how does a tachomter work?

Hi all.

I'd be intersted to know how an analog tachomoter typically works on a
motorcycle? The one on my Spada intermittantly seems to either display
approx. double the revs that it's doing, or drop off the dial completely.
In order to track down the source of the problem, it lead me to wondering
how the bugger works.

Thanks,

Paul C - Honda Spada.

 

In aus.motorcycles on Thu, 24 Jul 2003 11:59:00 +1000

cable driven? Cam (usually) rotates, gears rotate cable, cable rotates
gears inside tacho, gears inside tacho move dial pointer.

electronic? On mine, coils fire, send signal to tacho, electronics
drive little motor that moves dial pointer.

Zebee

 

is it digitally controlled ?, or is there is torque cable going into the back
of it ?

digitally controlled, there would be lots of ways. i've never pulled one
of these part to know.

torque controlled operates by a magnetic clutch. the torque cable turns a
little fly wheel with magnets on it. within the fly wheel is another
smaller wheel also with magnets on it.. but is spring loaded. when fly wheel
turns, it induces smaller wheel to turn as well, but as it's spring
loaded it resists being turned.. at a given speed the small wheel is wound
round at different amounts... hence the needle action you see.

well, that's how the leyland mini speedo's and tacho's are designed !
(which i used to pull apart when i was about 5-6 years old -- dad would
bring them home for me to pull apart .

-f

 

http://www.classictiger.com/techtips/motach.html

The Spada one will be something pretty similar to that...

big

 

I had a similar problem with my Spada. It ended up being battery
problems. It took a while, but in the end the following electrics were
bad (and the combination caused that prob, as well as many others).

- Not earthing currectly.
- Battery not recharging - due to bad earth (ended up putting earth
straight to neg on bat)
- Bat buggered (after months and months of manual recharging)
- possible problems with alternator (or whatever it is called on the bike)
- some bad wiring.

so though I know nothing about how it works...I know it went wrong for me.

Mike

 

Actually the main reason for the "little dance" is so that the controller for the stepper motor can
"zero" itself, and so it knows it's range.

i.e. depending on circumstance the number of "steps" the motor may have to take today to get
to the end of its range[1] may be 320 steps today, but tomorrow due to temp differences and sheer
dumb luck it could be 318 or even 322 steps

Think about the old bubblejet and even dot matrix printers, they do the little "zoom zoom" to the end
and back whenever turned on/reset, and even sometimes halfway through printing a page may decide
to re-centre themselves in case they start loosing track of where they are. This is exactly the same little
dance as when you first turn on a bike with a stepper motor powered tacho or speedo.

[1] This is actuated by a microswitch that is at the end of the range, giving "feedback" i guess you
could say as to the number of steps until the motor hits the end limit.

Mike.S

 

I actually did know that, but its a little hard to turn into a cutting remark...
It _is_ still Harsh Week, remember???

Spectacularly unlikely - it'll use current sensing on the stepper motor drive -
why waste money on something mechanical like that when you can do it in the
sofyware?

big

 

Is *intermittently* meaning regularly like clockwork ('scuse the pun) and at
certain intervals or just sporadically at odd times..??

I know nothing of the Spada's electrics, but my YZF1000 does that to alert
you of other electrical problems such as running to certain RPM (3k, 5k and
8k iirc) when there is problems with the throttle position sensor, EXUP
powervalve etc...

Say I am riding @ 4000rpm, the tacho shows this for 3sec then drops down to
3000rpm for 2 sec and then to 0rpm for 3 sec and then back to 4000rpm in a
continuous cycle until I adjust the TPS...

Anyway, is a thought just in case......

Cheers,
Gavern

 

Yes.

**** Off Spammer Bitch!!

big (day 31 of Harsh Week...)

 

Interesting. I have noticed that after starting, the lights get brighter
when revving the engine. The starter seems to spin plenty fast enough - so
I haven't bothered about it.


..

 

I had a quick squiz at it during lunch. There doesn't seem to be cable
going into the back (but there is for the speedo).

 

Thanks.

I'll have a read through that tonight. I may get brave, and attack it with
a screw driver - see what's inside.

 

Word of advice - you're going to need to take the headlight out of its housing
to get to all the wiring so you can unplug the instrument cluster - take a good
long look at how it all fits in - it can be a bugger to get the headlight back
in if you've got the plugs and wiring sitting in the wrong spot...

big

 

Nope. Sometimes it doesn't work for an entire day. Sometimes it starts off
working, then jumps to approx double the revs. It will even change while
cruising along at constant speed on the freeway.

 

Are there any weather proof seals that I would need to especially carefull
of ?
[/QUOTE]

Nope... its just the jigsaw of knowing which plug needs to sit where, and which
bit of wiring harness needs to bend which way...

big

 

That sounds awfully like a broken/loose electrical connection. Unless you
*really* feel like pulling your tacho apart I'd start the bike and jiggle
all the bits of the wiring harness you can reach, to see if you can induce
it to start/stop working. If you can, narrow it down from there.

Does it crank over well? If it seems to be labouring then start off by
cleaning the connections to your battery.

Best of luck!


BTH

 

Hi Paul,

If it is electronic, and it is jumping to double revs, I would suspect a
problem with the electronic ignition (assuming you have one). The tach
drive should be a wire taking pulses from the input side of the coil. the
Tacho counts these (using one of a number of techniques).

I had some problems a couple of years ago where the tacho would jump to read
double revs for a while then the motor would lose power. Turned out in my
case to be an air leak in a carby that was making the engine overheat. This
caused the electronic ignition unit (or the sender unit - never found out
which) to get too hot and it started 'stuttering'. Initially it would put
out double sparks, which didn't affect the motor much, just wasted a few
electrons. When it got really hot, the spark became eratic and I lost
power. Removing a fairing section fixed it temporarily. Getting the carbs
fixed fixed it permanently.

In your case it may not be a carby problem, but I'd have a look at the
electronic ignition module, check the airflow to its heat sink and make sure
it isn't getting too hot. Also check for loose electrical connections.
Other than that I have no idea what diagnostics or remedies might be
applied.

If you have an automotive multimeter that has a tacho setting, you might
take it with you (even gaffer tape it to the 'bars). If both your tacho and
the multimeter show the same behaviour, then the problem is not inside the
tacho (otherwise it probably is).

I wouldn't be exploring inside the tacho until I was pretty sure it was
defective. There are some delicate bits in there, and exploratory
dismantling could be disruptive.

Hope this helps.

Mike

 

What happens when you cut the ignition on a "stepper" tacho?
Do they zero or stay where they were?

(I was on my third XJ900, a total of about 6 years and 150,000 km ownership, before I noticed it had no
tacho-cable......)

 

~
I can understand you being a bit "edgy" after months and months of "manual recharging" but you really should
leave the native fauna alone!!
(Get yourself a booth!)

 

This i do not know, perhaps the tacho has a light spring that can zero it back?
Printers use "worm drives" so they stay where they are, tacho's i imagine use direct drive,
so i can safely say "fucknose".

Mike.S