Tach fluctuation

[WWhunter]

Well, finally had a few spare minutes to readjust the pitch on my prop.
Started up and tried to get a static RPM but unfortunately my tach was bouncing all over the place. It finally settled down a bit but still varied by 200-400 rpm. It does seem to stabilize a bit at higher RPM's. Bad Tach?

Oh, I should probably mention that my voltage regulator/rectifier was bad also. Everything went crazy for a second and then my low voltage light came on. I shut it down and replaced the rectifier before restarting.

Anyone ever had this issue?

Didn't have time to diagnose it but I am guessing the tach is bad. It is a VDO electronic type. Haven't had anytime to fly yet this summer and it will be fall soon.

[Av8r3400]

Sweep needle tachometers are notoriously inaccurate on Rotax engines. Both the two strokes and the 912 series.

I had a VDO and a Rotax tach in my old plane that were both terrible. I replaced the Rotax with a digital tach from ACS that was great. Spot on compared to an optical tach.

I have a Dynon panel in the Mangy. That checked perfect to the optical, too.

[WWhunter]

Thank you sir! I went through the major ordeal of getting it out today. What a friggin job!!! It's a 20 year old VDO and has always worked well until I had that Voltage rectifier issue. May have had a voltage spike that messed it up a bit.

I was doing some research a little bit ago and found a Technical Bulletin dated Aug 03 with the description 'ERRATIC OPERATION' it says to add a diode in line of the ignition lead. The diode is #1N4005 Luckily, (which is not in my usual day), I had one in a box!! I built test computers for FA-18 Hornets, that went on aircraft carriers, many years ago and as the average guy....always had a few spare components in my electronics stash. Score one for me!!

I'll get it soldered together tonight and hopefully it will cure the problem. Much cheaper than buying a new tach.

I will most likely get a digital as you suggested. I have seen this advice several times over the years.

[Norm]

https://www.amazon.ca/Qiilu-Inductive-Tachometer-Motorcycle-Snowmobile/dp/B072ZW5WPW/ref=sr_1_14?ie=UTF8&qid=1507380147&sr=8-14&keywords=digital+tachometer

This is the tach I use on my 912. I used one in a 582 for years. I have seen them as low as $10 so not a big ticket item. You could run it as a backup to your current tach. Inexpensive way to check your current tach.

Norm

[WWhunter]

Norm,
I am sitting here laughing!!! I just ordered and got confirmation that a similar tach was shipped. I was looking for a digital as an 'extra' and found a lot of 4 the whooping price of $28.99 shipped on ebay. Seller allwinshop sells them in various lot sizes. I figure I'll have some spares for other stuff around my property that I can use them on. I can use them on generators etc. to keep track of the hours for maintenance.

[Kahle76]

Sweep needle tachometers are notoriously inaccurate on Rotax engines. Both the two strokes and the 912 series.

I had a VDO and a Rotax tach in my old plane that were both terrible. I replaced the Rotax with a digital tach from ACS that was great. Spot on compared to an optical tach.

I have a Dynon panel in the Mangy. That checked perfect to the optical, too.

Do you remember the exact tach that you used?