a long overdue edit and an additional post at the bottom.
Points and condenser ignition has a rising edge or rising voltage at terminal one on the coil (green wire) as the points open, the moment of ignition.
DigiFant and CIS-E with knock sensor also have a rising edge on the green/white Hall generator signal wire.
CIS lambda has a falling edge on the green/white Hall generator signal wire.
Suggest reading the entire thread for full explanations.
Sorry, I'm about a quarter century late coming up with this....
1st, some VW standard tuning history and some basic ignition operational principles.
Way back in the air cooled days, for some models, as recent as the early 1970s, VW mechanics (we weren't known as "techs" yet, that euphonism came much later) set ignition timing with a test light, strobes were around, they just weren't yet necessary.
Ignition coils, then and now, are transformers. There are two sets of windings, the low voltage (nominally 12 VDC) primary and the high voltage (10k VAC to 50k VAC) secondary.
On most all cars the primary windings have power available at all times when the key is on (terminal 15 on Euro cars).
When the ground side is pulled to ground, current flows through the primary and a magnetic field builds up in the primary and the secondary windings.
When the ground circuit is opened, current stops flowing, the magnetic field collapses over both sets of windings. This is the big moment when the high voltage secondary voltage is produced and sent to the plug, this is the ignition event, this is the moment we want synchronized with crankshaft position for correct ignition timing.
Back to the '50s, '60s, '70s, test lights and points.
Since the spark moment was when the points opened, a mechanic could turn the crankshaft by hand up to the timing marks with a test light on coil terminal one, as the points opened, the light would light. The distributor was rotated to synchronize the lighting of the light with the crankshaft timing marks.
This method worked OK enough to time millions of cars for decades. Results compare to strobe light timing within about 2°.
Points were the complete coil switching system. With our electronic ignition, points functionality is handled (primarily) by two components, the Hall generator for a low current signal, the Hall control unit to switch the high current of the primary windings. Dwell was a fixed compromise with points, dwell is completely variable according to engine RPM with our Hall system.
On to our (slightly) more modern electronic ignition systems. Coils are still transformers, terminal 15 is still coil power, terminal 1 is still the ground side.
But we can't use a test light on terminal 1 anymore, at hand cranked engine speeds the power stage (Hall control unit) releases the ground to prevent key on battery draw and coil overheating.
What we can use is the Hall generator signal straight off the distributor, if and once we understand what it does.
The Hall signal is an on/off signal much like points.
The shutter wheel inside the distributor has four apertures (windows). When the aperture is open, allowing allowing both sides of the Hall pickup to "see" each other, the Hall signal voltage on the green/white wire is at zero. As the Hall "window" closes the Hall pickup path, the Hall signal voltage on the green/white wire goes high to approximately 10 VDC. This spike is the signal the Hall control uses to release the coil ground at terminal 1.
This is THE moment we're looking for!! We can use it just like was done in the days of points and test lights, we do want a DVOM or LED tester (not bulb type) though.
Remove the distributor cap, we don't want this engine to inadvertently start, we also want to see when the rotor points to the #1 notch.
Prepare to turn the engine by hand; a socket on the crank pulley bolt; well adjusted v-belts and a screwdriver in the alternator fan blades or a wrench on the alternator fan bolt, my preference is 5th gear with the park brake off and roll the car.
Connect a DVOM or LED (not a bulb type) test light to the green/white signal wire from the Hall generator, scale should read to at least 11 volts, we should see 10, 10.5.
Turn the engine clockwise, same as normal running rotation, until the rotor approaches the #1 notch, now, slowly creep up to the notch watching the voltage indicator closely. As soon as voltage suddenly goes from zero to 10 VDC, STOP! Check the timing marks, if you are at or very near the 6° BTDC mark, you're done.
If things don't line up just yet, bring the crank timing marks together, ALWAYS turn the crank in the running direction as you check, if you overshoot, overshoot backwards an 1/8th turn or so, then creep back clockwise.
Crank marks lined up now? Good! Loosen the distributor base clamp and rotate the distributor body watching the voltage indicator for the voltage spike to 10 VDC, tighten the clamp. Visually, viewed clockwise from the top, the Hall window should have just closed. If you get the opposite slope, the car is guaranteed to run awful, if at all.
Repeat backing the crank up 1/8th turn then creeping back to the timing marks until the voltage spike is as perfectly synchronized to the timing marks as possible.
I just tested this procedure on my DigiFant I car, marks were very close to where they were set with a timing light at 2,000 RPM, blue CTS disconnected.
CIS engines should take to this just as well, early, double vacuum advance chamber distributors require an additional step.
The double vacuum advance chamber distributors require manifold vacuum applied to the rear, flat chamber when strobe timing. This static method also requires that vacuum chamber operating. A MityVac or similar, if handy is the most obvious choice. Here's an untested alternative, off to AutoZone for a length of vacuum hose (sorry, you're Cabby DIYers, of course you have some on hand already), you will need a check valve for this anyway.
Long hose to engine side of check valve, short hose to other side of check valve, from there to the back chamber. Suck on the long hose, vacuum diaphragm should operate, check valve should hold vacuum. Re-check frequently that vacuum is holding as you time. You want to use the -3° timing mark (3°ATDC).
It is more accurate and far simpler to time with a strobe light, strobe is preferred. This static method gets close enough to get an engine running and very much drivable, for some DIYers it will be good enough as is for many thousands of miles.
Slightly off topic, this whole write up sprang from another Hall test I finally got around to today. You DIY CIS owners take note. Should you find questionable dwell meter operation while setting frequency valve duty cycle, connect your dwell test lead the to Hall generator green/white wire and test your tester. Mine read a very steady 36-37° on the 4 cylinder scale(I think it meant 36.5°), 40.5-40.6% duty cycle, yours should too.
Modified by tolusina at 9:01 PM 12-19-2008