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.

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.

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.

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,
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.

Quote:

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Originally Posted by 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.

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Do you remember the exact tach that you used?