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Tach reads perfect until 4k, then goes to 7k??

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cschmokel

Joined: 04 Aug 2009
Posts: 23
Location: Pullman, WA

Posted: Sat Jun 26, 2010 10:28 pm Post subject: Tach reads perfect until 4k, then goes to 7k??

Hey guys,



I tore my engine down for a sprag clutch replacement last Winter.  Everything is running top notch now, except this damn tachometer.  It reads steady from 0 to 4k, then beyond that it runs straight to 7k.  Literally, I can move the throttle 1 mm past the 4k point and WHAM, 7k.  The engine sounds perfectly normal, so I'm sure it's not actually at 7k (pretty sure it would tear itself apart and make one hell of a racket).  Any ideas?  It's a Rotax 912 ULS on a Kitfox 5 .. and I'm stumped.  Anybody run into this before?  Thanks!



Carl

brian.davies(at)clara.co.
Guest

Posted: Sat Jun 26, 2010 11:34 pm Post subject: Tach reads perfect until 4k, then goes to 7k??

Carl,



This is a fairly common problem with the Rotax tach.  It can often be cured

by placing a 300 ohm resister across two of the terminals. There is a Rotax

service letter on the subject. 



Brian Davies

The Europa Club  



--

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Sun Jun 27, 2010 5:23 am Post subject: Tach reads perfect until 4k, then goes to 7k??

Sounds like a westach which is supposed to read the pulses in the tach

line...Problem seems to be the voltage on the tach line isn't stable it

increases with rpm.



The best bet for you would be to get either a mechanical tack ( expensive )

or a digital tiny tach that picks up impulses to the spark plugs...  be

aware there are several different models of Tiny Tach so be sure to get the

right one for your engine.



Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

cschmokel

Joined: 04 Aug 2009
Posts: 23
Location: Pullman, WA

Posted: Sun Jun 27, 2010 9:22 am Post subject: Re: Tach reads perfect until 4k, then goes to 7k??

Brian,



Thanks, I found the service bulletin.  Not sure which tach I've got, will have to check Monday.  I don't recall seeing a resistor on the stator the last time I tore the engine apart, so I'm thinking it's the "new" type with the integrated resistor.  Which makes me think the tach is broken ..



Noel,



I checked out the tiny tach website .. it looks like all these little guys do is count ignition pulses, and the number of pulses/rev can be programmed.  Seems to me any one of them would work .. right?

brian.davies(at)clara.co.
Guest

Posted: Sun Jun 27, 2010 10:40 pm Post subject: Tach reads perfect until 4k, then goes to 7k??

I don't know why but the Rotax tach pulses seem to be a problem for many

tachs.  I have changed a couple that would not respond to the resistor "fix"

and found that the UMA tach specifically programmed for the Rotax 912 works

very well. 



Regards



Brian



--

Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Mon Jun 28, 2010 4:58 am Post subject: Tach reads perfect until 4k, then goes to 7k??

Not quite.



You have the right idea with the counting of the pulses but two stroke

engines is completely different than four stroke engines.  The two stroke

engine will spark at the top and the bottom of the piston travel.  That's

twice per revoloution.  The four stroke engine is far different in that it

only sparks once every two revolutions.  



Tiny Tach has two models available one for two stroke engines...  and one

for four stroke.  



BTW if you get a tiny tach, I used one on my 582 UL with great success, be

sure to wrap both plug leads going to any cylinder.  That will insure the

tach doesn't blank out when you shut off either mag.



Noel



--

_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

ricklach

Joined: 20 Jun 2006
Posts: 127
Location: Kernville, Calif.

Posted: Mon Jun 28, 2010 1:48 pm Post subject: Re: Tach reads perfect until 4k, then goes to 7k??

Carl and Brian,



I’ve been working with the same problem as Carl. My Rotax rev-counter is solid until it reaches 4800-5000 then it goes to 7000, like Carl’s. I’ve been searching through all the SB’s, SI’s, SA’s and SL’s and I only found Service Instruction “SI-13-1996 R2” that refers to installing a tack. But I find no reference to a resister across any terminals in this document. Have I missed it or is there another document you’re referring to? Also Brian what two terminals are you putting the resister across? Service Instruction “SI-13-1996 R2” has six installation configurations. I’m like most people Fig. 3 is my installation, the simple one. All other “FIG” are for a Rotax Fly-Dat or a 914 with a TCU. 



After review all the Schematics in the 912 Installation Manual and the Heavy Maintenance Manual all schematics showed a simple direct connection, without a ground, between the tack #2 “-“ terminal and the rev pick up. I tried that and the tack did not work at all. I then replaced the ground between the tack #2 “-“ terminal as shown in Fig. 3 of “SI-13-1996 R2” and I’m now back in the same spot, straight to 7000 from 5000 so not pass go and do not collect $200.



Any thoughts.



Rick

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701Driver
N35 26.700, W118 16.743

cschmokel

Joined: 04 Aug 2009
Posts: 23
Location: Pullman, WA

Posted: Mon Jun 28, 2010 9:13 pm Post subject: Re: Tach reads perfect until 4k, then goes to 7k??

Noel,



The TT2A Tiny Tach can be set for 1 spark / 2 revs.  Hopefully, it'll do the trick.



Rick,



Glad to know I'm not the only one!  What's odd is this whole nuttiness started after replacing my sprag clutch.  Oh well.  I'll let you know if the TT2A works.  Kernville, CA .. I used to go there as a kid .. paradise!

brian.davies(at)clara.co.
Guest

Posted: Tue Jun 29, 2010 12:09 am Post subject: Tach reads perfect until 4k, then goes to 7k??

Hi Rick,



Sorry about the vague reply I gave.  I was away from my office so could not get to the reference.  I have attached a file from Rotax Owner Assistance Network (ROAN) which gives the guidance.  It suggests using a 200 ohm resistor across the trigger coil on the stator.  I have had just as much success using a resistor across the equivalent terminals on the back of the Tach. On the Rotax tach it goes across terminals 2 and 3. 



I have also cured this problem by swapping over the blue and white wires in the connecting plug.  In some cases the only cure was changing the tacho.



You will see from all of this that there is something a bit marginal about the Rotax 912 tacho  set up. The pick up seems to produce a relatively low voltage (3v) strangely shaped pulse when properly loaded but can give out as much as 100volts if not properly loaded.  That is why the 200 ohm resistor sometimes fixes the problem.  By the way, the trigger coil produces 1 pulse per revolution. 



 I have never had a problem with the UMA replacement for the "official" Rotax Tach.



Regards



Brian



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grahamsingleton(at)btinte
Guest

Posted: Tue Jun 29, 2010 12:18 am Post subject: Tach reads perfect until 4k, then goes to 7k??

I had the same problem years ago, it happened when I reset the sensor gaps on the flywheel trigger coils to manual figures, they were very wide although the engine ran fine. After readjustment the symptoms were as you all describe, fine up to 4K then full scale deflection. 

Graham

From: Brian Davies <brian.davies(at)clara.co.uk>

To: rotaxengines-list(at)matronics.com

Sent: Tuesday, 29 June, 2010 9:08:09

Subject: RE: Re: Tach reads perfect until 4k, then goes to 7k??

Hi Rick,

Sorry about the vague reply I gave.  I was away from my office so could not get to the reference.  I have attached a file from Rotax Owner Assistance Network (ROAN) which gives the guidance.  It suggests using a 200 ohm resistor across the trigger coil on the stator.  I have had just as much success using a resistor across the equivalent terminals on the back of the Tach. On the Rotax tach it goes across terminals 2 and 3. 

I have also cured this problem by swapping over the blue and white wires in the connecting plug.  In some cases the only cure was changing the tacho.

You will see from all of this that there is something a bit marginal about the Rotax 912 tacho  set up. The pick up seems to produce a relatively low voltage (3v) strangely shaped pulse when properly loaded but can  give out as much as 100volts if not properly loaded.  That is why the 200 ohm resistor sometimes fixes the problem.  By the way, the trigger coil produces 1 pulse per revolution. 

 I have never had a problem with the UMA replacement for the "official" Rotax Tach.

Regards

Brian

--

ricklach

Joined: 20 Jun 2006
Posts: 127
Location: Kernville, Calif.

Posted: Tue Jun 29, 2010 7:33 am Post subject: Re: Tach reads perfect until 4k, then goes to 7k??

Thanks Brian,



I now have a list of things to try. Your time is very appreciated. I’ll keep you updated as to my luck with this problem.



Rick

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N35 26.700, W118 16.743

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