Attached is a picture of the transmission range switch. You have to remove the cover to see the connector and more of the wiring.

You may have a short there.

 

Thanks! I did see that unit and thought that was the range switch. So I think there is a separate connector towards the front of the car that sends something to it? Maybe not and I am confused.
So I could probably figure out a way to get cover off, but I'm working on the city street so to speak... and then is there an actual switch I can pull out and test?

I have a multimeter but my ohm-ing apparently sucks as I've gotten strange readings on other things and verified things were good when they weren't and vice versa. Specifically resistance testing only. Voltage metering I'm just fine at.

I think it was the transmission switch harness connector that I disconnected down low in the front of the car on the AT, wondering if I leave this disconnected, OR the 'ACC' relay which may only operate the reverse lights or it may do a whole lot more. I guess it doesn't make a lot of sense a relay would do anything useful to operate a radio, moon roof, windows etc etc... now that I think about it.
Maybe I'll test both on my last 2 fuses and see what happens.

Attached is the connector I am pretty sure I pulled earlier today. Found a how to diagnose for an older honda civic but it looks the same...

 

That didn't work.

Pulled back out 'ACC relay' aka the reverse light relay and everything in the car worked just fine with new #21 fuse in. Gears all shift fine. AND the dashboard gear selector works for all the gears N D D3 2 1
notice I didn't say R. As soon as shifter selector went into R you've seen this movie before - bam! - out goes fuse #21 yet again.

Also, had wrong connector out and car could not drive. Re-hooked mystery front lower connector back and car can drive again.

This is getting more than a little tiring. Good part of 3 days on this online and in the car and only verified that certain parts work and nothing solved.

One thing though, since car Transmission will hit all the gears and work, the shift indicator works, and the dashboard gauges shift indicator all work correctly (can't verify R works in dashboard since it instantly takes out my whole dash, and I remember P worked right on dash pre blown fuse), I would think that my Transmission Range Switch is working correctly simply based on that alone.
Is the only possibility left there is the wire to reverse that is shorting somewhere under the hood or near fusebox area? Thank you for any wisdom sent this way previously and in the future.

 

PAhonda:
I got the self diagnostic to work! It took many tries but got it. The thing stopped after a minute or so and continued flashing all the dash light and never stopped.
It did however sweep the needles on all the gauges, flash a bunch of stuff and made some beeps. The odo always read or came up reading '8888888' both times I ran it.
So I guess whatever that means - there isn't an error there. What's that for? Diagnosing simply the gauges etc??

I also did verify 'P' works in dash when AT in park. And I now noticed I have a CEL active both with key in position II and with car running. It did not go away after dash self check either. Before all this started did not have a CEL on the dash. 99.5% sure of that.

 

My transmission switch has some wacky continuity readings. I was only able to compare all the values to a posted 2.3L 2002 Accord.
I am sort of guess they are the same 10 pin connectors and values, and if so my switch does not look right.

Key On, Engine Off, for all combos in gear.
Park, Neutral, D4, D1 all tested the same as was published and had continuity in proper combos and no continuity where it should not be.

Reverse, D2 and D3 posted bad results (All were compared to published 2002 Accord values I found).
Reverse: 3 - 9 Continuity = good value. But pin 3 -6 tested with continuity also (wrong).
D2: 2- 3 Continuity = good value. But 2 -4 Continuity tested with continuity also (wrong); 2 - 5 No continuity (bad); 5 -3 no continuity (bad).
D3: 2 -3 Continuity = good value. But 2 -5 tested with continuity (wrong); 2 - 6 No Continuity (bad); 3 - 6 No Continuity (bad).

So this is seeming to indicate bad Transmission Range Switch. I'm going to try to find a replacement and post results.

INCIDENTALLY: this connector and the cover are VERY easy to access laying next to the outside driver edge of car and going between front wheel well arch and the front tire (car not up on jack or lift). Turning wheel left all way gives a lot of nice clearance! I also strongly recommend to simply unbolt the (2) 10mm bolts that hold in the wiring harness FIRST, and then work on disconnecting the damn wire harness connector, otherwise it's near impossible for a normal person without a lift. With those out you can fish the wiring all out towards the top of engine bay and move it around to find the stupid release points. You MUST release the metal part that hold the wire in place first, then press same release point a then press the 1 large obvious button on the other side to release the green connector from the black connector. I spent several hrs on this and it could have taken minutes and a lot less swearing had I known a good cheat procedure like this. Screw the manual method.

 

I tested the switch with key on engine off.
All tens pins out of transmission range switch (aka neutral safety switch), tested Infinite resistance to the frame member motor side metal (same frame where AT grounds itself), and not reading any short or 0 or close to zero resistance on any of those pins.

DOES anyone have the 04 wiring test diagram for this exact model? It's possible the 10 pin diagram is the same for the 02 and earlier BUT it does show up as a different part when ordering.

No one stocks part in Oakland CA so I can't replace and re test for 1-3 days

 

Pulled the transmission range switch / neutral safety switch just now. Took it apart and lightly cleaned some dirty grease off it. But i am almost certain this isn't the problem. Switch looks good. I am totally stumped over here.

 

It's definitely not the transmission range switch.
Here's how I know:
I unplugged wire harness green connector from black. I powered up car with all new light bulbs in and everything else connected. Did not blow fuse #21.
However, I then jumpered pin 3 and 9 which would signal the car that it was in reverse. Result? With key in position II and switching AT through gears (car off), fuse #21 blows yet again instantly when I hit "reverse" with shifter. In Neutral and D3 and drive gears no reverse lights but dashboard gauges still working. (I start tests in neutral now always to start before putting in new fuse 21).

So SUMMARY:
Still blowing fuse #21 in reverse when everything is connected.
Transmission Range Switch not the issue
Brake pedal switch not the issue.
Found very solid grounds in tail area behind trunk plastic liner after popping out plastic fasteners. About 7-10 black wires tied in solidly there.
Also with proper green wire multi connector pulled at rear seat driver side shoulder area still blowing fuse.
Blowing fuse with old Reverse bulbs burnt out in, new bulbs in, no bulbs connected, and bulb plastic holders removed. Wiring from rear seat connector to front of car looks solid, non exposed spots anywhere in plastic loom sleeve but haven't torn that all up either.

Not really sure what else to look at now. Very stumped. MCU or MICU? PCM? TPS? Is there a neutral safety switch behind glove box?? I don't think so, it should be the transmission range switch up front.

One thing, a year ago I removed the center armrest console, passenger seat and rear bench seat, to ah, transport very large artwork and pretend I owned a minivan. Was very careful, all been working 1 year and 10,000 miles since, but is there something in that center armrest tunnel area that could cause this? I know the shift lock solenoid is there, and I have to manually press the slot to unlock the transmission when fuse #21 is blown but I think that's normal there and means solenoid is working. And if start new tests in neutral can shift and start car without pressing in solenoid bypass Anything else there??

 

Disassembled armrest / center console area.
Tested Shift Lock Solenoid per the Mitchell book and it tests fine.
Disconnected seat heaters, Shift Lock Solenoid and Pin lock Solenoid.
Will all bulbs in at rear of car, still blows fuse #21 instantly the second I hit reverse or row through it with the shifter.

I have really no idea how to solve this and it continues to be a big problem.

 

Here's the Transmission Range Switch as I metered it with my ohm Meter.
It is different from the published 2002 2.3L Accord diagram I found.
Does this look wrong and a problem?

ESPECIALLY reverse circuit? Should it only have continuity between #3 and #9 pins and not also #3 to #6 pins, and #9 to #6 pins? Is that an issue and causing a short and blowing my fuse?
In my drawn diagram of continuity findings, the left √ checked column mean it was the same as 2002 2.3L Accord diagram. X means it was different.
So D3 and D2 did not have continuity with pins 6 and 5 respectively as the 2002 Accord has, but rather had continuity with pins 5 and 4 respectively.

The reason I think it isn't is the switch looked fine taken apart
and I was thinking I also shifted into reverse and still blew the fuse with Switch totally disconnected, but now I am wondering if I remembered that wrong? Can you shift car with that out? I think you can and I did, but I think main thing is I can't start the car and I cannot remove the key with it unplugged (which makes sense).

2nd pic is diagram I found for the Transmission Range Switch on the 2002 Accord 2.3L I understand these are different part #s to replace.