retro

Yeah, jumpering battery voltage to the coil would bypass a high resistance joint in the harness. It would do nothing for a shorted wire though. As you said a harness and an ECU should fix it. Your logic is solid too, swap the harness then the ECU.... it's big bucks and a lot of tedious work but it would be fixed right.

I am back home now so I'll read back through this thread later tonight after my work day is done. My intuition is firing off so I might have missed something.

This is off-topic but I have an idea that could help us all. Honda stopped including resistance tests in service manuals long ago, so I am thinking about measuring resistance on Ignition coils that I have laying around that are known to be good coils. We could each make a list of resistance tests of coil part numbers that we have on hand and then publish them in a sticky maybe...? @Mach 1 needs those resistance tests right now.

As explained above.... maybe, maybe not. Again maybe, but maybe not. That harness junction could have an overheated pair of wires inside it where they connect together that measures good, but does not pass enough current while under load. Similar to a fuse that overheats the terminals where the fuse plugs in inside a fuse box.... you've seen those failures before.... so you can't rule that possibility out until you get your eyes on the joints in question. I have never tried to open one of those harness junctions so I'm ready to learn what it looks like inside of them, if you are? All along I have suspected that the ECM is at fault but I've been staying away from that talk because I have no idea how it could be tested.

Well, continuity and resistance tests are meaningful only when a wire repair bonds every copper strand inside the wire together and you're 100% sure of that. So for instance, if a broken wire is soldered back together but several of the strands did not get bonded, but two or three strands did get bonded, then that wire repair would test as a good one, but would not necessarily conduct all of the current that the wire (repair) is required to handle during operation under it's normal full current load. In that case the wire would only measure a higher resistance than normal while under load. That is the reason why @Melatv suggested that you redo the repair that has the all-in-one solder/heat shrink gimmick on it. You can never be sure that the wire strands got hot enough for solder to flow into them to create a bond when using those sort of shortcut gizmos.... the wire conducts heat away quickly along it's length and commonly leaves you with a pretty pool of melted solder --- but a cold solder joint. A good temperature controlled soldering station is a must-have for harness repairs. Weller is one of the best affordable brands...

Sorry I've been away doing RLS for a while.... If I were you I would measure the resistance of the coil windings of your new OEM coil before going any further. Your coil is getting hot so it might be shorted. So, unplug the coil from the harness and measure resistance between the two connectors. Write that down.... then measure between the Black/Red terminal and ground (the stack of steel plates on the coil) and write that down. Then measure between the Green/Yellow terminal and ground and write that down. Then between the Black/Red and the spark plug boot and between the Green/Yellow and spark plug boot and finally, between the spark plug boot and ground. Let us know those measurements.

That is my thinking too. In my mind I had narrowed it down to either the JC 2 junction in the harness and/or the ECM. I was trying to stay away from the ECM... don't feel comfortable making those calls on the Internet. That is a good price for a used harness it's and loaded with parts. What do you think Mel?

It sounds like wiring may be melted together shorting. I'd look for overheating evidence then measure voltages and grounds until the problem is found. Unfortunately I can't help ya sort it out right now since I am well along on a bathroom rebuild project. I'll be gone for another 3 weeks or so.

I've never seen a failure inside a harness junction box yet but it's looking like I'm about to. Thanks Mel for jumping in.

The coil has to be bolted down too.

Yeah the coil is marked for each wire color. Do not plug them in wrong, they'll get hot and short.

So with the coil plugged in and the Gray ECM connector unplugged there is still a voltage drop measured at the coil Bl/R terminal?

A boiling water bath helps a bit but usually not quite enough... a razor blade scraper is usually what I end up with in my hand.

That's what I would do.

Yeah it sounds like the coil windings are shorted. The manual does not list resistance specs for the coil so the only way to know for sure is plug another coil in.

Even with the ECM gray connector unplugged there should be battery voltage at the coil, fuel injector, display meter, DLC connector, ECM gray connector, ignition switch, etc. since all of those Bl/R wires are fed battery voltage at the JC 2 junction connector. Hopefully the issue is as easy to trace down as it logically sounds.

It looks like a wiring issue to me, since you had the gray connector unplugged from the ECU and still measured a 1 volt drop at the Bl/R coil terminal. Check out the wiring diagram below.... trace the coil Bl/R wire and notice it joins the Fuel injector Bl/R wire inside the wiring harness. Then that Bl/R wire continues to J/C 2 junction connector where it is joined with fuel pump Bl/R, power Bl/R, DLC Bl/R, ECM Bl/R etc. Unplug and check voltage at the Fuel injector, in case the issue is the internal harness junction where the coil BL/R joins the Fuel injector BL/R. I'd put my money on that internal harness crimp joint at this point.... but if the Fuel injector Bl/R measures the same voltage drop as the coil Bl/R, then check voltage on all of the BL/R wires inside the junction connector if you can. I am working on a major bathroom demo & enlarge/replace project for my daughter so I can't be here very often, but I'll try to check back here at least once a day. Have fun tracing!

Yeah it looks like there is a possible current drain somewhere in the ignition circuit. If battery voltage goes through the Ignition switch before reaching the 10 amp fuse (as the diagram shows) then the drain is downstream from the ignition switch and the 10 amp fuse.

Yeah you can go ahead and check the Ignition switch. Let me know how that goes.

Use a stock NGK spark plug. It sounds like your float valve is leaking causing it to flood. The Shindy kit for your '87 TRX350D is #03-023 according to Shindy. Jeep beat me to it.... https://www.shindypro.com/product-page/carburetor-repair-kits A Shindy kit should fix it unless your fuel pump isn't shutting off. You can test that by plugging the fuel hose and turn the key on. The pump should run briefly then shut down at about 5 PSI or so.

Yeah there are two pairs of wires, Red/black and pink is one pair and Red and Black is the other pair. With the key off there should be no continuity between the Red/black and pink and no continuity between the Red and Black. With the key turned on there should be continuity between the Red/black and Pink and continuity between the Red and Black. You're working inside the connector on the switch side, not the harness side.... let me know those test results. Leave the battery disconnected for now, but go ahead and plug in your new OEM relay module and OEM coil and plug your ECM/PCM back in carefully, so everything is ready to go except for the Ignition switch and battery.

You guessed it.... your carb is flooding fuel after you put a china carb kit in the carb. You'll need to remove all of the All Balls carb kit parts and either put all of the original Honda OEM parts back in the carb, or replace them using a genuine Shindy carb kit. All Balls kits are china garbage and china garbage does not ever work on a Honda. It sounds like your fuel pump is fine. Let us know how it goes after the china parts are removed.

Ok, it's been a while so I went back and read this thread from the beginning. It looks like we left off with no power to anything, the display, lights etc. all dead. So nevermind the Relay tests for now, we need to find out if the Ignition switch has failed or not. As stated a few times earlier, DO NOT PLUG ANY CHINA PART IN AT ANY TIME. Throw them all away. Remove the front rack and the front fender and find the Ignition switch 4p connector and unclip it from the frame. Put your multimeter in Continuity mode or Resistance mode and check continuity at the switch-side of the connector on the two wire color pairs as shown in this diagram. Test with the Ignition switch turned Off and then test again with the switch turned ON. Report those results.

You can disconnect the positive battery cable and set your multimeter up to measure amps current with the red meter lead on the positive battery post and the black meter lead on the positive battery cable. If there is any current with the key off, unplug (& plug back in) component connectors one at a time until you find the culprit. Note that the display meter may draw a milliamp or so while the key is off - thats normal.

What year is it? Is the oil clean and the oil level right on the money?