SNIP

I got another problem. For some strange reason I think my tach is off.
Any of you EE types have any suggestions for making a cheap tach
that will work with mags? Preferably digital.

My local aircraft instrument shop does not want to work on the Cessna
mechanical tach because it is sealed. Apparently Cessna buys a cheap diesel

truck

tach from Stewart Warner and has it labeled "aircraft".

Paul Lamar

Paul,

Buy digital tach from Whitney for about $40.  It has an inductive
pickup. I peeled back a bit of the shielding, clamped the pickup on a
plug wire and then wrapped the whole thing in foil wrapped with a
few wraps of safety wire to keep the shielding intact.  Tie-wrap
the "lump" to a solid point on the engine (pushrod tubes on my
Lyc) and you're good.  I'll look up the part number if you
want.  Very nice for leaning as it works in 10 RPM increments,
lean until you get a 20 RPM drop, below 75% power on
my O-235 C2C.  This is in addition to my regular analog
electrical tach.  A bit flakey at idle, but very stable in cruise.
Probably a Federal crime to do on a certified aircraft, tho.

Might be able to get a 337 one-time field approval as a
secondary tach, as long as the piece of crap mech tach is
still the primary tach, since it is non-invasive on the mag wire.
Probably need to show that it doesn't cause radio noise,
and can't cause the ign to fail, then follow proper wiring
practices.  If your local FSDO isn't insane, but some are,
some aren't.

No way would I put any regular tach on the low voltage
side of a mag - too much chance to kill the mag if the tach
fails.  My VERY expensive (and only moderately accurate)
B&D (IIRC) tach uses a 1 megohm resistor to "isolate" the tach,
yet it can still read the pulses on the mag switch wire.  Theoretically,
even a dead short in the mag won't flow enough current thru 1
million ohms to actually affect the mag.  Seems reasonable.  This is
an aircraft approved tach, I think used on some Mooneys.
Cost nearly $400 15 yrs ago, probably hasn't gotten cheaper.
2 1/4" size was mandatory in LE panel, plus too long for a
mech tach cable.

Digital reads 50-60 RPM higher than the analog at cruise.  This
is about 2/3 of a needle width on the small analog tach.

Bill

Thanks Bill.
I notice AC Spruce has an optical one on page 328... 2000 cat that 
looks at the prop through the widshield for $79.95. Called
a Tru-Tach. No hookup required. PN 10-24745 Every FBO should
have one of these.

Paul Lamar
 
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