Erratic Tachometer

DensityDog · Post by **DensityDog** » Mon Jan 12, 2004 2:40 am

Mike, could you let us know how this turned out? Have you fixed it by now?
Sounds a lot like a cable problem, but with the numbers moving and no RPM indication sounds like it could only be inside the tach.
Max

Mike Smith · Post by **Mike Smith** » Wed Jan 14, 2004 2:29 am

Max,
This took a back burner when I got busy getting into a hangar and starting the annual ... and life in general. I've picked it back up and discovered the cables exterior near the engine end of it was broken open and had oil in it. I disconnected it from the engine and found oil in the connection area. I then looked under the panel and oil was all around the connection point to the Tachometer. I disconnected it and sure enough the oil had climbed up into the rear of the instrument.

So, a new cable is a must. The tach drive seal (if it's seperate) will need to be replaced. And I'm taking the Tachometer to the local guys (The Gyro House in Auburn, Ca) to see what the diagnosis on a rebuild/overhaul/replacement might be.

I'll update ya on the outcome.

Post by **GAHorn** » Wed Jan 14, 2004 9:07 pm

Mike Smith wrote:Max,
This took a back burner when I got busy getting into a hangar and starting the annual ... and life in general. I've picked it back up and discovered the cables exterior near the engine end of it was broken open and had oil in it. I disconnected it from the engine and found oil in the connection area. I then looked under the panel and oil was all around the connection point to the Tachometer. I disconnected it and sure enough the oil had climbed up into the rear of the instrument.

So, a new cable is a must. The tach drive seal (if it's seperate) will need to be replaced. And I'm taking the Tachometer to the local guys (The Gyro House in Auburn, Ca) to see what the diagnosis on a rebuild/overhaul/replacement might be.

I'll update ya on the outcome.

The tach drive seal is contained within the accessory -case tach drive housing and resembles a garlock seal. Remember to install it's "open" end towards the engine. If you remove the tach housing to replace the inner seal, be careful to check for damage to the gen/tach-housing gasket (PN 652072). If it is damaged, do not attempt to cut it, modify it, or re-install only a partial amount of it.
From a previous post I made on this subject about a year ago:
Make certain you replace the entire generator/tach housing gasket (PN 652072) as a single piece unit. Do NOT cut it or alter it. Make certain you also replace the tachometer drive housing-shaft seal (PN 642714) just above the gen mounting. If you don't you'll likely suffer oil leaks.
Install the new gasket with Permatex gasket sealer (very light film on both sides of gasket.)

zero.one.victor · Post by **zero.one.victor** » Thu Jan 15, 2004 1:19 am

George,where did you come up with the generator/tach drive gasket part number (652072)? My Continental IPC shows p/n 36100 for that gasket.

Eric

Post by **GAHorn** » Thu Jan 15, 2004 5:25 pm

Your part number is obsolete. The correct PN is 652072.

Mike Smith · Post by **Mike Smith** » Sun Jan 18, 2004 8:24 pm

Thanks George, I'll be ordering those parts on Tuesday. Aircraft spruce is also sending me a cataloge, but I wasn't sure about the PN for the tach cable and cable housing. I ordered them (with the help of the order taker on the phone) but do you have those PN's so I can make sure I receive the right part?

Also, I can't find a part number for the O-rings/gaskets for the glass gascolater. I've been told by Sacramento and Aircraft Spruce that they don't have these ... do I need to call Cessna?

Dave Clark · Post by **Dave Clark** » Mon Jan 19, 2004 3:13 pm

Mike

Look at Sacramento for the 33-199-6 gasket. I think it's the one you want. They have 1 only as of 1-16-03. Good luck!

Mike Smith · Post by **Mike Smith** » Mon Jan 19, 2004 11:53 pm

George,
I just recieved my Aircraft Spruce cataloge and cannot find the Tach Housing Gasket. I called Aircraft Spruce today and tried to order it (PN 652072) and the lady on the phone could not find it in their computer. I was wondering what part of the cataloge I could look this up and find the "new" part number (if they've changed it).

Thanks

Dave Clark · Post by **Dave Clark** » Tue Jan 20, 2004 12:35 am

Mike

I was referring to the gascolator gasket.

Post by **GAHorn** » Tue Jan 20, 2004 9:30 pm

PN 652072 is a Continental (TCM) part number. Spruce does not carry all TCM parts. Try Sacramento Sky Ranch 916 421-7672 http://www.sacskyranch.com/ not only for this, but also your gascolator gasket. (Note: there were several gascolator substitutions made over the years. Remove your gasket and measure it's ID, OD, and thickness for confirmation when you order new gaskets. NAPA also handles gascolator gaskets, as the same parts were used on many tractors. Their PN's 730-9506 or 730-9510 are most likely the correct ones.)

Mike Smith · Post by **Mike Smith** » Tue Mar 09, 2004 5:46 pm

UPDATE on the tach stuff:

I finally got the cable installed and it didn't fix the tach problem. So, I secured a "new" (to me) tach and it works wonderfully. I didn't replace the Tach/generater drive seal and gasket due to the hassle factor. When I checked the bolts for the fitting where the tach cable goes into the engine, I discovered they were not very tight. I tightened them and reattached the new tach cable. I have since disconneted the cable at the engine end a couple of times to see if there is any oil making it's way up the cable. None is, so I figure that I didn't need to replace the tach drive seal and gasket. All now appears to be well and right.

Post by **GAHorn** » Tue Mar 09, 2004 10:40 pm

BTW, here's a "helpful" if not exactly FAA approved modification I made to my tach-drive cable. (I dare 'em to find fault with this.)
At the engine end, pull the inner-cable out of the housing and lay it on clean newspaper. (Keep it clean!)
Using a 1/8" drill, drill a hole in the 6 o'clock position of the "bell-housing" that the knurled nut holds against the tach drive. This hole will serve as an oil-drain should the tach-drive seal ever leak, and allow the oil to go out the cowl instead of up the cable and into the tach.
Re-install the inner-cable and install the complete cable in the airplane. Be certain the drain hold is pointed downward.

Dave Clark · Post by **Dave Clark** » Wed Mar 10, 2004 1:21 am

Great tip George!

Joe Moilanen · Post by **Joe Moilanen** » Wed Mar 10, 2004 4:41 am

Just curious George, Why couldn't you separate the gen/tach gasket in half to prelude removing the uneffected. Seems the gasket was made in one piece for simplicity in manufacture but must be a reason that I'm overlooking. Guess I just cannot see the reasoning.

Joe

zero.one.victor · Post by **zero.one.victor** » Wed Mar 10, 2004 4:15 pm

Mike Smith wrote:UPDATE on the tach stuff:

I finally got the cable installed and it didn't fix the tach problem. So, I secured a "new" (to me) tach and it works wonderfully............

Some previous owner(s) replaced the tach & airspeed indicator in my ragwing with aftermarket units,instead of overhauling the original items. Sure wish they hadn't. Even if it was off a few hours from flight with a loaner,I'd rather have the original tach with the (almost) true hours recorded. Also rather have the stock tach & ASI for the factory color-coded limitation markings,and for that cool "original" look. My panel isn't strictly original,so it isn't that big a deal,but it is something I regret had been done. Maybe the original factor ain't no big deal to you,but it might be to the next "guardian" of your 170. FWIW
And before anyone gets on me about any other non-original items--gear legs,Cleveland brakes,radio's,paint schemes,etc-- IMHO an aftermarket tach is not an upgrade so why not just leave it original? Also,I would imagine that it's cheaper to overhaul the original tach than to buy a new (approved) tach.

Eric