ok, so real world testing incoming...this is a lot of text, but whatever.
My 25 hp merc died suddenly (again). 2 years ago had similar symptoms, turned out to be the stator. Information provided to chatgpt:
1. Weak spark to both cylinders, unable to jump gap on tester
2. DVA voltage on both stator leads 180v while hand cranking (barely within spec, but presumably very low rpm). Resistance tests normal. (I was guessing stator since chinesium replacement and same symptoms, but seems ok. )
3. All connections checked, grounds cleaned
4. Trigger resistance 750k ohms, spec 6.5-8k ohm.
Suggestion:
"Since the spark is equally weak on both cylinders, the trigger is
not the leading suspect."
"
Most likely order in your case: switch-box or coil grounding → partially grounded kill circuit → switch box itself → two degraded coils/plug wires → trigger."
I had already mentioned kill circuit was disconnected.
It kept suggesting things and I would offer reasons why I didn't agree. It eventually turned 180':
"
790 kΩ is not just somewhat high—it is effectively an open trigger winding if you measured directly between the trigger leads with the trigger disconnected from the switch box."
"A normal reading for this style of 25 hp Mercury trigger is generally in the
hundreds of ohms, roughly around 650–850 Ω, not 790,000 Ω."
ME: "specs say trigger resistance is 6.5 k ohm, not 650 ohm. reading is trigger disconnected from switch box"
"
You’re right—I gave you the wrong resistance range for this ignition.
With a specified trigger resistance of about
6.5 kΩ, a measured
790 kΩ with the trigger disconnected is grossly out of specification—roughly
120 times too high. That indicates an almost-open trigger winding or a failing connection within the trigger leads.
That makes the trigger the strongest suspect, especially given:
- Sudden onset
- Weak spark on both cylinders
- Approximately 180 V DVA from the stator while connected
- No change with the stop circuit disconnected
- Both cylinders sharing the same trigger assembly"
"I would not spend time blaming both coils or the plug wires first. The coil-input DVA test would still be useful, but that trigger resistance is already a clear failure finding."
Now, I actually don't know what the problem is at this point. I ordered a new trigger assembly. However, had I just listened to AI I would not have really suspected it. I had to correct it multiple times (both reasoning and data), and then it agreed with me. I actually don't feel great about the trigger as the cause, but my diagnostics say otherwise. As a side note, this motor was running absolutely perfectly for the first time since I've owned it for exactly 2 weeks. Curious what the actual problem is, we'll see (hopefully).
