94 90 hp Force no spark top cylinder

pachouly1956

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Oct 13, 2012
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Swapped input wires on coils, failure followed input wire. I do not have a DVA for testing.
Also not much of a CDI guy, this is above my points and condenser skill level.
What are the common issues for this problem
 

pnwboat

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Oct 8, 2007
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There are two possible ignition systems used on that particular motor. The Prestolite ignition system which has the 3 Blue coils and 2 Blue CD Modules. The Mercury Thunder Bolt system has a Switch Box and 3 very Dark Blue almost black looking coils. What type does your motor have? If you can post a picture it would help.
 

pnwboat

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OK you have the Thunder Bolt system. So I am assuming the problem is the Green wire that goes to the first coil? If so I would say the the problem is probably the Switch Box. Unfortunately, I'm not aware of any way to test the Switch Box with a meter. Next best thing you can do is to make sure that everything else ....the stator and ignition trigger are reading within specs as far as resistance.

1. Disconnect the two Yellow wires that come from under the flywheel and go to the Regulator/Rectifier. Check for spark on #1. If it has good spark, replace the Regulator/Rectifier. Note: the resistance across the two Yellow wires that come out from under the flywheel is about .6 OHMs. Almost a dead short, but not quite. If no change, reconnect the wires and go on to the next step.

2. Disconnect the Blue and Red wires that come out from under the flywheel. Measure the resistance between the Red wire and ground (engine block), and the Blue wire and ground. On one, you should read between 3250 and 3650 OHMs. On the other, you should read between 2200 and 2400 OHMs. If you have a pair that reads way off, replace the stator. If they are within specs., go on to the next step.

3. Disconnect the Brown wire that comes out from under the flywheel and goes to the Switch Box. Measure between the Brown wire and the White/Black wire on the Switch Box. It should read between 800 - 1400 OHMs. Do the same thing with the White wire and Purple wire that also come out from under the flywheel and go to the Switch Box. They too should read between 800 - 1400 OHMS between the White wire and White/Black wire, and also between the Purple wire and White/Black wire on the Switch Box. If any reading on these wires are out of spec., then the ignition trigger is bad.

If all the items in steps 1 - 3 check out OK, then the Switch Box is probably the culprit.

Make sure all of your ground wires are connected and making good contact. Don't just do a visual check, remove the ground wire and clean it off making sure it has a good connection.
 
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pachouly1956

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Oct 13, 2012
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1. 1 ohm at yellow wires
2. blue & red to to ground 3830 & 3840
3. brown to wh/b 1397 / wh/b to pur 1248 / wh to wh/b 999 /

I through a DVA together seems to work well. The trigger voltage is wh/b to pur 5.2 / wh/b to wh 3.4 / wh/b to brn 4.9 book calls for 4v this was taken at cranking speed.
The book has a Cranking DVA for the switch box but doesn't tell what wires to take it from.
Any thoughts
I'm not comfortable trying to get running volt readings, without a terminal strip and having to un bolt the plate that holds sw- box and rectifier to gain access to the wires.

Thanks so much, you instructions are greatly appreciated and have made it easier to comprehend what is in this SELOC manual.Wish you would have wrote it.
 

pnwboat

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Oct 8, 2007
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Well I seemed to have made a mistake in my original resistance specs. for the Blue and Red stator leads. My apologies. On one, you should see 3250 - 3650 OHMs to ground. On the other, you should see 75 - 90 OHMs to ground. Double check those stator lead resistance readings again to be sure. Check each wire separately disconnected from the Switch Box to ground. Your stator has a low speed winding, and a separate high speed winding. I don't know which color is which, but based on your initial resistance readings, it looks like your stator windings are not reading within specs.

Forgot to mention. You check the ignition cranking voltage at the stator leads. Blue and Red wires. On one you should see 180VDC, the other you should see about 25VDC. The DVA converts the AC voltage output of the stator to DC, so you want to set your meter to DC voltage. The manuals don't cover this little bit of info very well.
 
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pachouly1956

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Oct 13, 2012
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Stator red to grnd 28 v blue to grnd 160 v , both were climbing voltages these are the first solid numbers that appeared but kept climbing with each spin.
I didn't have this issue reading the trigger volts, I think I see an issue on a charge circuit with my DVA .
Ohms red to grnd 109.6 blue to grnd 3860 and just for the heck of it red to blue 3750
 

pnwboat

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Oct 8, 2007
Messages
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OK.... those resistance and voltage readings on your stator are more in line. It is normal for the stator voltage to increase with each turn of the flywheel. I don't think that there is anything wrong with your DIY DVA. They look pretty much within normal tolerance. Based on what you're seeing, I would bet that the problem is the Switch Box itself.

Again, make sure all of the ground connections are good. There should be a Black ground wire that goes from the Coil/Switch Box mounting plate to the engine block. Check resistance reading between the Coil/Switch Box mounting plate and the engine block. It should read a dead short.
 
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