Limited Coil Tachometers

[Vintage Engine Repairs]

Can the EDT 9 by stihl give you the limited coil reading on other brands like husqvarna?


Is there a better tach that can read limited coils (faster refresh rates?)

Thanks guys.

 

[cookies]

The proline one I got from sawzilla has a really fast refresh rate, fraction of a second refresh

 

[Vintage Engine Repairs]

The proline one I got from sawzilla has a really fast refresh rate, fraction of a second refresh

Oh cool!Does it read limited coils?

 

[cookies]

It reads any ignition coil I have tried it on

 

[Vintage Engine Repairs]

It reads any ignition coil I have tried it on

Thanks mate, yeah my current one does too, everything apart from Limited coils.

 

[huskihl]

Thanks mate, yeah my current one does too, everything apart from Limited coils.

I haven’t seen a tach yet that will work once the limited coil RPM is reached. They go haywire and read 5K to 18K. But all of them I’ve used so far work below the limited rpm spec.

 

[SteveSr]

By definition these tachometers are just counting sparks. When the coil limits and starts cutting off sparks your reading goes out the window! The faster the refresh rate the less accurate the reading and I really doubt that it would read a limited coil once it started limiting.

 

[Vintage Engine Repairs]

I haven’t seen a tach yet that will work once the limited coil RPM is reached. They go haywire and read 5K to 18K. But all of them I’ve used so far work below the limited rpm spec.

By definition these tachometers are just counting sparks. When the coil limits and starts cutting off sparks your reading goes out the window! The faster the refresh rate the less accurate the reading and I really doubt that it would read a limited coil once it started limiting.

I’ll update this tomorrow with a video of the stihl edt 9 reading a limited coil holding the rpm without going all whacky.

 

[huskihl]

I’ll update this tomorrow with a video of the stihl edt 9 reading a limited coil holding the rpm without going all whacky.

Which engine?

 

[Vintage Engine Repairs]

Which engine?

I have a stihl fs55 just come in so I’ll do a video on that with the tachs before I repair and on-sell it.

 

[KostasVolos]

Those tachometers use a MOSFET transistor as a capacitative sensor of high voltage pulses .Capacitative and not inductive as more than often are advertised.An inductive method would require a grounding.
The MOSFET sensor is connected
on a counter which counts real time the spark high voltage pulses - no "sampling rate" - and the counter is signaling a microcontroller in order for the signals to be further processed and visualized on a screen .
The two above need time ,thus the
"refresh rate" .It's the time that the
programmed "loop" starts ,does it's thing ,ends and restarts a new "loop" .

In case of the tachometer the counter signals the MCU (micro controller unit ) on how many high voltage pulses have been triggering the sensor .
The MCU inserts "time" and depending on which engine operation is selected it calculates the sparks/sec into rev/sec and then into rpms .All this processing usually takes about a second or half a second (or even less )on tachometers with faster MCUs .

But still that won't help to read correctly a limited coil .
What helps is the "variable engine type " programming .

 

[KostasVolos]

For example such a tach is adjusted to read for " one spark per revolution " .
Meaning that the MCU will assign
a full crankshaft revolution for every high voltage pulse that is triggering the sensor.
All good ,till the rev limiter of the coil hits ! Then our "limited coil approved " tach sensors the wide discrepancy between the measured sparks -since the rev limited coil now is powering the spark plug EVERY OTHER REVOLUTION -and automatically
readjusts the way it calculates rpms from pulses/sec ,thus the steady output measurement on the screen .It's an extra programming sub-routine issue and does not have anything to do with the refresh rate of one or half a sec ( which is rather slow to be used for measuring every other rev at say 14,000 rpm ...) .

 

[Vintage Engine Repairs]

For example such a tach is adjusted to read for " one spark per revolution " .
Meaning that the MCU will assign
a full crankshaft revolution for every high voltage pulse that is triggering the sensor.
All good ,till the rev limiter of the coil hits ! Then our "limited coil approved " tach sensors the wide discrepancy between the measured sparks -since the rev limited coil now is powering the spark plug EVERY OTHER REVOLUTION -and automatically
readjusts the way it calculates rpms from pulses/sec ,thus the steady output mwadurement on the screen .It's an extra programming sub-routine issue and does not have anything to do with the refresh rate of one or half a sec ( which is rather slow to be used for measuring every other rev at say 14,000 rpm ...) .

This is absolutely fascinating, THANK YOU! I have been trying to find out how the stihl tach can read the limited coils and my other one can’t!

I need to read it a few more times to understand everything though.

 

[KostasVolos]

It's a software thing .
Stihl tach has added programming
which enables the tach to be
"aware" when the coil's rev limiter is reached *and then re-adjust the method of rpm calculating .
No hardware is involved .

* A sudden discrepancy in calculated rpms ,(i.e at one loop
the output is 14,000 rpm and
half a sec later at the following
system loop the output falls to 6500 rpm ) if sensored is triggering the MCU to automatically move from
calculating as "1 spark pulse = 1 rev " to calculating as " 1 spark pulse = 2 revs " . If the opposite discrepancy occurs ( rpms fall below set coil limit ) then the MCU will switch back to default
rpm measuring ,being " 1 spark pulse = 1 rev " .

 

[Vintage Engine Repairs]

This is such useful info. I had presumed it would be some little added electronic device to be able to process the limiting of the ignition. But no!

 

[KostasVolos]

With some aluminium foil tape these tachs can be hacked to pick up wirelessly spark plug signals from a 10-20 cm distance.

Remember.The MOSFET sensor is triggered when a sudden change in capacitance is made .
Capacitance of which capacitor ?
One might ask ...

The spark plug wire ( yes the copper wire ) is one of the two capacitor armatures .The polymer isolation of the spark plug cable ,the air thickness ( aka distance ) between the two capacitor armatures and the polymer isolation of the second armature ( the wire of the tach ) is the dielectric between the armatures.
And finally the second armature ,
is the wire inside the cable that you connect to the tach.

In order for the distance to be greater ,the surface of the armatures have to increase .
You ca not increase the surface of the spark plug wire ,but you can increase the surface of the tach wire.
Now a bit of aluminium foil tape ,made into a loop ,a ring ,or something similar ,connected to the tach can act as a strong pick up antenna .

 

[Vintage Engine Repairs]

Gunna have to re read your post a few times again, but funnily enough, I did this video today, any thoughts?

 

[KostasVolos]

So ,another way to work with limited coils is to use...two ordinary tachs !
Can be of the el cheapo $10 type!
They are more than precise and accurate .

One set for " 1 spark = 1rev " and
the other set " 1spark = 2revs " .
Until the rev limiter is reached the
first ,the 1:1 type will show the correct values .When the rev limiter will come into play ,the second tach ,the " 1:2" one ,will show the correct readings ...
Messy ,but in theory it works .
They can even be sharing the same "tach wire" .Just pass it through the small holes on both tachs.

 

[KostasVolos]

Gunna have to re read your post a few times again, but funnily enough, I did this video today, any thoughts?

At the other site ,I 've responded with a link ...
;-)

 

[Vintage Engine Repairs]

What site? I can’t find a comment. I’ll read this in the morning. It’s 2 am in Australia and I better sleep lol thanks again for the info!