Oilheads Have Two Hall Sensors--Why?

I'm debugging a problem at the moment and realized I've got no idea why there are two hall sensors on the crankshaft. The plugs fire once per revolution, the spark fires once per revolution but there are two sensors on the plate. Does anyone know why? ...

Dana Hagar's page on rebuilding the HES plate http://advwisdom.hogranch.com/Wisdom/oilhead_hall_sensors.pdf shows on the second page of text that one sensor triggers at TDC and the other at 180˚ of crankshaft rotation. As to why the Motronic was apparently designed to need both, I don't know.

Read through it. Thanks. Still searching for a description of the reason for two.

Popular conceptions are that
1) on some bikes, having a waste spark (at the top of the exhaust stroke) may reduce emissions
2) having both plugs fire simultaneously might reduce the complexity of the ignition system timing & wiring.

With today's electronics, both of the above become somewhat moot. It also consumes more electrical energy, and creates more heat. Personally, I prefer the sound of an engine that doesn't have the extra firing.

On the Oilheads since both injectors and both plugs fire together, you'd think one hall sensor would be enough. I'm hoping to learn why they included two.

I can see that one sensor is indicating TDC TDC TDC
While the other is indicating BDC BDC BDC

You'd think that one or the other would be enough. Some systems only have one.

I've never actually tested it, but I have seen indications that one is used by the injection control and one by the ignition control. You could test this pretty easily. Thinking about it, it's actually more likely that the ignition uses both (for better prediction of crank position on an engine with big crank speed variation) and the injection uses one (could be a simple trigger).

Some people should keep in mind that this system is not like points; ignition does not happen simply because one gate closes or opens. The sensors simply tell the Motronic where the crank is, and the Motronic figures out the crank speed and then decides when to spark. On the K75, there are only two sensors (120? apart) for three cylinders. So, if the Motronic wants to throw in another spark or three somewhere, it doesn't need another sensor to do it.

AntonLargiader said:

I've never actually tested it, but I have seen indications that one is used by the injection control and one by the ignition control. You could test this pretty easily. Thinking about it, it's actually more likely that the ignition uses both (for better prediction of crank position on an engine with big crank speed variation) and the injection uses one (could be a simple trigger).

Some people should keep in mind that this system is not like points; ignition does not happen simply because one gate closes or opens. The sensors simply tell the Motronic where the crank is, and the Motronic figures out the crank speed and then decides when to spark. On the K75, there are only two sensors (120? apart) for three cylinders. So, if the Motronic wants to throw in another spark or three somewhere, it doesn't need another sensor to do it.

Click to expand...

Thank you Anton, it seems like your explanation is on the mark, I appreciate it. I will plan to measure which sensor controls what when I get some time.

As you said, calculating when to fire the plugs next requires prediction including calculating the rate of acceleration or deceleration of the crankshaft. By having two sensors, the Motronic gets twice as many datapoints and knows both TDC and BDC which must be useful during cranking and low RPMs.

I've inadvertently learned that the Motronic can tell the difference between a smooth start and one with mis-firing. It holds the spark advance at zero degrees until both cylinders are firing reliably and then increases the spark advance to about 6 degrees. I believe the only sensor that could detect the uneven running of misfiring is the HES, since the O2 sensor is offline until the bike warms up. Here's a link to how I blundered into that knowledge: R1150RT Slow Start.