Removing and replacing the tachometer cable (7ECA with O-235)

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kubark42

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Massachusetts
I'm seeing a teensy amount of ticking/trembling in my tach reading and the folks on the Lycoming forum suggest it might be the tach cable needs to be pulled and relubed. Sounds easy, but the devil can still be in the details. I can't find any mention of it in the service manual I've got, aside from as a part to replace if the tach gets shaky.

  1. Do I have to remove both ends of the tach cable to slide the inner core out?
  2. If I have to remove both ends, is there anything to know about correctly resealing the bulkhead or do I just put an ugly glob of silicone on it like there is right now?
 
Not positive, but I think you just unscrew the cap on the back of the engine and pull the cable out, Careful putting it back in - the square part needs to slip back in to the tach. Then align the key with the engine slot before screwing the cap back on.
 
Yep it's pretty idiot proof. Unscrew the cable housing at the engine, pull the cable out, lube it, stick it back in, twist it a bit to seat in the tach, and screw the housing back on.

You won't have to mess with the firewall. The cable sheath stays in place.

I wouldn't get your hopes up that this will solve your issue though. Mechanical tachs wear out and need to be replaced. A new Mitchell tach with factory screen printed arcs is $250 and will give you rock solid stable readings.
 
While you have the cable out, take a rag (not your bare hand) and run it over the cable in both directions...if the rag catches on anything it means the cable has broken strands and no amount of lube is going to help. The cable would need replacing.
Chris
 
Excellent, thanks everyone! I'd hoped to get to it today, but it'll have to wait until this hurricane goes past. I'll report back on how the experience went, and if there are any meat hooks (aka broken metal strands) in the cable.
 
kubark42 said:
View attachment 4107

Wow, you guys were spot on, it was completely idiot-proof. Under 15 seconds and it was out.
Click to expand...

Looks to be in excellent condition. I strongly doubt that is the cause of your bounce. IMO the damping system in your tach is getting worn. If it doesn't bother you, there is no pressing need to fix it. If it gets to the point where you cannot accurately set power or do a proper runup, it is an easy fix.

Get the part number off your old tach and look it up on Aircraft Spruce. If you have a Stewart Warner, they are no longer produced, but Mitchell makes a drop in replacement. It is reasonably priced as airplane parts go, and can be ordered with hours preset and arcs printed on the face.

I ordered mine last fall. A day later, Mitchell called me and asked if I wanted the ACA factory face. I said heck yeah. I love it, looks great and rock solid stable.

PXL_20210823_003510185.webp
 
Yeah. Too bad you are not experimental - $35 to eBay plus $160 to the speedometer shop would do it. The Mitchell is probably not much more expensive.
 
I think mine was about $250. Money well spent. Labor is about as simple as it gets. 4 screws and the cable. My A&P checked my work and did a log entry

I looked at digital options but they cost about twice as much and involved more work.
 
The bouncing is definitely the engine RPM changing. I took a flight afterwards and played around a bit. Someone at the Lycoming forum now thinks it might be an induction leak. That's the point where I pass it off to an A&P for a quick starter-fluid induction leak test or however they test for that stuff when there's a spinning blade of death inches away.
 
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