Run away Tachometer

rebelflier · #1 07-06-2014, 02:04 PM

Friday I was returning from my first flight to the Rocky Mountains to my home north of Houston. It was a 5 hour flight, and as I was coming around Fort Worth, I noticed the Tach was reading near red line. Not sure what was going on, I reduced power and it came down. After I figured it was a faulty reading from the tach, I continued home. By the time I decended to land, the Tach was reading 3400 RPM's. It is an old school, mechanical tach with 257 hrs. on it, made by Jet Electronics and Technology, Inc. Thinking that the little coiled tension spring had come loose I took it apart and everything is intact, nothing broken or loose. I put a small drive cable in it, and drove it with my CNC lathe at 1250 RPMs which should read 2500 on tach. It indicated a perfect 2500 RPMs. Now I'm not sure the problem is the Tach. Anyone ever seen this before? Don't see how the tach cable could turn faster if broken or something.
RV-6
O320 E2C
Fixed pitch prop.

PaulT · #2 07-09-2014, 07:33 AM

It's the tacho' for sure. I've seen it many times from a variety of manufacturers.
What you describe is the result of some contact between the rotating magnet assembly, and the drag cup, (which the pointer is attached to).
It is usually due to wear, but can also be from some foreign matter (metallic swarf or some grease/oil) has got to where it shouldn't be.
It can also occur when the magnet assy' shifts a bit on the shaft which can happen on the VDO style tacho's. Generally though, the calibration would alter a bit, which you mention is still okay.
Check the endplay in that main shaft. The bit that sticks out the end that you can spin. Should only be a few thou or so.
Also when you do your lathe speed test, try tilting the tach for and aft. This may help reproduce the fault on the bench.
I think many repair shops these days will suggest replacement, but some of the new stuff is junk. A lot of the old Stewart Warner, and AC tacho's are still out there, some with many thousands of hours that have never been opened up. They made 'em better back then.
Hope this helps.
Paul.

rebelflier · #3 07-09-2014, 08:43 AM

Thanks for the help Paul. Everything felt good, not a lot of slop except for a little end play on the shaft that drives the hour meter gears. I wounder if some grease might have migrated from the tach cable onto the magnet assy. Considering I was at a high altitude for so long, maybe it pulled some in when decending (low pressure to high). I will pull it back apart and check for grease or foreign matter.