Erratic TinyTach

[eschrom]

Lately the TinyTach display has been drifting erratically while the engine is idling but then latches onto the speed when I advance the throttle. Does anyone have an idea why?

Ed

 

[oahupilot]

Sounds like a weak signal on the wound pickup coil, maybe add more turns.

 

[eschrom]

Thanks, OP.

Ed

 

[Pttim]

Agree with what oahupilot says

 

[eschrom]

Had a crack at this today. The TT pickup wire was wrapped two and a fraction turns around the lead so I increased it to three and the problem ... got worse. Tomorrow I'll back it off to two turns and see what harpoons.

Ed

 

[eschrom]

At two turns I'm back about where I started - erratic readings at idle but accurate, or at least believable, displays at higher power settings. I should clarify that the erratic readings occurred before the flight while the engine was relatively cool. Afterwards, when the cowl was good and hot, the idle readings seemed about right. Next time I pull the top cowl I'll adjust it down to one and a half turns.

Ed

 

[sonerainut]

I had an issue years ago where the tiny tach sensor wire that was wrapped around one secondary ignition lead was too close to the lead for the adjacent cylinder and it was occasionally seeing the other wire firing as well. RPM was all over the place and out of whack...

Repositioned the other wire (not much, like 1/2 away) and problem solved.

Also, after replacing my second tiny tach due to dead internal battery, I decided to carefully cut one open at the seam and soldered on a couple of wires that I connected to a radio shack AA battery holder. Now I have a replaceable battery and with a couple of Energizer Lithium batteries, way longer (its been 5+ years) lasting.

Lee

 

[Tom H]

I have tried several models of TinyTachs used on VW engines. I have had mixed success with them. One works fine on our half VW. Others had issues with stability on the full VW. I think sonerainut's theory of detecting the other ignition wires could be one of the issues.

Later, we tried one of the Trail Tech TTO Tachs. There are several versions available. It has an internal battery (5 years, likely not designed for replacement, but I have opened up a TinyTach and brought out wires for external battery, TTO may be able to have this done, too), and it senses one of the ignition wires like a Tiny Tach. But, it works great, rock solid on first try.

TTO Tachs are less expensive than Tiny Tach, also, I believe.

Also, my son bought a dozen Chinese Tiny Tach copies, which he bought for about $5 each. He gave me one to try. It also works really well, but not quite as stable as the TTO.

 

[eschrom]

Thanks for the replies. The TT pickup wraps around the lead under the Slick's wire harness cap so there's nothing I can do about spacing. If more fiddling with the number of turns doesn't fix the problem maybe I'll try to shield the pickup.

This is actually my second TinyTach. The first one has already been cut apart for a new battery and will be reinstalled when number two dies.

Ed

 

[kennyw]

The TinyTach that was on 994SP when I bought it had replaceable AAA batteries. When it stopped working, I tried replacing the batteries and the thing was so erratic throughout the RPM range, I just assumed it had failed and bought another one. I didn't like it anyway because it always alternated with RPM and tach time.

The new one has a 5-year non-replaceable battery. Its not really rock steady. But, its useable. And, when the engine is running, it stays on the RPM readout. It only shows tach time when the engine is stopped.

I don't think they designed it with any kind of data smoothing. Its just an instantaneous readout that continuously changing about +/- 50rpm . I rely on the manifold pressure guage to set the throttle. The corresponding RPMs are pretty consistent in the pattern. At altitude, it takes a minute with throttle and mixture to get it trimmed for cruise.

 

[eschrom]

At altitude, it takes a minute with throttle and mixture to get it trimmed for cruise.

Same here. I like the AeroCarb but even the slightest throttle changes require a mixture adjustment.

Your TT with the external batteries was Scott's doing. IIRC he posted a how-to about it.

Ed

 

[Tom H]

...The TT pickup wraps around the lead under the Slick's wire harness cap so there's nothing I can do about spacing....

Ed

Ed, the one Tiny Tach that I am using is connected like you say, inside the Slick mag cap. The original shielded ignition wires have been replaced with an automotive noise suppressing type wire, for com radio noise reduction (helps a lot). When the engine gets to a steady setting, the tach will calm down. RPM changes cause it to become erratic.

Regarding shielding - Good luck on that! I went to great lengths to shield one of the Tiny Tachs, including: opened and replaced the tiny tach wire lead with a length of very small diameter coax cable (can't immediately find the number), grounded the shield at the engine, I believe. No good.

I also put a braided shield over the Tiny Tach lead and made a metal box to install the Tiny Tach in, which I hoped would shield it from most noise signals. Did not work.

I tried various resistors in the signal lead, both at the engine end and at the Tiny Tach end. No success. We tried few wraps, many wraps, several locations, trying to keep the sense wire clear of any other wires.

Then, probably from a recommendation on this site, tried the Trail Tech TTO tach. Problem solved. (This was on the second ignition, we have two tachs).

I know many have great success with Tiny Tach, but have heard many stories of problems with them, too.

Hope this helps.

 

[eschrom]

Thanks, Tom. I hadn't thought much about how I would implement the shielding but after your experience I think I'll just scratch that side track off the list.

Do you have any advice on how to connect automotive wires to the Slick? Radio noise is another of my electrical problems.

Ed

 

[Tom H]

Here is the mod to install automotive ignition wires to the Slick.

I purchased a spare mag cap off of Ebay for this mod because I was not sure if the mod would work, and did not want to sacrifice the original mag cap and wires.

 

[eschrom]

Thanks much, Tom!

 

[oahupilot]

Shielding will not work as the noise is being transmitted on the ignition wire/ cap itself.

However I would try a filter or ferrite bead , the black cylinder thing that gets clipped on power cord of electronics.

A filter can be created with capacitor and resistor together, google for RC filter.

 

[JuergenA]

Hi,
here is my TinyTach installation:

http://www.sonerai.net/gallery/thumbnails.php?album=119

with best regards
Juergen

 

[eschrom]

Thanks, Ohau and Juergen.

...noise is being transmitted on the ignition wire/ cap itself

My installation is the same as Juergen describes except for the extra isolation through the hole. I wonder whether that points to the solution? Next time I have it apart I'll add the isolation.

Thanks again to all who responded. This site is the best!

Ed

 

[oahupilot]

Do you have any advice on how to connect automotive wires to the Slick? Radio noise is another of my electrical problems.

Make sure you use suppressed spark plug cable, as un-suppressed automotive cable is very noisy electrically. You will hear it on the radio as tick sound.