1995 90hp mercury (3cyl.) trigger box/cdi/power pack failure on one cylinder

[enginepower]

New to boating but I'm learning quick. I have no spark on top cylinder so I switched coil trigger wire coming from trigger box from the center cylinder that fires to top cylinder that didn't and that coil is now capable of producing spark so I am suspecting a faulty trigger box/power pack. The trigger ohm spec is 800-1400 and I have about 1250 between white to white/black and purple to white/black but brown to white/black is 980. They are all open circuit to ground. I haven't checked voltage output because I don't have dva but does the switch box provide voltage to trigger then gets returned (1 wire for each cylinder)? Can it be possible that a bad trigger could cause the trigger box to fail? I have one on the way but hate to fry that one too if trigger should be replaced.

 

[Texasmark]

Starting over. Software hiccup. The trigger pickup is a low voltage version of a magneto like you have in your lawn mower to fire your spark plug. You have a coil of wire wrapped around an iron core (to increase inductance) with one end of the wire grounded and the other open to output voltage when excited. As the magnet on the flywheel passes each respective pickup coil, the passing magnet induces a pulse of energy into the coil which is delivered to the switch box. I don't know what the switch box does with it but I suppose it amplifies it to a sufficient amplitude to drive the high voltage transformer located in each of the 3 ignition coil modules.

Since you switched the output of the trigger to another "switchbox amplifier" I can't help but assume that is what failed. On whether or not the trigger could cause the box to fail, I doubt it. You are talking about a fixed magnetic circuit: Insulated wire, laminated soft iron core and a passing magnet, all of which have a very long life and pretty constant outputs considering temp and age.

The DVA is not rocket science. It is just a sample and hold circuit. The trigger pulse is very narrow and occurs once per cylinder per revolution.....I would think. Never looked at one but the technology is there. Since the "energy" in the pulse is low, most voltmeters are not sensitive enough to do their own sample and holding. With an Oscilloscope, you could look at the pulse and wouldn't need the DVA.

The sample and hold function is a rectifying diode, a storage capacitor, and a bleeder resistor across the capacitor. Cathode of the diode, one side of the cap and one side of the resistor are connected to a standard DVM on the 400v +DCV scale. The anode of the diode is connected to the engine wire and the other side of the cap and resistor connect to engine ground where the meter neg lead is connected. Crank the engine to get the reading.

Values for the components are 600+ for a diode, 0.47 uf paper or plastic cap and a 1 meg ohm bleeder. Radio shack can fix you up with the parts.

HTH,
Mark

 

[enginepower]

Thanks for reply. Helped me understand it better. I didn't know if trigger was hall effect type or magneto inductive type. I had switched the coil driving wires from center coil to top coil to see if the coil was good and I got good spark. I did try checking trigger with my fluke 77 and didn't display anything but the bar graph at bottom peaked up somewhere around 6-8 VAC on all 3. Yes, I found schematic to make my own DVA with 22uf cap. All 4 wires go directly to switch box so i take it one is a common ground supply and the other 3 are trigger pickups positioned 120 degrees from each other? (1 trigger per cyl) If that is the case, I guess it could be possible I have a trigger problem instead of a bad switch box. (too late, already ordered one). I still plan on making a dva and checking output of each trigger before I hook up the switch box.

 

[Texasmark]

Finding a 600v .47 uf is a lot easier than finding a 600v 22 uf. Energy stored in the cap is 1/2 CV exp 2. Either will charge up to the peak, time depending on the internal resistance and size in uf of the C and the size of the bleeder. Obviously the larger the cap the longer it takes to charge and the lower the resistance of the bleeder cane be for the same voltage reading. But comparing a 100k R and a 1 Meg, in the same wattage and type is the same size and price. The C is not that easy as you will probably realize.

With the DVMs available, input impedance to the meter is in the Meg ohms on up as compared to yester year where it was something like 20k ohms/volt. So they don't load the circuit. The reason I assumed the sense circuit was a magneto was the fact that the pickup was a coil whereas the Hall sensor is of a different physical configuration and more expensive. Other thing is the Hall devices are primarily used for sense circuits rather than what I call working circuits, at least the ones I to which I have been accustomed.

Good luck.

Mark