The infamous distributor shutter wheel. This bent piece of tin along with the Hall effect sensor in the distributor sends the timing signals to the ECM so it can compute when to fire the upcoming plug. It also has one of its paddles narrower than all of the rest. By comparing the duty cycle of each sequence of paddle/gap the ECM determines (or at least is supposed to - see discussion on the Injector Sync Problems) which injector pair to fire off next. This little piece of tin is also at the heart of the Ignition Jitter and Spark Scatter problems that this systems has. There are many sources of errors when you try to use the distributor as the device to computed when the next firing event will occur in an electronic control system. CAM gear or chain noise, possible CAM twist (?), distributor gear to CAM gear interface, vertical distributor shaft movement (helical drive gears cause rotational errors), bearing clearances (radial wobble) in the distributor, shutter paddle's not being in perfect radial alignment, paddle leading edges not being exactly 90 crank degrees apart, plus I'm sure more that I haven't thought of.
Well now that I have replaced this with a Crank Trigger
I can measure it more accurately outside of its natural environment.
The below table is what I came up with. The measurements are tangent
distances so there is a slight error but should be fairly close to the
actual circumference distances. If nothing else they can be used
for comparisons. I tried to be consistent in the way I made each
measurement. Poor construction made it difficult to be too precise.
Things like the paddles NOT
being square to the rotational plane or perfectly rectangular (more spark
scatter !) didn't help any !
| Paddle | Paddle Width | Gap Width | Edge-to-Edge | Degree Scatter | Scatter @2500 |
|---|---|---|---|---|---|
| 1 | .3270" | .7565" | 1.0690" | +.10 | 751 - 2,497 |
| 8 | .5025" | .5895" | 1.0715" | +.31 | 753 - 2,490 |
| 4 | .4990" | .5960" | 1.0755" | +.65 | 756 - 2,480 |
| 3 | .5005" | .5900" | 1.0695" | +.14 | 751 - 2,497 |
| 6 | .5020" | .5905" | 1.0785" | +.90 | 758 - 2,474 |
| 5 | .5035" | .5840" | 1.0590" | -.74 | 744 - 2,520 |
| 7 | .4995" | .5865" | 1.0605" | -.62 | 745 - 2,517 |
| 2 | .4995" | .5830" | 1.0590" | -.74 | 744 - 2,520 |
| Degree Scatter: | Took the average of the leading Edge-to-Edge values and computed crank degree errors. |
|---|---|
| Scatter @ 2500: | Assumed 2500 RPM, which would give us 750 timer counts (8us tics), and computed against the average Edge-to-Edge values how much error in ECM timer counts and RPM would be contributed by the shutter wheel alone. |
Well it looks like the poor construction of the wheel itself may be contributing to 30% to 50% of the total errors I have been seeing. There is a peak-to-peak RPM error of 46 RPM here alone. I have no way of knowing the contribution of the shutter paddles not being square to the rotational plane, not being perfectly rectangular, or radial placement errors affect when the Hall Effect will sense a change of state. Measuring the diameter across each paddle pair there is .015" worth of variation there alone. Certainly varying the distance between the magnet and the sensor must have some affect on the timing.