1972 Johnson 125 - no spark

[imported_scott_r]

Hi,

I'm pouring through the Clymer shop manual and I've read through a lot of posts on troubleshooting OMC ignition systems, but I'm hitting a few roadblocks and looking for some suggestions. I have a 1972 Johnson 125 V4 that is not producing spark on any cylinder, with plugs in or out.

I've confirmed 12 V at the input to the pulse pack (terminal 8/9) but did not get any voltage at the output of the pulse pack (looking for 250+ volts at terminal 5).

The next step in the manual's pulse pack output test is to connect a "known-good sensor" to the pulse pack connector. I assume that means an optical sensor tester device, which I don't have, so I can't follow the manual troubleshooting procedures from that point on.

Are there tests I can perform with a simple multimeter to troubleshoot the pulse pack, sensor, coil, and other ignition components, and try to determine where the problem is? In the posts I've read, and in the shop manual for that matter, it's difficult to confirm that the suggested tests will apply to this exact model, so I'm looking for some fresh input. I've tried to consider all of the simple things that might be causing a problem, such as poor or missing grounds, broken wires, etc., but I still might be missing something obvious, so any suggestions in that area will be appreciated as well.

Thanks for the help,

Scott

 

[tashasdaddy]

Re: 1972 Johnson 125 - no spark

do some reading here. http://www.boatpartstore.com/page29.asp

 

[imported_scott_r]

Re: 1972 Johnson 125 - no spark

Thanks. I've been there but I didn't think this was an alternator-driven CD ignition system, as you linked to. The only section I could find that seemed to apply to this engine was on page 28 labeled "Johnson/Evirude Prestolite Battery Ignitions with Pickup Sensors." Can you confirm exactly which procedure I should follow for a 1972 Johnson 125?

Thanks again,
Scott

 

[tashasdaddy]

Re: 1972 Johnson 125 - no spark

Scott to be honest, i can't remember. will ask someone who may look at this.

 

[F_R]

Re: 1972 Johnson 125 - no spark

Thanks. I've been there but I didn't think this was an alternator-driven CD ignition system, as you linked to. The only section I could find that seemed to apply to this engine was on page 28 labeled "Johnson/Evirude Prestolite Battery Ignitions with Pickup Sensors." Can you confirm exactly which procedure I should follow for a 1972 Johnson 125?

Thanks again,
Scott

Confirming it is a Prestolite battery ignition system with a pickup sensor.

 

[Joe Reeves]

Re: 1972 Johnson 125 - no 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.

 

[imported_scott_r]

Re: 1972 Johnson 125 - no spark

Where should I look to purchase an appropriate diode?

Thanks for the help!

Scott

 

[Joe Reeves]

Re: 1972 Johnson 125 - no spark

Any electronic parts store, Radio Shack, etc.

 

[imported_scott_r]

Re: 1972 Johnson 125 - no spark

Just wondering if someone can explain the following test, from
http://www.boatpartstore.com/page28.asp under "Johnson/Evinrude
Prestolite Battery Ignitions with Pickup Sensors:"

5. Disconnect the black/white sensor wires. Turn the ignition switch on and strike the black/white wires together. The unit should fire each time. If it does, this means the CD module is usually good and the sensor needs to be checked.

Q. Where should the black/white sensor wires be disconnected? They appear to be hard-wired from the pulse pack plug all the way to the base.

Q. What does "the unit should fire each time ..." refer to?

Q. Are we sure this test applies to the 1972 125 hp?

I've ordered a DVA adapter to help with testing but in the meantime since I've got everything apart I'm trying to see if there are other simple tests I can perform to isolate the source of the no-spark problem.

I've also checked and do seem to be getting close to 10 volts at input to the pulse pack pack while cranking, although I need to test that again with a more accurate meter.

Thanks,
Scott