Rev counter on CB400 Superfour

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Allrover
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Bike owned: CB400 Super Four NC31
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Re: Rev counter on CB400 Superfour

Post by Allrover » Mon May 11, 2015 3:49 pm

Ok, first bit of investigation done.
I got the tank off just fine, and set about first of all just trying to measure volts from the negative terminal to something in the instrument binnacle to give me a reference point. I took the headlight off to expose the connector blocks where the main loom splits and goes off to the various lights and instruments.

I didn't know which of the connector blocks did what, so I switched the ignition on and just disconnected them in turn until I found the one that made the lights in the clocks go out. So far so good.

I then connected the black lead off my multimeter to the negative terminal on the battery, and touched the red lead against each contact in the connector block in turn watching to see 12V spring up. First contact - nothing, second, nothing and so until the last contact (sadly I can't remember which or which orientation I had it in) I swore I saw 24V flash up, and then disappear. When I tested again, nothing.

Anyway, reconnecting the connector block showed no lights, so I figured I had blown a fuse. Checked the fuse box and I had.

Replaced the fuse and all back to normal, however I am now at a loss as to how to proceed. I did test from the neg on the battery to the pos on the back of the headlamp and that showed 24v consistently.

Perhaps I just got unlucky and accidentally shorted something, but I thought that first step, getting some volts from the battery to an instrument light would be easy.

Why on earth am I seeing 24v to the headlight? Surely it should be 12v. Am I using the multimeter wrong?

Hmm I am going to go back to it and double check things, but think I can do more harm than good here!

WIth regards to the other end of the tacho cable, I tried to find it entering the ignition control module - I couldn't even get the connectors off the module they where clipped in so tight. So going to go back to that one too!

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Mon May 11, 2015 5:11 pm

If your Multimeter is set so it can pull enough amps to pop the bike's lighting and instruments fuse then you're using it wrong.

I'm by no means an electrical expert. In fact, for anything beyond simple voltage checks I always reference my multimeter's instruction booklet to make sure I'm doing it right. For example, on my multimeter the negative (black) lead only has one port to plug into, but the red lead has two ports on my multimeter and one of those ports will let the multimeter pull a lot of amps (sounds like your situation).

Don't make assumptions about your multimeter, read the instruction booklet (if you can't find it go online and Google search the meter's make and model, there should be a PDF version available to download and print).

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Mon May 11, 2015 5:26 pm

Cheers, my father is a retired electronics engineer so he'll know about the multimeter. Sadly he doesn't live near me other wise I'd have him round. Found the wiring diagrams on page 130 and 131 and studied them more, and figured out how to work out which bit of each connector each wire is, so I'll look again.

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Mon May 11, 2015 10:07 pm

Had the meter set to AC not DC. Damn it.

V. Frustrating as I can turn my hand to pretty much anything practical round the house, but motor mechanic is a new field of interest for me so I'm having to make those silly first time mistakes.

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Mon May 11, 2015 10:26 pm

sounds like progress, keep at it.

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Mon May 18, 2015 9:29 am

Made decent progress on this over the weekend. Although still very tricky to trace the wires, I think I established that there is
a) no connectivity issue between the tacho wire at the instrument end, and the end that plugs into the CDI / ignition module. That assumes that the green and yellow wire is the right one.

Despite having the wiring diagram, it's still tricky as on the diagram the connector blocks are drawn as e.g. 9 pins in a row, whereas the physical connector block is actually 3 rows of 3 pins, so you can't just count across to find the lead you are interested in. You have to rely on the colour chart, which is very vague.

b) No voltage on that tacho signal wire when the ignition is on, engine running or not.

That seems to suggest the CDI unit itself is faulty, so given they are pretty cheap on eBay and a breakers had one available, I ordered a replacement CDI. On the assumption the wiring itself is ok, its a straightforward unplug one unit, plug the replacement in, and try it job.

Took the opportunity to remove a crappy old set of heated grips from the wiring whilst I was in there, it had been wired in like a crazy spider web, so glad to get that out!

I'll update when the replacement arrives and see if it makes any difference.

SevenThreeSeven
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Re: Rev counter on CB400 Superfour

Post by SevenThreeSeven » Mon May 18, 2015 11:58 am

b) No voltage on that tacho signal wire when the ignition is on, engine running or not.
How did you attempt to test that wire? In case you tried testing it by putting the multimeter Red probe on the wire and the Black probe on the battery negative terminal, I don't think that would work.

Remember that the yellow/with green trace wire from tachometer to ignition control box isn't a voltage source, it's just an intermittent ground. On most bikes with this sort of tachometer design this wire would be grounded every time the bike's cylinder 1 & 4 coil is grounded to fire those 2 plugs (your Superfour almost certainly has a "wasted spark" ignition setup). So all this wire does is present an intermittent path-to-ground for the 12volt power that is supplied to the bike's tachometer gauge by the other wire that connects to the tachometer.

Because of this situation I'd imagine it wouldn't be all that easy to verify this wire is functioning as intended. I guess you'd have to hook up the multimeter with the Red probe to the bike's battery positive (to provide a source of 12volt power) and then connect the Black probe to the Yellow/with green trace wire. Then start the bike and see if the multimeter registers anything. Hopefully the intermittent flow of 12volt power would show up on your multimeter's screen.

I think that the best device for testing a situation like this wire would be an oscilloscope (to detect and display the "square wave" pattern created by the on-off-on-off... of 12volt power flow), but I'm betting you don't have one of those laying around the house.

But did you check the yellow/with green trace wire for continuity from the tachometer gauge to the ignition control box plug? Just in case there's a break somewhere along it's length as it runs through the bike's wiring harness. You could do this by disconnecting the wire at both of its ends (at the ignition box and at the tachometer) and then running 12volt power across it from the bike's battery to one end of the wire and then back to battery from the wire's other end with your multimeter included as part of the loop to verify that it shows voltage.

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Wed May 20, 2015 9:09 am

You're right, I did just test for voltage on that wire, so now I understand how it works, then I can see it would have showed nothing regardless.

I did check for continuity though - I disconnected the big wiring loom plug into the CDI, and the 9 pin connector block behind the headlamp, and stuck my multimeter on the yellow and green wire at either end. There is a setting on the meter that looks like a capital O, but more like a greek letter O (Ohms?). Thats how I have always tested circuits and fuses for basic connectivity - from memory it shows 1 when there is no connection, and it changes to zero or very close to zero when you make a circuit, or vice versa. I always just touch the red and black probes together to start to check which way it goes.

I still think I'm taking the right approach - I cant think of a better next step than to bang a replacement CDI box on, and see if its any different. If it fixes it then great, if it doesn't then I jut stick the CDI back on eBay and go back to the drawing board.

Allrover
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Re: Rev counter on CB400 Superfour

Post by Allrover » Fri May 22, 2015 1:21 pm

It LIVES!!!

The replacement CDI box from the breakers (£35) turned up in the post today. A very simple unplug of the old one and plug in of the new one and lo and behold the tacho needle lifted off zero for the first time since I got the bike. It works perfectly.

So in hindsight it would have been a quick and easy fix from the start, but then I would never have learnt so much about the bike in the process. Thank you to everyone that contributed to helping me on this thread, especially SevenThreeSeven.

This also highlights the pros and cons of having an oldish bike, especially an import. The cons is there is no bloody owners manual except for suitably cryptic russian one, and no youtube videos doing exactly the thing you want to do that you can just copy. The Pro's, spare parts from breakers yards are available and generally cheap, so swapping parts to help diagnose the problem is easy to do.

Now, what to do on the bike next.... :smile:

Hinch99
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Re: Rev counter on CB400 Superfour

Post by Hinch99 » Sat Jan 13, 2018 6:06 pm

Hi did you ever establish which wire comes from the ignition coil? Was it the yellow and green? There are only two wires going into the back of the tachometer a green one which I believe is earth and the yellow and green which must be the ignition coil I presume. Ive been trying to install an aftermarket tachometer and have connected the yellow and green to the ignition wire from the meter but the needle does not move. The original one I removed works fine so it seems to be a wiring issue. Can you shed any light?


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