Assuming like your name says, that you have a 1989 740GL (and not a Turbo, because Turbos and NA models use different systems in 1989- only!) the timing settings are programmed into the EZK ignition computer.

The base timing settings come from the crank sensor, mounted on top of the bell housing and reading a "tone ring" with little holes around the flywheel.

The reference point or TDC is given to the sensor by a lack of a hole or window- the sensor observes a longer space between pulses once per rotation. After it establishes the location of that window, it then counts holes to generate pulses for the ignition computer to send to the power stage (Bosch) or Regina ignition coil/module.

Of course, once per rev the zero point passes by the sensor, so it can continually reset the base timing if it ever misses a count.

The timing advance curve is built into the ignition unit, and also processes engine vacuum and signals from the knock sensor, screwed to the cylinder head. Make sure that the small vacuum line coming off the very center of the intake is connected- not the one for the flame trap but one that has a small rubber 90° piece then connects to a plastic tube. Inspect the rubber 90 for cracks which would give bad vacuum readings to the computer.

Also check if you can that the connector to the knock sensor is secure- it's under the intake back by cylinder 2 and hard to get at.

And last consider that the outer shell of the harmonic balancer sometimes slips- see if the rubber has separated between the inside hub and the belt grooves. Use a mirror to inspect it or look at it from below.

Good luck with it!
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::: Rob Bareiss, New London CT ::: 92 244 M47 212K ::: 90 745GL M47 275K ::: 90 745T AW71 213K ::: 90 745T Parts Car ::: 88 245DL SOLD! ::: 84 242DL Project ::: 70 VW Bus ::: 70 VW Pickup Project ::: 71 VW Notchback :::