Rev counter on CB400 Superfour

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Allrover
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Bike owned: CB400 Super Four NC31
Location: Manchester, UK
Rev counter on CB400 Superfour

Post by Allrover » Mon Apr 27, 2015 6:40 pm

Hi, I recently bought a 1992 CB400 Superfour NC31 and am loving it.

The rev counter didnt work on it when I bought it, but all the other instruments are fine. I bought a replacement instrument binnacle and fitted that, but it made no difference leading me to the obvious conclusion that the problem is elsewhere.

I have no idea how a rev counter actually works - as there is only one physical cable into the back of the instruments, that leads me to assume it uses an electrical signal to work out what the revs are.

Can anyone explain how the rev counter works? and therefore where else I can check on the bike to try to find the source of the problem?

Thanks, Chris

tomigun
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Re: Rev counter on CB400 Superfour

Post by tomigun » Wed Apr 29, 2015 6:29 pm

I think our motorcycle tachometer works by taking positive pulse from the electronic ignition(cdi). So you must first check out the cable that gives signal from cdi to tachometer. I hope that your cdi have no problem.

billgrundies
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Re: Rev counter on CB400 Superfour

Post by billgrundies » Wed Apr 29, 2015 9:02 pm

hi Allrover, Tomigun is correct it does operate using a pulse. just found some info that may be useful. inside the headlight there is one red 6 way and one white 6 way connector, unplug them. on the red plug is a black/brown wire +ve. on the white plug is a green wire -ve. these wires are on the main loom not the one going to the clocks. if you switch on the ignition and meter between these wires you should get a voltage reading. if not do the usual checks on the loom for damage, corrosion, loose connectors etc back to cdi unit. if it does meter out ok your looking to the wiring to the clock or the clock itself to be faulty. these colours do match my bike, on all my wiring there is a greenish dab of colour at the connectors.

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Thu Apr 30, 2015 2:10 am

First, a question for you about your Honda: What kind of spark plug ignition coil does the bike use? Does your CB400 use CDI (Capacitive Discharge Ignition) or does it use TCI (Transistor Controlled Ignition)? It's not hard to tell, you just have to determine whether the bike's coils are connected to 12volt power. If the coils have 12volt power inputs, the system is TCI, but if the coils only connect to the ignition control box, the system is CDI.

If it turns out that your Honda has a TCI system (coils are connected to 12volt power) then read the rest of this post. If not, if your ignition system is CDI, then disregard the rest of this post.

I've re-wired several older Japanese street motorcycles (examples: 2005 Kawasaki EX250, Suzuki GSF400). In every case they were TCI ignition and their tachometer gauges were the same with regard to how they operate.

The analog rev-counters on these older motorcycles are "old school" instruments, they don't "count engine revolutions" like today's totally digital, computer-controlled cars and motorcycles. This type of tachometer is actually a "Galvanometer" (which is a type of ammeter).

The method this instrument uses to accurately indicate engine speed is by flowing a known amount of electricity (DC current) through a small set of copper windings that act as electromagnets to exert a pull-force against the return (to zero) spring-loaded tachometer needle.

The source (control) of the electricity flow for the tachometer's galvanometer is a wire connected to one of the coils.

The flow of electric current on this wire is controlled by the Ignitor Box (or ignition control box). The coils in a TCI system always have a 12 volt supply source but they don't flow any amperage unless the CDI grounds the coil signal wire, which is does for about 2.5 milliseconds before each firing of a spark plug (this 2.5 milliseconds of 12 volt DC power flow is called "Dwell" and this is how the coil's primary winding is "powered up" in preparation for the firing of a spark plug).

The engineers who designed these old tachometers used the known coil Dwell-time-constant to calibrate/set the amount of pull-force the electromagnet in the tachometer will exert so that it shows accurate RPMs throughout the rev-range.

If you take one of these tachometers apart you will find it has a voltage input line (voltage supply) from the bike's wiring harness (12volt probably) and a electrical line to one coil. There's also a small circuit board that contains several resistors that were used to calibrate the electrical current flow which calibrates the tachometer's movement across the instruments rev-range.

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Sun May 03, 2015 10:57 pm

As I suspected, the CB400's coils are connected directly to 12volt power, which means the CB400 has a TCI type of ignition.

Image

Also, in the overall wiring diagram for the CB400 you can see that the CB400's Tachometer gauge has only two wires attached to it (if you don't count the wires that run to the lighting inside the gauge). One of the two wires is 12volt power. The other runs directly to the CB400's ignition control module where it is almost certainly attached to the signal source for one of the coils (the source that alternately grounds then releases the coil's continuous 12volt power).

This means that my earlier description of the operating principle of your CB400's tachometer is correct.

So you only have to trace and test 2 wires to troubleshoot your tachometer.

The 12volt wire is easy to test simply by using a multi meter to see if it has 12volt power when the bike is switched "on" (positive probe on the wire, negative probe to the bike battery negative terminal). This wire should be the common color of all the bike's wiring that is "12volt power".

The other wire, the one that connects the tachometer to the ignition module controlled "coil ground" will be a bit harder to trace because after the binnacle multi-connector plug it runs through the wiring harness to the bike's ignition control module.

Because you've had the same result (no tachometer operation) with two separate instrument binnacles I'm guessing the problem is in the wiring beyond the binnacle's connector plug. And to take that guess even further I'd bet that the problem is with the coil signal wire rather than the 12volt power supply wire (because the gauge lights in the binnacle must be working, otherwise you would have mentioned that, and those lights share the same 12volt power supply as the tachometer gauge).

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Re: Rev counter on CB400 Superfour

Post by Superjack44 » Mon May 04, 2015 7:00 pm

:worthy: 2 great and useful posts

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Tue May 05, 2015 11:59 am

Fantastic responses all, thank you, especially SevenThreeSeven. To log on when I got into work this morning and find all those was just amazing.

I absolutely love this bike, and want to restore it to its former glory. I am immediately realising the key problem of running an import though - very little documentation to help. If I'd picked any UK supplied Honda, there would be Haynes manuals and youtube videos galore covering every possible issue I'm sure.

@seventhreeseven
- I presume you have got that wiring diagram from the russian - english translation handbook? Thats the only english language version I'm aware of. I do have a copy of that, but not the background in motorcycle fettling to understand it.

Right, I understand the principles, and know what I am looking for, now time to go looking. I suspect I may very rapidly run into the problem that I can't see very well without removing the petrol tank, so that might be my next post!

I'll come back with an update, as nothing worse than posting some help on a forum and never hearing if it helped.... :whistle:

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Wed May 06, 2015 9:40 am

I will remove the petrol tank this weekend and get a good look at the wiring. In the meantime I found this diagram in the trusty russian translation manual. I can see a lead on here entitled '8 - A Wire of the generator of pulses', which going on the description in the previous posts sounds like it might be the lead providing the signal the rev counter measures.

Image

I know I won't know whats going on until I get in there, but if anyone looks at that and knows its got nothing to do with it, let me know...

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Wed May 06, 2015 5:23 pm

I can see a lead on here entitled '8 - A Wire of the generator of pulses', which going on the description in the previous posts sounds like it might be the lead providing the signal the rev counter measures.
No...

That No.8 wire isn't the wire you're looking for.

In the diagram you posted the No.8 "Wire of a Generator of Pulses" is referring to not one but a pair of wires that are connected to the engine's Variable Reluctance sensor (also known as a VR sensor, or "Pulse Generator").

In case you're interested, here's a bit of clarification about No.8, the "Wire of a Generator of Pulses"...

All motorcycles that have TCI (Transistor Controlled Ignition, also known in plain-old english as "electronic ignition") have a metal toothed-wheel that is connected to the crankshaft (on your Super Four it is located under the left engine side cover). The toothed-wheel turns as the engine's crankshaft rotates which moves its teeth past the VR sensor. The toothed-wheel always has a pattern (a certain number of teeth, with some teeth missing). As each of the toothed-wheel's metal teeth passes the VR sensor a "pulse" is generated and the pattern of pulses that the VR sensor sends out as this toothed-wheel spins is recognized by the Ignition Control Box circuitry. This is how the Ignition Control Box knows where the crankshaft is (within each 360 degrees engine rotation) and that's how it determines when fire the spark plugs.

Now back to talking about the wire you're looking for... In the diagram of the Super Four Ignition Control Box wiring (page 114 in the Russian translated manual) the wire you're looking for is number (No.15).

Diagram of the Super Four Ignition Control Box wiring:
Image

Now you need to figure out what color that wire is. In the diagram the wire is color-coded with two characters: the crazy "X" looking thing, then a "/" and then the odd number "3" looking thing. On the very last page of the manual there is a color code key. From looking at this key I'd bet your Tachometer-to-Ignition Control Box wire is Yellow with a Green Stripe on it.

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Thu May 07, 2015 9:36 am

Got it, that makes perfect sense. Frustrating too as I was looking at that ignition diagram in the manual yesterday but didn't spot the tacho lead. Useful tip about the colour code chart.

Well it doesn't get any clearer than that, I now just need to get a few hours free from family duties at the weekend to start investigating!


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