Tach cable routing

[Ed Haywood]

Before I dropped my Decathlon off for avionics work about 3 weeks ago, the mechanical tach had a bit of bouncing, maybe 50 RPM at run-up speed. When I got it back, the tach was much worse. It sticks at 1400, then jumps past 1800, and won't stabilize under 2000. I cannot do an accurate mag drop check or cycle the prop 300 RPM on runup.

It appears the tach cable is too long. The logs show that 30 years ago the tach was relocated from the left side of the panel (where the MP gauge is now) to the current location near the center of the panel, a distance of approx 8". The firewall thru-fitting is directly forward of the new position, so the cable length required to reach to the side of the panel is not needed. Now there is slack in the cable, a semi-circle maybe 6 inches in radius.

The avionics shop removed my Turn Coord gyro and replaced it with an electronic AI. The TC body was in the space between the tach and the firewall thru-fitting. My theory is that the tach cable was held steady by contact with the TC. Now that the TC is gone, the cable has repositioned in such a way that negatively affects function.

Should the tach cable be secured at various points? If the bend is free to move laterally, might that cause sticking?

 

[Dan Thomas]

The cable needs to be rerouted to reduce the radii of bends. And if it's that old it should just be replaced with one that's shorter anyway. The lube in the housing gets gummy and the cables starts sticking and the tach bounces around.

Get someone to check the tach accuracy with a tach checker. If it's off much it should be replaced, too, and I would instead install an STC'd electronic tach and be done with it. No cables to stick or reroute.

 

[Ed Haywood]

Rerouting the cable resolved the issue sufficiently for now. The bend in the cable was pressing on the underside of the glare shield in a way that stressed it. Flipping the bend to the other side allowed it to run from the firewall to the gauge without making contact with anything else. I had previously cleaned and lubed it.

Tach is still a hair jumpy and sticky at certain spots, but not at run-up or cruise rpm. That takes away the need for an immediate fix, so I can take my time to look at options and schedule the install when I'm having other work done.