what causes no spark on atv

First thing I'd check is the ground connection on the coil. I had a no-spark on my YFZ450 and it was just a corroded ground wire. Clean all the frame grounds and re-tighten. Also verify you have 12V at the coil primary when cranking.

Those TRX stators are known to fail at the pickup coil. You tested stator resistance but not the pulser coil. It's a small coil near the flywheel. Should read 150-250 ohms. If it's open or out of spec, you'll get no spark.

Don't forget the kill switch circuit. Even if the lever looks fine, those can short out internally. Unplug the kill switch wires at the handlebar and see if it sparks. Simple test that saved me a lot of headache.

Could be a bad CDI even though you swapped one. Not all CDIs are created equal. Make sure the one you tried is actually known working on your exact model. Some aftermarket ones have different timing curves and won't spark.

I'd look at the key switch if you have one. water gets in there and shorts out. Bypass the key switch temporarily to rule it out. That's a common issue on older ATVs.

Stator ohm readings sound low for a TRX. I think spec is around 200-300 ohms between those wires. If you're only getting 80, that stator might be cooked. Check the resistance again with the wires disconnected from the harness.

One thing people overlook is the neutral safety switch. On some Hondas, if that switch is faulty, it kills spark. Try starting in neutral with the clutch pulled in. Or jump the two wires on the switch connector.

I had a no-spark once because the spark plug cap was bad. It looked fine but had broken internally. Pop a new NGK plug and cap on before you go any further. Cheap and easy to rule out.

Check the voltage at the CDI input. You need to see battery voltage on the red/white wire when the key is on. If that's missing, the problem is in the wiring harness or main fuse. Use a multimeter, don't just assume.

Before you condemn the stator, check the flywheel key. If the flywheel slipped, timing is off and spark might still happen but at wrong time. Actually with no spark, that's less likely but worth verifying if you've got the flywheel off.

I disagree about the stator resistance being the main issue. Those specs vary by year. My 03 manual says 80-120 ohms for the charge coil. So 80 might be fine. Check the pulser coil resistance instead. That's the most common failure point.

Stupid question: is the battery good? Low voltage can cause weak spark or no spark. Even if it cranks, the voltage may drop below the CDI's minimum threshold. Load test the battery.

I've seen the spark plug itself cause no spark. I know it sounds basic, but I once had a brand new plug that was dead out of the box. Swap in a known good plug first.

If you've checked all the usual suspects, I'd pull the flywheel and inspect the pickup coil and its wiring. Sometimes the wire chafes against the engine case and shorts to ground. Hard to see without removal.

Another idea: the ignition coil primary resistance should be around 0.5-2 ohms. Secondary around 10-15k ohms. Measure those. A shorted coil can prevent spark even with everything else good.

Don't forget the clutch safety switch. On your TRX, if that switch is open, no spark. You can bypass it by connecting the two wires together. Easy test.

I had a similar problem and it turned out to be a corroded connector at the CDI. Pull apart every connector in the ignition system, clean with contact cleaner, add dielectric grease. Corrosion can cause intermittent no-spark.

My advice: download the service manual for your exact year. The diagnostic flow chart is worth its weight in gold. You're guessing now, but a systematic approach will save you time and money.

Check the cranking RPM. Some CDI units won't fire below a certain RPM if the starter is weak. Measure battery voltage while cranking. If it drops below 10V, the CDI may not spark. Try jump starting from another battery.

One last thing: make sure the engine is properly grounded to the frame. There's a strap from the engine to the frame that can break or rust. If that ground is missing, the coil won't get a return path. I've seen it happen.