Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser CPS Parts EarthX Batteries
Results 1 to 5 of 5

Thread: Tach fluctuation

  1. #1
    Senior Member
    Join Date
    May 2011
    Location
    MN
    Posts
    400

    Default Tach fluctuation

    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.

  2. #2
    Super Moderator Av8r3400's Avatar
    Join Date
    Jan 2009
    Location
    Merrill, WI
    Posts
    2,746

    Default Re: Tach fluctuation

    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.
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  3. #3
    Senior Member
    Join Date
    May 2011
    Location
    MN
    Posts
    400

    Default Re: Tach fluctuation

    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.

  4. #4
    Senior Member Norm's Avatar
    Join Date
    May 2013
    Location
    Airdrie, North of Calgary
    Posts
    320

    Default Re: Tach fluctuation

    https://www.amazon.ca/Qiilu-Inductiv...tal+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
    Norm
    Airdrie Ab, Can
    North of Calgary
    Flying SuperFox Model IV

  5. #5
    Senior Member
    Join Date
    May 2011
    Location
    MN
    Posts
    400

    Default Re: Tach fluctuation

    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •