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mike_beresford(at)yahoo.c
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 Posted: Fri Mar 20, 2009 8:49 am Post subject: Questions on Tachometer Drive |
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Hi all
Over the years I have had a few problems with the tachoeter in my Yak-52, which usually seemed to be resolved by cleaig contacts. Recently the tachometer failed, and no amount of contact cleaning helped. I suspected a loose wire, so I checked the cannon plug at the generator side. When I opened it one of the wires was loose, so I resoldered it and hecked the wiring. During this it became obvious that I was not the first person to do this - there is a join in the wiring in the engine bay, as though this section has previously been replaced.
I still cannot get the tachometer to work. I did a bit of work with my multimeter this morning, and found:
- with both the generator and instrument ends disconnected, I get continuity between pin 1 of the generator with pin 1 of the instrument, likewise pin 2 to pin 2, pin 3 to pin 3, earth to earth. No continuity between pin 1 generator and any other pin, except instrument pin 1. Likewise for pins two and three. Pin 4 on the instrument does not appear to be connected to anything. From this I believe that the wiring is correct and intact.
- When running the engine, I measured the AC voltages on the connector at the tachometer in the front cockpit. Pin 1 to earth had a small voltage, pin 3 to earth had a small voltage. No voltage on pin 2 to earth.
| Quote: | From these results, it seems to me that the fault lies with the tachometer drive and not with the wiring or tachometer instrument. It seems to have lost a phase. Before I start removing the drive unit, are there any other tests I can do to confirm my diagnosis?
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ALso, is my assumptinon that each pin on the generator connects to the same number pin on the instrument valid? It seems logical to me, but there may be some reason why the order may differ?
Any advice would be appreciated.
As a matter of interest, at least one previous owner of this aircraft had hassles with tachometer readings.
Thanks
Blue skies
Mike Beresford
South Africa
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mark.bitterlich(at)navy.m
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| Posted: Fri Mar 20, 2009 11:48 am Post subject: Questions on Tachometer Drive |
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Mike, you have done good tests. It has been my experience that the Russian
wire and their solder is not up to the same quality as one might hope. Here
is a list of things:
1. Replace the wire that actually solders to the tach generator cannon plug
with aircraft grade multi-strand Teflon covered wire. Then SPLICE it is
with a environmental crimp splice to the original Russian wiring about a
foot back. MAKE DARN SURE YOU MAINTAIN TOTAL SHIELDING OF THESE WIRES AND
THAT THE SHIELD IS WELL CONNECTED TO THE CANNON PLUG ITSELF. Loss of
shielding to the tach generator will result in the most horrific noise you
can imagine to the radio and can actually result in burning out the receiver
itself. THAT HAPPENED right here at my airport!
2. Any repairs or previous splices should also be shielded.
Onto your particular problem. Remove (UNSCREW) the Tach Generator from the
engine. It is EASY to do. Remove the cannon plug and simply look for the
bright steel colored ring that has finger holds to unscrew it. Be sure you
know what you are doing here and DO NOT disassemble the accessory plate
itself! Simply unscrew the ring and remove the tach generator. Ok, now go
over to your bench and get a normal variable speed electric drill. Connect
the tachometer generator shaft into the chuck of the electric drill. Have
someone else hold this contraption for you. Now start up the electric drill
and measure from each pin to case ground and between each TWO pins... Such
as 1-2 1-3 2-3 At any given RPM of the drill, you should see equal pin to
pin voltage, and all pin to ground voltages should be close to equal. It's
an easy test, and very effective in finding out if your tach gen is bad,
which from what you describe most likely is.
3. If you lose any single wire, or have the wiring backwards from the tach
gen to the tachometer movement itself in the cockpit, the thing will not
just lay there dead. It will move BACKWARDS, or it will move correctly but
be "way off" from the proper normal pointer indication for the given RPM.
Since you have checked the wiring properly, I would first check the tach gen
just like I mentioned above... If it actually checks GOOD, then try the
airplane with one tach indicator DISCONNECTED and the other one working and
then reverse. It is possible for a bad tach indicator in the cockpit to
load down the voltage making both read zero, but that is extremely rare.
Most likely... You have a bad tach gen, but the drill test will verify it in
about 30 minutes total time.
Good Luck,
Mark Bitterlich
N50YK
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