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Unstable Tach

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tshank(at)core.com
Guest

Posted: Wed Mar 08, 2006 6:18 pm Post subject: Unstable Tach

I remember this being discussed in the past but I have not been able to 

locate anything with the search engine. I have a Status Suburu and a VDO 

tach. Above 3000 RPM's the tach become unstable, jumping 1000 RPM's or 

more at a time. I have tried some filtering on the line with no success. 

When I switch the tach to the secondary ignition it is stable. I was 

hoping someone had solved this before, so  I have to cart my Tectronics 

scope out to the shop and try to track this down.



Tim Shankland

thilo.kind(at)gmx.net
Guest

Posted: Wed Mar 08, 2006 8:32 pm Post subject: Unstable Tach

Hi Tim,



I had a similar experience with my Micro Tach 100 on a Rotax 912. Finally

installed a resitor in the sensing wire - problem solved. I'm not home right

now, but I think it was 47 K.



Good luck



Thilo Kind> --- Ursprngliche Nachricht ---

 	  Quote: 	
		   Von: Tim & Diane Shankland <tshank(at)core.com>

 An: Zenith List <zenith-list(at)matronics.com>

 Betreff: Unstable Tach

 Datum: Wed, 08 Mar 2006 21:11:29 -0500

 

 

 

 I remember this being discussed in the past but I have not been able to 

 locate anything with the search engine. I have a Status Suburu and a VDO 

 tach. Above 3000 RPM's the tach become unstable, jumping 1000 RPM's or 

 more at a time. I have tried some filtering on the line with no success. 

 When I switch the tach to the secondary ignition it is stable. I was 

 hoping someone had solved this before, so  I have to cart my Tectronics 

 scope out to the shop and try to track this down.

 

 Tim Shankland

 

 

 

 

 

 

  

  

  

 

 

 


	


-- 

"Feel free" mit GMX FreeMail!

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frank.hinde(at)hp.com
Guest

Posted: Thu Mar 09, 2006 6:58 am Post subject: Unstable Tach

Sometimes (quite often actually) the coils go bad (low resisatnce on the

primary side...It usually frys the $300 ignitor chip too so you need to

check this out before your next flight).



I could how this might cause it...Check your primary coil resistance,

should be about 1.1 ohms I believe.



Try swapping just the coils and see if the problem follows the coil.



Frank 



--

tshank(at)core.com
Guest

Posted: Thu Mar 09, 2006 6:52 pm Post subject: Unstable Tach

Thanks for the suggestions, I all ready have tried resistors, either no 

effect or no tach, I'll check the coil although the engine only has 

about 6 hours on it and I haven't flown it yet. And I will check for 

inductive coupling and maybe swap the coils.



Tim Shankland



Hinde, Frank George (Corvallis) wrote:



[quote]



Sometimes (quite often actually) the coils go bad (low resisatnce on the

primary side...It usually frys the $300 ignitor chip too so you need to

check this out before your next flight).



I could how this might cause it...Check your primary coil resistance,

should be about 1.1 ohms I believe.



Try swapping just the coils and see if the problem follows the coil.



Frank 



--

frank.hinde(at)hp.com
Guest

Posted: Fri Mar 10, 2006 7:20 am Post subject: Unstable Tach

The good thing is there aren't many things it can be. You know it is not

the tach, so it has to be something to do with an ignition component.



Just a thought Tim, are you using a changeover switch between one

ignition system and the other?



If you have you do realise you have just connected the ignitions

together. It is not impossible for that switch to short to ground and it

could ground out both your ingitons and stop you engine running.



Likely?...No, but it is generally not a good idea to rely on a single

component in a critical application.



Frank 



--

tshank(at)core.com
Guest

Posted: Fri Mar 10, 2006 5:19 pm Post subject: Unstable Tach

I am using a switch to change the tach form one ignition to the other, 

true a shorting of the switch could cause the selected one to fail.  I 

guess I could include a 1 or 2 K resistor in series with those lead to 

protect from that possibility.



Tim



Hinde, Frank George (Corvallis) wrote:



[quote]



The good thing is there aren't many things it can be. You know it is not

the tach, so it has to be something to do with an ignition component.



Just a thought Tim, are you using a changeover switch between one

ignition system and the other?



If you have you do realise you have just connected the ignitions

together. It is not impossible for that switch to short to ground and it

could ground out both your ingitons and stop you engine running.



Likely?...No, but it is generally not a good idea to rely on a single

component in a critical application.



Frank 



--

tshank(at)core.com
Guest

Posted: Thu Mar 16, 2006 8:01 pm Post subject: Unstable Tach

This is just a continuation of my previous posts. As suggested I checked 

for inductive couplings etc. No repositioning of the wiring made any 

difference. It was time for the big guns, I carted my Techtronics 4 

channel rack mount scope out to the plane, it might have handles but it 

is not very portable. Anyway the normal wave form is a 10 msec step down 

to about -10 volts followed by an upward step to + 20 volts for 5 msec 

then back to 0. There is some ringing on both ends of the 20 volt step. 

The problem appears to be rather high voltage (15-20 volt) high 

frequency (5-10 microsec) noise that occurs above 3000 RPM  usually in 

the middle of the -10 volt step. Since there was a significant frequency 

difference between the good and bad signal I constructed a filter, a 10K 

resistor connected to one end of two 12 volt zener diodes back to back 

with the other end to ground and a 0.2 uF capacitor across the the 

diodes. Checked it out today and all tack operation is smooth.



TIm Shankland

 	  Quote:

rdewees(at)mindspring.com
Guest

Posted: Fri Mar 17, 2006 5:40 am Post subject: Unstable Tach

Wow Tim,

You really got into the guts of the problem didn't you?  Thanks so 

much.  One nagging question tho-- my tach is unstable (inoperative) 

below 1000 rpm.  Any ideas what's going on?  Voltage too low as opposed 

to too high?  Suggestions?

Thanks

Ron DeWees

do not archive

Tim & Diane Shankland wrote:

 	  Quote: 	

This is just a continuation of my previous posts. As suggested I checked 

for inductive couplings etc. No repositioning of the wiring made any 

difference. It was time for the big guns, I carted my Techtronics 4 

channel rack mount scope out to the plane, it might have handles but it 

is not very portable. Anyway the normal wave form is a 10 msec step down 

to about -10 volts followed by an upward step to + 20 volts for 5 msec 

then back to 0. There is some ringing on both ends of the 20 volt step. 

The problem appears to be rather high voltage (15-20 volt) high 

frequency (5-10 microsec) noise that occurs above 3000 RPM  usually in 

the middle of the -10 volt step. Since there was a significant frequency 

difference between the good and bad signal I constructed a filter, a 10K 

resistor connected to one end of two 12 volt zener diodes back to back 

with the other end to ground and a 0.2 uF capacitor across the the 

diodes. Checked it out today and all tack operation is smooth.

TIm Shankland

>

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JAPhillipsGA(at)aol.com
Guest

Posted: Fri Mar 17, 2006 8:31 am Post subject: Unstable Tach

Tim, for dim fellow like myself with the same tac problem how do I fix it, 

again??? You got a drawing or some photos ? Hope to hear from you, Bill of 

Georgia N505WP 601XL-3300

frank.hinde(at)hp.com
Guest

Posted: Fri Mar 17, 2006 8:56 am Post subject: Unstable Tach

Are we sure the coil on this ignition system is not going south...there

have been a number of failures of the IC107 coil used extensively on

Subaru powered airplanes.



Frank 



--

tshank(at)core.com
Guest

Posted: Fri Mar 17, 2006 5:15 pm Post subject: Unstable Tach

Ron,

It's really hard to tell without looking at the signal, that's why I 

used the scope. (Funny I looked at a portable scope at Osh. last year in 

the consignment test and almost bought it, carting the big one out makes 

me wish I had.)  As I described the tach, at least in my case sees  a 

square wave at each plug firing and the amplitude did not change 

significantly with RPM. If you do not get any tach operation below 1000 

it could mean that you have a problem with your tach. If it is a 

problem  with insufficient  signal it will be much more difficult  in 

that an  active addition ( amplifier) would be necessary rather than a 

simple passive filter.



Tim Shankland



ron dewees wrote:



 	  Quote: 	
		  



Wow Tim,

You really got into the guts of the problem didn't you?  Thanks so 

much.  One nagging question tho-- my tach is unstable (inoperative) 

below 1000 rpm.  Any ideas what's going on?  Voltage too low as opposed 

to too high?  Suggestions?

Thanks

Ron DeWees



do not archive



Tim & Diane Shankland wrote:



  



>

>

>This is just a continuation of my previous posts. As suggested I checked 

>for inductive couplings etc. No repositioning of the wiring made any 

>difference. It was time for the big guns, I carted my Techtronics 4 

>channel rack mount scope out to the plane, it might have handles but it 

>is not very portable. Anyway the normal wave form is a 10 msec step down 

>to about -10 volts followed by an upward step to + 20 volts for 5 msec 

>then back to 0. There is some ringing on both ends of the 20 volt step. 

>The problem appears to be rather high voltage (15-20 volt) high 

>frequency (5-10 microsec) noise that occurs above 3000 RPM  usually in 

>the middle of the -10 volt step. Since there was a significant frequency 

>difference between the good and bad signal I constructed a filter, a 10K 

>resistor connected to one end of two 12 volt zener diodes back to back 

>with the other end to ground and a 0.2 uF capacitor across the the 

>diodes. Checked it out today and all tack operation is smooth.

>

>TIm Shankland

> 

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>>

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>>

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tshank(at)core.com
Guest

Posted: Fri Mar 17, 2006 5:44 pm Post subject: Unstable Tach

Frank,

As I mentioned in an earlier post the airplane has not flown yet. It 

would seem though if there is a problem that it would be with the 

ignition module. The extra noise pulse comes right in the middle of the  

-10 volt excursion. but doesn't effect the level. That is I have a 

straight line at -10 volts with the noise pulse in the middle but still 

a steady straight level after it. Since it occurs exactly at 3000 RPM 

every time I'm wondering if it has something to do with the centrifugal 

spark advance? The information I have on the engine gives me lost of 

info about the cooling system but nothing about the spark advance. I 

guess I could use a timing light to determine when the spark advance 

starts to see if there is any relationship.



Tim Shankland



Hinde, Frank George (Corvallis) wrote:



[quote]



Are we sure the coil on this ignition system is not going south...there

have been a number of failures of the IC107 coil used extensively on

Subaru powered airplanes.



Frank 



--

tshank(at)core.com
Guest

Posted: Fri Mar 17, 2006 5:44 pm Post subject: Unstable Tach

It's a real simple circuit, I'll draw it up and post it. It might take a 

day to get to it.



Tim Shankland



JAPhillipsGA(at)aol.com wrote:



 	  Quote: 	
		  



Tim, for dim fellow like myself with the same tac problem how do I fix it, 

again??? You got a drawing or some photos ? Hope to hear from you, Bill of 

Georgia N505WP 601XL-3300

frank.hinde(at)hp.com
Guest

Posted: Sat Mar 18, 2006 9:09 am Post subject: Unstable Tach

Are U running the standard Sooby ignition chip? (I think you said you

have a Stratus right?) If so they are notoriously feeble. Normally they

either work or they don't though.



I've junked mine in favour of the TP45 NAPA unit.



Frank 



--

tshank(at)core.com
Guest

Posted: Sat Mar 18, 2006 7:22 pm Post subject: Unstable Tach

Frank,

Yes, I'm running a Stratus, so far the standard ignition starts and 

runs. As I mentioned in a previous post my current working theory is a 

vibration in the centrifugal advance. I do notice what seemed to be a 

slight roughness that occurs with the tach instability. I'm going to put 

the prop back on and run the engine up to a higher RPM to see if the 

roughness is just in the mid range. If my theory is correct it will end 

when the centrifugal advance hits it's high end stop.



Tim



Hinde, Frank George (Corvallis) wrote:



[quote]



Are U running the standard Sooby ignition chip? (I think you said you

have a Stratus right?) If so they are notoriously feeble. Normally they

either work or they don't though.



I've junked mine in favour of the TP45 NAPA unit.



Frank 



--

frank.hinde(at)hp.com
Guest

Posted: Mon Mar 20, 2006 7:45 am Post subject: Unstable Tach

Check to see how much side play you have on the distributor shaft and if

it might be coming into contact with the pick up...Or indeed could be

getting too far away. There should be very little side play (None!) but

the reality is a lot of them do. I'm thinking you could be hittng a

resonance frequency that pulls the shaft to one side.



Have you swapped the coils just for fun?...If you have a leaky low

tension side then this could in effect be grounding out the chip...It

would would be a quick test in any case if only to discount the theory.



Frank



--

tshank(at)core.com
Guest

Posted: Mon Mar 20, 2006 6:52 pm Post subject: Unstable Tach

Frank,

I had previously checked the shaft for play and found none, and it 

doesn't appear to be coming into contact with the pickup. I did put the 

prop back on yesterday and ran up the engine. I have my filter on now so 

the tach is steady at all speeds. Engine smooth at 4000 RPM's  and up 

maybe a little roughness between 3000 and 4000, hard to say. Ran up to 

5400 RPM, going to have to add a little pitch in the prop, I still had 

some throttle left. I looked into swapping coils, but I didn't want to 

have to dig into that wiring at this point. I might if I get some other 

things done.



Tim



Hinde, Frank George (Corvallis) wrote:



[quote]



Check to see how much side play you have on the distributor shaft and if

it might be coming into contact with the pick up...Or indeed could be

getting too far away. There should be very little side play (None!) but

the reality is a lot of them do. I'm thinking you could be hittng a

resonance frequency that pulls the shaft to one side.



Have you swapped the coils just for fun?...If you have a leaky low

tension side then this could in effect be grounding out the chip...It

would would be a quick test in any case if only to discount the theory.



Frank



--

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