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So finally figured it out at long last. It was some resistance in the big round connector I think it’s got 9 wires. The red wire had corroded in the plug in the past, and someone cut the red wire out of the connector, and used an insulated stab on connector. Made me look at the rest of the connectors inside the plug and there was small traces of corrosion. The green/red wire a piece of the connector inside had corroded off. I used an exacto knife and scraped all the connectors inside the plug. Viola, got spark and fired up. Pretty sure that green/red wire was the issue, have to see what it goes to in the schematic when I get a minute. Might have to redo that wire, like someone
So finally figured it out at long last. It was some resistance in the big round connector I think it’s got 9 wires. The red wire had corroded in the plug in the past, and someone cut the red wire out of the connector, and used an insulated stab on connector. Made me look at the rest of the connectors inside the plug and there was small traces of corrosion. The green/red wire a piece of the connector inside had corroded off. I used an exacto knife and scraped all the connectors inside the plug. Viola, got spark and fired up. Pretty sure that green/red wire was the issue, have to see what it goes to in the schematic when I get a minute. Might have to redo that wire, like someone else did with the red
hi, I am having the same problem with my 2001 Honda trx300ex. I have replaced the cdi box and the ignitions coil. What connector are you talking about that I can check on mine? Thanks
 

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I am having the same no-spark problem and am hoping someone can offer some new insight.

The machine, a 2001 300EX, belongs to my daughter. It wasn't charging the battery, I traced that down to the Regulator/Rectifier unit and replaced that. All was fine during some test rides, battery was now charging, engine running well.

She picked it up and set off to ride it home. It got about 3 miles and suddenly misfired a few times and died. I trailered it back to my place and found no spark.

I have checked continuity in all wires. From the CDI connector, I have 0.3Ω to the coil primaries. I have 300Ω to the ignition pulse generator. Coil secondary is 7KΩ to ground. I have a strong 12v power supply (using a test light, not just a high-impedance mutli-meter which can be fooled by corroded connections). Similarly, I have good grounds at the coil and the CDI using a test light. The reverse inhibit supplies a solid 6v "high" logic signal in reverse and 0v "low" logic signal in neutral or forward* from the harness. The alternator sense lead provides about 2.5 volts RMS to ground when cranking, with enough current to dimly light the test light. The pulse generator also produces an AC voltage. I confirmed both AC signals with a scope (yellow trace is generator output, blue is pulse generator, 2V per division):

4800



I would expect a CDI type ignition to put a few hundred volts into the coil primary, but I get no output from the CDI whatsoever (green, 100V per division):

4801


As a result, I believed that the problem was in the CDI module. My daughter obtained a replacement (via Amazon, 1 day delivery). However, the results are the same. So either I am missing something critical, or the replacement is defective.

I wish I had access to another 300EX to try swapping in the CDI. The closest thing available is her husband's 2002 400EX. Does anyone know if that is pin-compatible? Not necessarily the same timing curve, I'm sure, but sufficient to see if the CDI can output voltage to the coil?

*with regards to the debate about NC vs NO contacts on the reverse switch earlier in this thread: The wiring diagram shows both neutral and reverse as NO switches, however if that is so, then the "normal" position for the reverse switch is in Reverse. The neutral light goes out and the reverse light illuminates when the switch is unplugged. Grounding the wire puts the reverse light out. I theorize that the 6v signal tells the CDI to limit RPM while in reverse. If I can get the dang thing running, I may investigate that hypothesis further, but first things first...
 

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No it is not - the 300EX has a DC ignition and the 400EX has an AC ignition - put the 400EX on the 300 and it will burn out the unit.

Both the neutral switch and reverse switch are NO - reverse is closed (connected to ground) when in reverse and the neutral switch is closed when in neutral - neither has continuity to ground when in any forward gear. 12v+ goes to the light and from the light to the switch.

4802

This is the 300EX CDI unit - from left to right
Bl/W (black with white stripe) wire is 12v+
Gr (green) is ground through the reverse switch
Bl/Y (black with a yellow stripe) is the discharge to the coil wire
Bu/Y (blue with yellow stripe) is the pulse generator wire
G/W (green wire with white stripe) is frame ground
Y/W (yellow with a white stripe) is to one phase of the stator - used to trigger reverse speed limiting

4803


The neutral and reverse light switches are the same (#19) - if you unplug either one, it should not affect the other - unplug both and check the voltage of each wire unplugged from the switch it will have 12v - ground them one at a time and both together - if either light is on when unplugged from the switch, there is a short in the wire - if the neutral wire is plugged onto the reverse switch and the neutral light glows, the switch is faulty. With both light s out, and the clutch pulled in activating the clutch switch, the starter motor should spin the engine over - look for spark. If the CDI unit has ground on the green wire and is running over a preset RPM, it will kill the spark to a preset limit, but the engine will still run.

If you check the peak voltage of the pulse coil, it should be .7v minimum (point 7 = 7/10 volt)
Output from the CDI unit to the coil should be 100v minimum

If the pulse coil is not triggering the CDI, then no output to the coil - if you have the correct pulse voltage and no voltage to the coil, then I will guess you got a bad CDI unit from Amazon - I advise people to buy 5 and if one works return the other 4.

Let me know if this helps
 

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No it is not - the 300EX has a DC ignition and the 400EX has an AC ignition - put the 400EX on the 300 and it will burn out the unit.
I was afraid of that. Oh well...

Both the neutral switch and reverse switch are NO - reverse is closed (connected to ground) when in reverse and the neutral switch is closed when in neutral - neither has continuity to ground when in any forward gear. 12v+ goes to the light and from the light to the switch.
I checked several times. On this machine, when the unit is put in reverse (from neutral), the green light goes out and the red light comes on. If the reverse switch is unplugged, the green light goes out and the red light comes on. Which makes sense if you think about it -- the machine is no longer in neutral when it is in reverse, so both lights should never be on at the same time.

Checking continuity with an ohmmeter, the switch is closed in forward/neutral and open in reverse. They do indeed appear to be the same switch, and your part diagram supports that, but the switch appears to be actuated differently -- ie the neutral switch is activated in neutral, but the reverse switch is DE-activated in reverse. The reverse inhibitor unit must incorporate a relay or flip-flop to invert the output to the light (and CDI).

Now, I am in Canada so it is possible that this is a unique Canadian market legal requirement and that the reverse indicator operates differently in other countries.

If you check the peak voltage of the pulse coil, it should be .7v minimum (point 7 = 7/10 volt)
I forget the exact voltage by the multi-meter, but think it was around 3-4vac RMS. The scope showed almost 12v peak to peak, but of course a multi-meter cannot capture that resolution.

Output from the CDI unit to the coil should be 100v minimum
Yeah, not even a ripple there. On the scope or the meter.

If the pulse coil is not triggering the CDI, then no output to the coil - if you have the correct pulse voltage and no voltage to the coil, then I will guess you got a bad CDI unit from Amazon - I advise people to buy 5 and if one works return the other 4.
I was afraid of that, too. :-(

Thanks for your help. I appreciate the time it took for such a detailed response.
 

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Check with a local Honda dealer, but I'm betting the switches are still the same and wired the same. The button on the shift drum that contacts the neutral switch also contacts the reverse switch - when in contact with reverse switch the reverse light is on, when shifted out of reverse to neutral it contacts the neutral switch and the neutral light comes on - nothing else contacts the button on the shift drum.

Tip the machine to the left to prevent oil from running out and swap the switches - check their continuity when out of the engine - I've seen the switches short out internally causing a problem similar to what you describe.
 

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Update:

The ignition module is NLA from Honda. I bought a used one from the auction site. It lasted all of 15 minutes. Worked fine at low speeds in the bush, then I took it for a run down the road, winding it out a bit and it died. No spark again.

It was used, so maybe just it's time was up. But also possible that the system was overcharging -- no way to test unless the machine is running. So I bought a new aftermarket module (Rick's Hot Shot), installed it, put a voltmeter on the battery and it fired right up. Charging around 14.2 volts at idle. Revving the engine up, the charge voltage quickly rose to 14.7v, but no higher. Let it warm up, revved the engine a few more times and it died. No spark again. I was watching the voltage at the moment it died -- never exceeded 14.7v. Total run time on that box: 5 minutes, tops.

Any insights would be welcome. I can't just keep throwing CDI boxes at this thing. Something else must be wrong. Would an intermittently shorting ignition coil cause this problem?
 

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After studying the traces, I believe the pulse coil is causing the problem. The pulse coil is supposed to output a spike in one direction only - it rises and falls without reversing direction. Makes no difference if it is + going or - going, but it should spike to about 3 to 5 volts once per crankshaft rotation. It appears the pulse coil output is higher than the alternator output - should be the other way around,

The alternator should be an AC output from about 20 volts at idle to 100 volts per phase whereas the pulse coil should be a 2 to 5 volt peak pulse.

Then the CDI unit will send about a 100v peak pulse to the coil.
 

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After a lot of research, I believe the pulse signal is normal. The negative-going spike is an artifact of the rotating magnet.

I bought this generic CDI system from Amazon. For less than $30, it came with a 4-pin DC CDI, coil and wiring connectors. I scavenged the connector from one of the failed Honda 300ex CDI modules, and made an adapter, wired like so (colours are the generic harness wires):



Then I set up a crude bench test, powered by a variable-voltage linear power supply with the pulse signal provided by a programmable arbitrary waveform generator and spark gap by an adjustable spark tester:



I experimented with a wide variety of input waveforms from pure sine waves to spikes and voltages from 1v amplitude up to 10v amplitude. I also experimented with supply voltages from 6 to 17VDC. Here are a couple of samples. The yellow trace is the input signal, the green trace is output to the coil. As you can see, the CDI is triggered by the falling edge of any shape positive-going pulse and ignores any negative pulses. Output volage peaks consistently at 98v, peak to peak. Note that this CDI actually outputs a long MSD burst rather than a single pulse. Makes an interesting buzz sound over a 1/4" gap. I have no idea if a working Honda CDI does the same.







I installed it in the machine, retaining the stock Honda connectors, coil and pickup so it is completely reversible. Seems to work fine so far:



(I'm not sure how to embed a video clip on this forum, but here is a short clip of it running)

One word of caution for anyone else about to emulate my solution: there is no rev limiter. I bench- tested this up to the equivalent of 10,000 rpm. In particular, there is no reverse gear rpm limit! You are your own safety switch if you go this route. Engine damage or personal injury could result from mis-use!
 

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Quite impressive and a viable solution to the dilemma - you could possibly source the connectors and offer the adapter wiring harness and parts as a kit to others for a moderate profit, but don't expect a huge demand - just a few with the same problem as the numbers of the old machines continues to decline. Thanks for your effort and detailed solution to a common but not widespread problem.

You sir are a few rungs above my pay grade.
 

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Thanks for the kind words. Hopefully anybody with a soldering iron and some heat-shrink tube can duplicate my adapter. After all, anybody who would be attempting this already has the connector in the form of a dead Honda CDI (Pro tip: soaking in gasoline helps soften the rubber potting material, making it easier to clean off the connector). And the adapter is not strictly necessary -- one could simply splice the wires that come with the generic kit into the Honda harness, bypassing the connector altogether.
 

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Update: One summer later and the machine still runs fine.

I just received a question by PM about the wiring. To clarify:

The colours shown below are the colours of the wires that came with the aftermarket connector. The pin locations are of the salvaged Honda connector (removed from the Honda CDI box) as seen from the solder (back) side. The aftermarket CDI box does not have provision to limit the RPM in reverse, so it only has two wires and the pins on the Honda Connector that provided the reverse switch signal are not connected.

If you are not salvaging the connector, then Green is connected to the bike's frame and/or battery - terminal (Honda wire colour Green/White). Blue/White is connected to the Pulse Generator (Honda wire colour: Blue/Yellow). Black/Yellow is connected to Honda Coil (Honda wire also Black/Yellow). Finally, the Black/Red wire is connected to switched 12v power from the Engine Stop switch (Honda colour Black/White).

 
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