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Discussion Starter #1
Got a new speedometer to replace my old gigantic cluster, when wiring in the speedo I just removed the adapter from the old one and put the relevant wires for the new speedo into the old adapter and plugged it back in.

However the old speedo had an oil light and my new speedo does not. Can I leave the spot on my new adapter empty where the oil wire would've been or will that ruin something?

I've plugged in the new Speedo and tested the wires, it works perfect with no issues - when i plug it into the bike it won't work at all. Bike battery is ~12.5v so its not a dead battery.

I'm also no electrician and ive been soldering together some wires to make things work

Can't figure out why the speedo will not work when I have all the relevant wires plugged into the adapter

Thank you for any help
 

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You said your bike is a 78 CB400T, but what speedometer are you using? Did it come without instructions? (common)
Could you post a diagram and some photos of how you have it connected? What adaptor?
 

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Honda color codes will not match any of the aftermarket stuff. Things to remember about Honda is Green is ground, Brown wires are related to lights, Red is usually a battery hot lead(unswitched) and Black is a main switched power lead.
 

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Discussion Starter #4 (Edited)
https://www.amazon.com/gp/product/B074P1SJ3M/ref=oh_aui_detailpage_o05_s01?ie=UTF8&psc=1 - This is the speedo i bought, the instructions were jenky af, it claimed to have yellow/brown wires for signals and there were only yellow's and I thought it was missing a 4th ground wire. I ended up dissecting the speedo to figure out which wires go where, So i had it wired correctly, everything was matching up with the correct wires in the harness but nothing was turning on and I wasn't sure if it was because there is a the oil pressure switch not having any power leading to it or something.

https://i.imgur.com/rNgcqpW.jpg
https://i.imgur.com/2sRLS0J.jpg
https://i.imgur.com/l0I5lzl.jpg


Thanks guys
 

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Pictures didn't load right so can't see them.
While we all like and appreciate a good deal, $17 for a speedo is way cheap and there's a reason it is.
 

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Discussion Starter #6
Pictures have been updated, yeah I knew it was dirt cheap, i figured why not its only 16 bucks...and it actually works. I just cant figure out why the rest of the bike wont work, some missing link somewhere and i cant figure it out
 

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Pictures have been updated, yeah I knew it was dirt cheap, i figured why not its only 16 bucks...and it actually works. I just cant figure out why the rest of the bike wont work, some missing link somewhere and i cant figure it out
Now see, if those were bulbs and not LEDs it would all be simple... :rolleyes:
 

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Discussion Starter #8
Now see, if those were bulbs and not LEDs it would all be simple... :rolleyes:
The LED's for Neutral/Signal/Headlight work perfectly, theres no issue with the LED's not flashing or anything, I replaced the relay to work with LED's.

The plugs that come from the wiring harness have a few extra wires - Blue+red(oil light) and Brown+white / Brown wires that go straight to the tach/Speedo - Any thoughts as to why the bike wouldnt work without those having something plugged into them?
 

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Don't know much at all about the SOHC 400/450 series, never worked on them as they came out after I was working on Harleys for the City of Tampa. Possibly something safety-related to the oil light circuit?
 

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Discussion Starter #10
Don't know much at all about the SOHC 400/450 series, never worked on them as they came out after I was working on Harleys for the City of Tampa. Possibly something safety-related to the oil light circuit?
See i thought so too, The wire runs directly from the oil light down the harness under the seat to the oil pressure switch. Its possible there could be a safety thing there.

Also - The Brown + Brown/Whte wires that go directly into the speedo/tach, could there be some safety feature there aswell?
 

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Also - The Brown + Brown/Whte wires that go directly into the speedo/tach, could there be some safety feature there aswell?
I think the brown and brown/white are illumination. Someone in the 400 crowd will be able to answer the oil light/safety question, I'm sure LDR will know
 

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There is no safety, this model will actually run w/o a battery since the ignition is a separate circuit powered by the stator.
So Honda wiring code is:
Green = ground
Brown and Brown w/White are the speedo/tach lighting
Lt. Green w/Red tracer is Neutral
Lt. Blue or Blue w/Red is oil pressure
Note that the oil pressure and neutral light are a grounding circuit, meaning that the switch controls the ground for the light so both lights should be powered with the key on.
 

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Discussion Starter #13
Okay so after several hours of tinkering and staring at the wiring diagram, I realized I had blown the 15A main fuse originally and thats why nothing was working... The Brown/White wire is what powers the tach and also I think i found out the tail light is connected to that wire also, as like an added feature, when the tail light goes out the tach/speedo light goes out too so you know, thats pretty neat.

The oil pressure/neutral wires are definitely grounds - Black is power from the ignition switch, so by leaving the Oil pressure wire untouched its not hurting anything.

Thanks again for the help
 

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Discussion Starter #14
Also - I plan on removing the Tach cable from the engine block since my new speedo doesn't have a tach assembly, Can i just remove the cable from the block and leave it be or should i fill it or block it with something?
 

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Can't speak to the 400T situation, but most Hondas (if not all) have an oil seal on the tach drive and unless you have a plastic cap or plug (depending on if it's an internal or external cable attachment) it should be fine
 

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You'll need to plug the tach drive, crankcase pressure is going to blow oil out the open hole. If you're going to keep it this was them I'd suggest getting a right case cover from a CB400T1 which had no tach.
You speedo should be powered from the Black wire from the ignition. It's a more voltage stable source than the Brown's.
 

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I stand corrected - did not know it would leave an opening in the crankcase, or even that the tach drive was in the crankcase on the 400 series... guess I should stop trying to answer 400 series questions! :D
 

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I stand corrected - did not know it would leave an opening in the crankcase, or even that the tach drive was in the crankcase on the 400 series... guess I should stop trying to answer 400 series questions! :D
They are a bit different than the earlier models but don't stop answering questions. So much of what we answer applies to any engine,
 
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