1985 Mercury 50 HP 4 cylinder - no spark

[barrythedude]

I did have spark on just #2 cylinder, but it too has now vanished. Everything seems to ohm out except the stator - red-red/white is 155ohms, as it should be, blue-blue/white should be 5000-7000 ohms and is 0 or open, so I think I have found my problem, unless the enlightened here can shed some info? I would really appreciate it. Compression good, whirls nicely. I wanted to check if the ignition was at fault by unplugging the connector at the key, but for the life of me I cannot get the control box apart. Thanx in advance.

 

[CharlieB]

Re: 1985 Mercury 50 HP 4 cylinder - no spark

Switchbox uses the black wire w/yellow stipe as the ground-to kill. disconnect this wire and retest for spark, if it now has spark there is a short in the harness, lanyard, or ignition switch killing spark.

Ohms test can be indicator, but a DVA test is a far better indicator. Since there is NO RETURN on electrical parts, verify your stator with the DVA tests. A DVA adaptor can be purchased or home-built very cheaply following the directions posted elsewhere in this forum.

 

[barrythedude]

Re: 1985 Mercury 50 HP 4 cylinder - no spark

Hi and thanks - I have done the mercury switch test, as per the CDI troubleshooting guide, also took the two yellow wires off the rectifier, same result. Not sure what a DVA does different than my multimeter other than smoothing out fast/slow peaks in voltage. I was told by a tech to take the connector off the back of the ignition switch and check for spark when using a pullstart rope, but I cant get it off/apart no matter what I try. Any other ides for me? Thanx in advance

 

[CharlieB]

Re: 1985 Mercury 50 HP 4 cylinder - no spark

DVA meter and/or adaptor has a resistor and a capacitor, together they place a measured load on the system to drive the charge voltage up which tests the capability to handle load, the cap slows the voltage swing long enough to stabilize the meter so you can read it.

A standard volt/ohm meter will not place any load on a charge coil, stator, or trigger, any voltage reading, even on a peak reading meter is not a true indicator if the system will handle any load.

A broken yet abutted winding in a coil will still generate voltage, yet once any load is place on it, will fail, voltage will drop to zero.

You can test for spark by disconnecting the kill wire (Black w/yellow) at the switchbox on the motor. Crank the motor and measure spark. If none, proceed with the test outlined with the DVA meter.

 

[barrythedude]

Re: 1985 Mercury 50 HP 4 cylinder - no spark

I do have 12-14 volts DC at the input of the switchbox, from the stator connections. I may have to make the DVA adapter, but thought there might be some additional tricks/tips, or better yet, someone hearing of a similar scenario.

 

[Exotic4x4]

Re: 1985 Mercury 50 HP 4 cylinder - no spark

Do you pull the wire from the power pack? Or where my 70 HP Johnson is doing the.same.thing but line intermittently loses spark at half throttle then comes back trying to track this demon down also...