Tacho ...............

Can someone explain how a tacho works .I believe it gets a pulse ,what sort of a pulse,
wattage ,or ohms, or a signal from planets beyond ours .

Coil I believe.

I got this info when I was trying to find the problem with Rhondas tacho.



I even bought an aftermarket tacho and tried to take signal from one of the four coils, negative is usual I think, to no avail. 

I had the tacho out and took it to an instrument specialist, but he quoted $90 just to look at it.

Help me Rhonda!

I assume it would be a voltage triggered tacho Gus - but my eyesight isn't up to the resolution of Bosco's wiring diagram (pity - it looks very useful).  In the simplest form, it would work on a signal from the coil (as Hammy said), like in this simplified diagram:



But from reading the Bosco post, I'd say the VFR tacho is not directly connected to the coil but to the IMU (the wiring diagram would confirm that if I could read it).  Can you carry out the diagnostic check as above?  That should isolate the problem.  In the simplest sense, either:
(a) the tacho is getting any power; or
(b) the tacho isn't getting any signal - if it's the latter it would be essential to be able to read the wiring diagram and find out if the IMU is putting out any signal - could be a bummer (eg, expensive).
(c) or both!

But all of the above assumes the input signal (ie, not the 12v power) to the tacho is coming from the IMU.  Can anyone read the wiring diagram?

Looking at the diagram, there's one earth, which goes back to a frame lug near the rear of the bike, and the other goes to the ECU.  Check the earth first as many a fault is caused by a dodgy earth point.  Then disconnect the feed from the ECU and check for continuity between that plug and the connection to the ECU.

Do you have any strangely acting bulbs?  These sorts of faults are often grunged up connectors or a harness break and I'd only suspect instrument failure if these are ruled out.

tacho is at top left of schematic, receives signal from ignition control module (top right) through yellow/green wire, and a green earth. 
It does run through a diode, by the looks. one thing that I didn't test. 
From memory I tried testing for a voltage at the ICM, but only with a multi meter. it could be a very small signal.

I can PM a copy of the diagram that you should then be able to expand, if that would help.

Bosco15 wrote:tacho is at top left of schematic, receives signal from ignition control module (top right) through yellow/green wire, and a green earth. 
It does run through a diode, by the looks. one thing that I didn't test. 
From memory I tried testing for a voltage at the ICM, but only with a multi meter. it could be a very small signal.

I can PM a copy of the diagram that you should then be able to expand, if that would help.

Looking at the blurry image I think what looks like a diode symbol is just a plug connector.

EXPANDED PICS.

I tried another tacho still no good . One wire is green (earth ) so I can forget that one as earth is good .
The other wire comes from the ic mod . Am I to read ohms or volts on that wire .
With dash removes and earth wire and pulse wire (both to tacho) I get a reading of 11.5 at idle . The strange bit to me is when I rev the engine it drops .
I would have thought it would rise .Yes that's just a plug connector

Reading voltage at signal wire tells you that it most likely is the tacho at fault. 
I did look inside the tacho and saw transistors. 

Switching Signals

• Modern tachometers use a "switching" or "pulse" configuration. Somewhere on the engine, you'll find some sort of gear-like or on-off switch that sends a pulse to the speedometer; this pulse comes from the points on a points-type distributor, the ignition module or optical sensor on a non-points unit or the crankshaft position sensor on electronically controlled engines. These sensors send a weak on-off signal to the tach; the faster the engine spins, the more frequent the pulse.
  
  

Pulse Tachometers

• The switching pulse goes into a small transistor, which works like a relay with no moving parts. The signal from the engine actually rises and falls in waves; these waves would mess with the tach mechanism, so the tach uses them to trigger a millisecond-long power burst to a larger transistor. This second transistor is essentially an amplifier, catching the weak signal from the first transistor and using it to trigger a more powerful pulse.
  
  

The Needle Mechanism

• A tach needle sits on a square metal plug with a coil wrapped around it; one end of the coil attaches to a ground while the other goes to the amplifier. A north-pole magnet sits on one side of the metal plug with a south pole on the other side. When an electrical pulse goes through the coil, it magnetizes the plug and gives the needle a shove to the right. The quicker these shoving pulses come, the harder the needle pushes against its return spring and the farther it moves.
  
  

Read more: http://www.ehow.com/info_12144401_automobile-tachometers-work.html#ixzz2sFWWM6Fw

gus wrote:I tried another tacho still no good . One wire is green (earth ) so I can forget that one as earth is good .
The other wire comes from the ic mod . Am I to read ohms or volts on that wire .
With dash removes and earth wire and pulse wire (both to tacho) I get a reading of 11.5 at idle . The strange bit to me is when I rev the engine it drops .
I would have thought it would rise .Yes that's just a plug connector

Ohms Gus, in answer to your first question.  Multimeter set to ohms and connected between the Y/G wire and then all pins of the ICU connector (one at a time obviously).  You should have no open circuit readings (ie infinite resistance) on any of the tests.

Bosco15 wrote:

I can't blame you for being a bit confused Gus, the diagrams don't appear to line up with the text. eg they say Y/G is positive and yet they show the negative terminal of the multi meter connected to it.

Basically they are saying that if you connect as per the top illustration you should see battery voltage from the Y/G to the G in the 6pin connector. I can't tell if that is the instrument cluster side, if so then the Tacho gets voltage from somewhere which seems at odds with the circuit diagram which only shows 2 wires going to the tacho.

In the bottom illustration they want you to check for continuity from where the Y/G wire connects to the ECU anr the connector at the dash. On Ohms range this should be zero ohms or very close to it. If you short your meter leads together on ohms setting you should get 0 ohms but some meters may read 2 ohms for example. My meter has a diode test/ continuity function that beeps when a short or very low resistance is across the probes.

I went looking for a Haynes manual or something similar that may have had a correct procedure, no luck so far.

I found a manual for an older VFR 750 that made more sense.



Maybe it will give you a clue.

To check if the tach is getting a signal you really need an oscilloscope, this is because the signal is a pulse that increases in frequency the more you rev the bike (rev = revolutions per minute = pulses per minute  ).  Unfortunately there are a myrid of ways to make this pulse, so it would be rare for a tach of one bike to be able to understand the pulses being generated by another  :


Seeing that most of us don't even own a multimeter, there are not too many home garages with one of these hanging around.

The two tests mentioned in the manual is to see if there is any voltage on the wire (DC test done on the multimeter on Volts) and if the wire is not broken (continuity test done on Ohms). In reality the signal is neither DC voltage (well if you get padantic it could be a rapidly changing on/off DC level) nor Ohms, but AC voltage.  The DC voltage test will change reading as the motor is reved as the signal pulsing confuses the DC setting of the meter.  The Ohm test should not be done with the bike switched on or running as the bike's voltage could interfer with the meter readings.  It also does not matter which lead goes where when testing continuity of a wire.  

It will be rare these days to find a signal from a coil going to the tach, the most common method is a pulsed signal from the ECU which appears to be the case here.

By the sounds of things I'd say the signal is getting to the tach if you see a voltage and it changes with revs (even though it goes down it is still a change).  As mentioned by others, check all connectors for tightness and corrosion before going out and getting another tach.

Thanks all ,I realised I'd never fix it if my understanding wasn't better .

Howdy Gus,

From the above discussion it seems to me that the signal to the tacho is OK.

I know you have already checked the wiring but perhaps there is one final test you might like to do.

1. Get hold of two dressmaking pins (no, where not making any more frocks today   )

2. Get as close to the wire entry points to the tacho as possible and push the pins through the insulation to make connection with the internal copper wire.

3. Connect your multimeter on DC volts to these pins.

4. Test1 ignition on you should get a reading somewhere near 12 volts

5. Test 2 with engine running observe the reading decreasing with an increase in engine revs.

6. If above tests both OK then we must conclude that the tacho is stuffed.

7. There is no step 7

THe wires going into the back of the Tacho have exposed metal eyes . That's where I have taken the readings .
I'm now checking ecu is the original Honda item from this model . The R\Rec has been changed so maybe the ecu shat itself at the same time .

Howdy Gus,

Well I reckon you've proved beyond reasonable doubt that you have power and data pulses going into the tacho.

Unless you find something really weird on the bike I consider that the fault is within the tacho         

Is the tacho something you would dismantle or is it a sealed unit??


All right one last question. If the two wires going to the tacho are attached via screw terminals are they specifically labelled or could they be swapped over??

Grd and pulse so no mistake there . BUT

The used tacho I received LOOKS the same, but from a newer model . I realise now, that that model has a different ig\n\control number on it so it is quite feasible
this tacho is not compatible  with my unite .
I have the correct tacho in the mail but I will be in Tasi when it arrives , unless I'm lucky and it arrives the day before I leave .