me just yesterday, believe it or not.

Jeff
 
fastjeff said:
If either of the circuits is open, it's defective. That said, if they both show okay it can still be defective--had that happen to me just yesterday, believe it or not.

Jeff
Click to expand...

Which was bad in your case? The switch box or the distributor? I still need to test this tonight....hoping in my case the distributor is what's bad lol
 
fastjeff said:
The distributor. Hope that's not the case 'cause they're rare and pricey.

Jeff
Click to expand...

Well I picked up a switch box that was referbished but the guy never used it on eBay for $150 now I have spark at the coil but nothing at the plug wires. Wondering if I need to replace the cap and rotor......and I looked at those as well and ya ure right far from cheap.... $106 for a cap and $190 something for a rotor.......not impressed......any other thoughts on other potential problems for the spark not trialing to the cylenders like it should? I see there is a trigger plate in the rotor assembly is that a possible suspect? I'm guessing that that she ha the 3 wires from the distributor cap that connect to the switch box have something to do with that trigger plate as well??
 
There's a spring-loaded carbon 'bullet' inside the cap that transfer juice from the cap to the rotor. See if it's missing (they fall out).

You're close to a runner!

Jeff
 
fastjeff said:
There's a spring-loaded carbon 'bullet' inside the cap that transfer juice from the cap to the rotor. See if it's missing (they fall out).

You're close to a runner!

Jeff
Click to expand...

I may be close to having a runner but I'm just a little ways off from a complete boat.....it needs to be redecked (not horribly rotten just soft in one spot) and I want to paint the floor with Tuff Coat, which is apparently rubberized marine flooring paint. Either way running and floating is half the battle.

My dad had mentioned something about their being graphite in the center of the rotor....is just the graphite and spring alone replaceable without the whole rotor?
 
Another thought.....should there be any other points of interest in my ignition system besides the cap and rotor at this point? I'm aware there is a trigger plate and such above the cap so is this something I should look into as well or am I safe to say it's either just a cap or rotor problem at this point?
 
..."is just the graphite and spring alone replaceable "

Yes, but the trick is finding it. I stole one from an old GM distributor.


Jeff
 
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