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Cable or tach?

acam37

acam37

Well Known Member
I have an issue with my mechanical tachometer. At full power (constant speed) it will only spool up to 2400, then after about a minute the needle will jump to 2600rpm with no noticeable pitch change from the prop. We verified the prop is turning 2670rpm static with a hand held rpm meter. So my question is, can the cable be the culprit or is the tachometer going bad?
 
Could be a cable problem as well. I replaced and repositioned my tach couple years ago which required a different cable length. I ordered a custom made cable and it was a bit longer. I routed it with a steeper bend. It caused the cable to disingage and slip. Problem was solved with correct length.
 
I am pretty sure it's the tach and not a cable but wanted to get a second opinion and see if anyone else had this problem and wound up being a cable. I've got a WTB in the classifieds but if no response in the next few days I will go ahead and buy one. At first I thought we had a more serious problem but after re-adjusting the fine pitch stops on the prop, nothing changed on the tach so we put it back where it was. There is a 200rpm drop across the board compared to the rpm meter.
 
I had a similar situation in the tach of my Cessna 120. It would jump up and then back to the actual reading. Gradually the jumps became more significant. It turned out there is an oil seal, I think on the engine side of the cable, that permitted oil to enter the cable and into the tach itself. He replaced the seal, cleaned the oil out of the "broken" tach, and it has been perfect since. So in my case, we did not end up replacing the tach, we replaced the oil seal and cleaned the tach.

Jerry
 
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