REFLECTOR: Checking Magneto Timing with a Digital Volt/Ohm Meter

[Ronnie Brown]

Several days ago, someone said you couldn't check mag timing with a good
voltmeter/ohm meter.

Actually you can:

I have a $50 Radioshack meter similar to
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F008%5F002%5F000&product%5Fid=22%2D812

If you have a digital ohm meter that will display ohms in 0.1 ohm
increments, it should work too.

MAKE SURE THERE IS NO FUEL IN THE FUEL/CARB.  THE REALLY SAFE WAY TO DO THIS
IS TO REMOVE ONE PLUG FROM EACH CYLINDER.

CAUTION - DO NOT HAVE THE OHM METER CONNECTED WHILE THE IMPULSE COUPLING
SNAPS - GOOD CHANCE YOU WILL DAMAGE YOUR OHM METER DUE THE HIGH ENERGY AT
THE P LEAD WHEN THE MAG FIRES.

1.  Turn the engine in the normal direction until the impulse coupling
snaps - #1 firing position - can be verified by holding your finger over the
#1 spark plug hole and turn until you get pressure.

2. Turn the prop back wards about 45 degrees.

3. Turn the ignition switch on.  Connect one lead of the ohm meter to the P
lead for the mag.  (I actually connect to the back of my mag/starter
switch - easier to get to in my plane).

4.  Connect the other ohm meter lead the engine case.

5.  My meter shows 0.7 ohms with the points closed.

6. Now, very slowly move the prop in the counter clockwise direction until
the resistance changes to 0.4- 0.5 ohms.  You will need to let the ohm meter
settle for a couple of seconds after each movement - the mag will supply
some voltage when it is moving.

7.  The timing marks are in two locations.  On the front of the flywheel
facing the engine case split.  Mine has markings at 0 TDC, 22 BTDC, and 25
BTDC (before top dead center).

8.  The other mark is on the starter ring - and there is a dimple on the
upper starter flange that is the zero point.  The dimple on the starter ring
is covered up by the prop extension cover - but I made a mark out on the
ring gear it self.

9.  TURN THE IGNITION BACK OFF, REMOVE THE DVM, PRIOR TO TURNING THE ENGINE
PAST THE FIRING POINT!!!

I have verified this works - two A&P's and their two buzz boxes were used to
verify my indications were correct.  Of course, if you have access to the
buzz boxes, by all means use it instead.  But if you have a good DVM, you
can use it instead.  WORKS!!!

Ronnie Brown