Mechanical tachometer diagnosis help

[LarryT]

This has me perplexed. My attempt to diagnose the problem has further confused me.

1. The tach is a Falcon TH-007C presumably purchased from Van's and quit working during flight this past Wednesday. Needle went from engine RPM to 0 and remained there the rest of the flight.

2. I disconnected the cable at both engine and the tach - anticipating either a broken cable or broken guage. I pulled the inner cable out of the cable sheath intact, and it came out smoothly, looked pretty clean and still had a little bit of grease on it.

3. I put a square drive bit in my cordless drill, stuck it into the back of the tach, triggered the drill, first at low RPM, then at higher speed. I got an indication on the tach, although it seemed slow to respond.

4. I stuck the engine end of the inner cable into the engine. I was unable to rotate the cable with what little torque I could apply.

Clearly I don't need to replace the inner or outer cable.

Do I need to replace the tach? Is there some additional diagnosis I can do with the tach, e.g., take it to a speedo shop? This tach only has 181.27 hours registered. If it is failed, it seems unreasonable for it to have failed in such a short time. Should I replace it with a like make and model? - Van's price is $119. Are there other, better alternatives?

How likely is it that the engine output failed? I assume it is some kind of gear set in the accessory case that runs off the gearing that drives the magnetos. Sorry I don't know the correct terminology.

Thanks for any knowledgeable help you can give me. After being AOG for 2.5 years, the 5 hours I have put on the aircraft in April has just whetted my appetite for more flying time.

Larry Tompkins
544WB -6A
W52 Battle Ground, WA

 

[jrs14855]

tach

The engine portion is very substantial. The shaft that the tach cable engages in has a slot for the "key" in the tach cable. The shaft has a cross pin that engages a slot in the camshaft.
If there are bends in the cable/housing assembly it is possible for the cable to disengage at either end.
I prefer the Mitchell tach.

 

[LarryT]

Cable Bends

The engine output is more or less on the engine centerline. Tach is on the left side of the instrument panel. Not a lot of elevation change but the cable necessarily makes a backward "S" bend in the plan view.

I guess I don't understand how, if the inner cable is the correct length, it could come out. Doesn't the engine end with the tang bottom out?

I guess I could reconnect the engine end, shove the inner cable all the way home and see how far it protrudes at the tach end.

Larry

 

[jrs14855]

tach

I believe it is Aircraft Spruce catalog that has a note of caution on bends in cable causing loss of length of inner cable.

 

[LarryT]

Inner Cable shortening

Thanks for the reference. I think? that caution relates to the installation bends reducing the inner cable length rather than shortening in service. The plane has nearly 500 hr.s total. the tach in question has been in the plane for ~ 181 hrs. when it quit working. It makes sense to me that bends would shorten the inner cable length -after all it is a close-wound spring. It does not make sense to me that the overall cable length would shorten in service unless the inner liner of the outer sheath wears significantly.

Just the same, I am going to reconnect at the engine end, measure the protrusion at the tach end and determine whether or not the cable is engaged into the tach.
If I there is engagement, I guess i will call Van's and troubleshoot by part replacement, i.e., a new tach. I also note, FWIW, that Falcon tach warranty is one year and Mitchell warranty is two years. It two Falcon tachs have gone TU in 500 hrs. it doesn't say much for their durability.

Larry

 

[LarryT]

Inner cable is probably too short

Just finished reinstalling the inner cable and attaching to the engine. I made sure the cable was bottomed out at that end.

the tach end was just about even with the cup that fits over the unthreaded post that protrudes beyond the threads. I measured the depth of the square hole as about 1" and the post sticks out past the threads about 1/4", therefore, the inner cable could be 3/4" longer than it is

I think I will go to a speedo shop and have an inner cable made up that is 1/2" longer than current. that way the cable will not have any axial load.

The only thing I can think of is that the cable length was barely adequate originally, but sheath liner wear resulted in just enough length reduction in service that it quit driving the tach.

Thank you so much for your help. This is not something that would have occurred to me to check. When I get the new cable installed, i will test and let you know how it turns out.

Larry

 

[DanBaier]

...that the cable length was barely adequate originally, but sheath liner wear resulted in just enough length reduction in service that it quit driving the tach....

Had exactly that happen once in a Cherokee with the installation of a new tach instrument. Worked for a bit - then not. A new, longer, drive cable did the trick.

Dan

 

[LarryT]

Follow up

I had a new inner cable made up that was 3/4" longer. That appears to have solved the issue. I made certain the new cable was bottomed out at the engine end and had a sufficient amount protruding from the tach end. The previous cable was apparently "floating" and gradually worked itself toward the engine end and out of contact with the tachometer end.

Larry Tompkins
544WB -6A
W52 Battle Ground, WA