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tachometer anomaly

J

JRo

Well Known Member
My RV-12, while flying, has an intermittent high rpm indication for just a fraction of a second, then back to normal. The engine itself is humming along beautifully with no audible pitch change. I feel certain it is just an indication problem. Trouble is, though, when the tach reads above 5500 rpm (yellow line) even for a second, it triggers both the visual and aural warning(s). Very disconcerting.

Has anyone seen this or have any thoughts. My display is the 10" SkyView. Would this be an issue for Vans, Rotax, or Dynon?

Thanks,
Jim
 
My TACH wire had the original Van's 30K ohm resistor installed. New harnesses have a 100K in place of the 30K. I replaced the original 30K with a 100K per guidance from Vans and my TACH now works great!

Look at the electrical schematic for your version of the plans versus the most recent schematic to locate the TACH resistor that needs to be changed.
 
Where is the tachometer resistor located? Is the 100K ohm resistor a common Radio Shack item, or a specialty item I need to track down?

Thanks,
Jim
 
Radio Shack should have it

Jim -

My nearby Radio Shack store still has some small electronic components in pull out drawers at the rear of the store. 100K ohms is a very common value so you should have no problem finding it at Radio Shack or any other establishment that sells electronic component parts.

If you need to go mail order, Mouser would have for sure.
 
You can look at the electrical schematic drawings and find the resistor on the Tach wire. As I recall, it was on one of the wires near one of the engine plugs that goes into the AV-50000A box aft of the firewall.

Let me check my plans and get back to you later tonight re: the exact location of my 100K resistor.

BTW, I bought my 100K resistor at Radio Shack.
 
OK, here is what I did to fix my jumpy tach:

Look at the applicable electrical schematic for your RV-12 version (My schematic is dated 3-27-12). On Wire #F776 (white/green) going to Pin #34 in 37-pin Plug #WH-00020 you should see a 30k resistor shown. It is spliced into the F776 wire near the forward portion of the harness that goes into the firewall-mounted EMS box. Carefully remove this 30K and replace it with a 100K (I used a 100K Radio Shack resistor). The newer harnesses apparently now use a 100K instead of the original 30K.

YMMV - Good Luck!

-- David (ex-RCAP CFI / RPI)
 
Thanks Dave!! This will get me on the right track. I will review my schematic, visit my local Radio Shack, and go from there.
 
I picked up a packet of 100k ohm resisters at Radio Shack so I now have 5, 4 too many. If someone wants or needs one send me your address at my personal email below and I will send you my extra.
 
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