100hp johnson intermittent spark

[jerni21]

hi i have a '72 100hp johnson model# 100ESL72R and for a couple of years i have had a constant intermittent spark problem, we changed the pulse pack, and it worked good, once and a while we would have the problem, now this year the same thing and we would like to figure it out instead of taking a chance every time we take the boat out. This year we fired her up for the first time and it ran pretty good, then after it was running for maybe ten minutes it just flat out died and we couldnt get it restarted so we have one of those spark testers and sure enough, no spark on any of the cylinders. We were looking over the motor and we noticed right below the coil there was some like rubbery stuff that looked like it melted so we pulled the coil off and the rubber sealant on the botton side of the coil was starting to melt, dont know when this started; so now we're thinking it might be a bad coil but we're not sure. what do you guys think?

 

[Joe Reeves]

Re: 100hp johnson intermittent spark

(Voltage Drop To Battery Capacitance Discharge)
( Pulsepack When Electric Starter Is Engaged)
(J. Reeves)

On the older Battery Capacitance Discharge ignition systems (1968-1972), the electric starter reaches a point, even with a top notch battery, whereas the starter will draw excess voltage/current/whatever which results in a voltage drop to the pulsepack. The cure is to purchase a diode which is capable of handling 12 volts and installing it between the starter terminal of the starter solenoid (NOT the battery cable terminal) and the wiring terminal that supplies voltage to the pulsepack.

The diode must be installed so that the current flows from the starter terminal of the solenoid to the pulsepack... NOT vice versa. When that diode is installed in this manner, when the key is turned to the start position, the voltage that is applied to the starter is also applied directly to the pulsepack via the diode effectively eliminating the voltage drop and energizing the pulsepack with the required voltage needed for its proper operation.

********************
(Battery Capacitance Dischage Powerpack Test)
Various OMC Engines - 1968 to 1972)
(J. Reeves)

Purchase a small 12v bulb at your local automotive parts store (the 12v bulb is to look like a flashlight bulb, not a headlight bulb). Solder two wires to that bulb, one to the side of the bulb (ground), and the other to the positive point. You might use a bulb of a somewhat lower voltage to obtain a brighter glow... just a suggestion.

Remove the spark plugs. With the key in the on position, make sure that you have 12v going to the pack at the terminal block (purple wire). Now, connect the ground wire from the bulb to any powerhead ground. Connect the wire from the positive point of that bulb to the powerpack wire that is connected to the coil wire on the terminal board (blue wire).

Crank the engine and observe that bulb closely (CLOSELY!). If that bulb glows even the slightest bit, the powerpack is okay. It may be a very dim glow... just so it glows! If it doesn't glow, the pack has failed.

Keep in mind, that type powerpack (Battery Capacitance Discharge) demands a top notch battery of at least 70 amp hours. Any less will, in time, cause powerpack failure.

The coil..... If anything is leaking out of it, replace it.

 

[jerni21]

Re: 100hp johnson intermittent spark

i replaced the coil, wasnt the problem still no spark, i had a volt meter on the battery side of the solenoid and i had 12.7 volts before i cranked but while cranking it dropped to 9.3 or so only while it's cranking, so is that the problem? and when you say from the starter side of the solenoid to the power pack supply, i still have to leave the wire that's supplying the pack at all other times correct? also i haven't tested the pulse pack yet, when you say put the ground from the bulb on any powerhead ground what do you mean by that? the plug wire?

 

[jerni21]

Re: 100hp johnson intermittent spark

also, do you know which terminal to go to for the pack?? i know the starter side of the solenoid, but where does the other side go? i have a schematic and it looks like terminal #9 but of course nothing is labeled, is it terminal #9?

 

[Joe Reeves]

Re: 100hp johnson intermittent spark

Correct..... Do not disconnect the voltage supply wire from the powerpack.

The voltage drop to 9.3 volts when cranking is the reason for installing that diode.

The "other" side of the solenoid is a large RED cable. That other large RED wire on the solenoid leads to the positive terminal of the battery.

The negative cable from the battery is connected to the powerhead effectively making the entire powerhead a "grounding point". When I say to ground the bulb wire...... simply attach it to any metal portion of the powerhead.

 

[jerni21]

Re: 100hp johnson intermittent spark

so do you think the diode will solve the problem?? i will try to test the pack tommorow, hopefully it's still good, also when i asked where the other side went, i was unclear, when im hooking up the diode, one side of it goes to the starter side of the solenoid, but where does the other side go? how do i find the power supply to the pack? i was looking at a schematic and it looks like it's terminal 9 but im not sure

 

[Joe Reeves]

Re: 100hp johnson intermittent spark

Have the key ON, engine NOT running. Using a volt meter, find the wire/terminal that leads to the powerpack that shows a 12 volt reading. This will be the terminal (usually a purple wire) that powers the powerpack and is the terminal that the other end of the diode must be attached to.

Be sure to turn the key off after this voltage test.

 

[jerni21]

Re: 100hp johnson intermittent spark

ok, i will try that today, thanks alot

 

[jerni21]

Re: 100hp johnson intermittent spark

ok heres an update, went out to work on the boat today, was trying to find the point on the terminal board to connect the other side of my diode, so i find that and in the midst of moving some wires i heard some arcing coming from under the flywheel so i figured i was getting some spark, so i pulled the pack off and one of the sensor wires was grounding out on the block so i fixed that. Then i went to radioshack to look for a diode and i couldnt find a 12v diode, so where do i get one? so at least i know the pack is good since i was getting a spark. Also why i was looking over the motor i noticed another box, and i was reading up on the motor and it looks like it's a clipper circuit so i was reading up on it and seen they were very problematic but i dont know if theres any problem with it.

 

[jerni21]

Re: 100hp johnson intermittent spark

any thoughts?

 

[F_R]

Re: 100hp johnson intermittent spark

You originally said the motor was running and just flat quit. The diode is to give higher voltage while cranking. Different scenario.

The grounded sensor wire is enough to kill it.

 

[emdsapmgr]

Re: 100hp johnson intermittent spark

Check the output on the sensor when your engine is warmed up. My 72 100 ran great when cold, for a few minutes-then quit and would not refire. Found the sensor was bad. When it got warmed up, it quit. Replaced it and the engine ran great for the rest of the time I owned it.

 

[jerni21]

Re: 100hp johnson intermittent spark

most of the time there is no spark while cranking, the just flat out quitting was a first for this motor but hasnt done it since, just that one time probably due to the grounded sensor wire but that's one problem fixed, we'll see is she sitll has spark in a couple days