The International Cessna 170 Association

Went to fly the ragwing pre-annual last week and the tach went nuts on startup.
I have a video which I can try to link, but basically the needle was all over the place, from zero to redline and beyond and just completely irregular.

Any tips for troubleshooting this? Is it more likely the instrument or cable?

As it turns out I was mistaken on the annual, we've been doing them in June for a while but last year signed it off in July so I have another month and would like to find a way to get airborne before she goes down for inspection.

Sounds like your tach cable is hanging up in the housing, winding up, and then letting go, making the needle jerk. The cable may be worn out or just needing a good clean and lube. I would check the cable first.

I wondered if that might be it.

Any tips/tricks to lubing the cable? What lube is preferred?

Thanks,

Remove the cable from the plane and if possible, pull the retainer clip, and pull the inner wire-wound drive cable out. Clean it up and check for any broken or worn-thru wire. If you find something wrong, buy a new cable, but if you find nothing there, then lube and reinstall and check the operation. My aviation guru friend says Lucas Red-N-Tacky is the best grease for the tach cable. Sold at auto supply stores.

Chances are the inner cable can't be or will be very difficult to remove. In that case, flush out the cable with lots of mineral spirits or penetrating oil, something like PB Blaster or Corrosion X. Then re-lube using a tube of Red-N-Tacky grease in a pump grease gun with a short piece of vinyl hose that will slide over the end fitting of the cable and grease gun head and can be clamped-off. Then pump until grease comes out the other end. Re-install and see what happens. Chances are a new cable will probably be needed.

Keystone Instruments of Lock Haven PA has recommended plain old petroleum jelly such as Vaseline brand and that is what they used for tack cables when I asked a few years ago.

I had a jumpy tach, even after sending it in for an overhaul. Tried all the lube tricks, and then ordered a new cable from Aircraft Spruce. That didn't solve it. The plastic cased cables had too much binding from the curve needed in routing.

Univair sells a metal cable, similar to the original. I ordered this, and it worked perfect. The needle is rock solid.

U1713-R325 UNIVAIR RECORDING TACHOMETER CABLE