Tip Over Sensor

ECU Inputs: Tip-Over Sensor

The Aprilia ECU has a sensor that determines if you have crashed or not. Basically, the switch uses gravity to close a switch if the bike is not in an upright position for a certain amount of time. If the ECU decides that you have crashed, it turns the engine off for you so that you don't have to go hunting for the kill switch as you slide down the road.

Circuit Notes

The Aprilia tip-over sensor circuitry looks like this:

The sensor is basically a switch, which makes a certain amount of sense since your bike is either tipped over, or it is not. The obvious question then becomes, why do they feed the output of a switch into an analog input where the processor has to do an analog to digital conversion just to read it?

We have two clues. The first clue is that the service manual states that the fall sensor should have a resistance of 62K Ohms. This is how we know that there must be a 62K resistor inside it. The other clue is provided by R772. Notice that it has the same resistance value as the resistor inside the tip-over sensor. This means that when the switch is open (bike is upright), R772 and RFS (the resistor inside the fall sensor) form a voltage divider. Since the resistor values are the same, the MUX_X3 analog input pin will see Vcc/2 volts or about 2.5 volts. If the bike falls over, the switch closes, and the MUX_X3 input will be pulled down to zero volts. We have complete functionality at this point: if the bike falls over, we can tell, and if it has not, we can also tell.

Now consider what would happen if the sensor became disconnected. The RFS resistor in the voltage divider circuit goes away, so R772 will be able to pull the analog input MUX_X3 all the way up to 5 volts (Vcc). So now with one single input pin, we have a three-state input:

  • 0.0V: Tipped over

  • 2.5V: Upright

  • 5.0V: Sensor disconnected

In a digital system, it is not possible to get three information states out of a single binary signal on one wire. The beauty of using an analog input is that the ECU is not limited to using binary signaling: if we want three input states, we arrange to have three input states.

The upshot is that using a voltage divider and an analog input has made the whole sensor wiring path testable. Missing sensors or high-resistance connections will be detected as voltages closer to 5.0 volts that to 2.5 volts. And in fact, that is exactly what the ECU software does. It tests for a 2.5 volt reading with a certain amount of slop to allow for manufacturing variations in both the R772 resistor and the resistor inside the sensor. Voltages outside of the allowable range of slop indicate that the bike has fallen over, or that the wiring and/or sensor is broken.

Very slick!

Unlike the tilt switch on a pinball machine, the ECU does not make any rash decisions in deciding if your bike fell over. In fact, the first line of defense regarding tip-over false alarms is the RC filter formed by R771/C771. The sensor switch must be closed for long enough to discharge the voltage on C771 to the point where the ECU thinks that the bike might have fallen over. Even when the ECU observes this initial tip-over event, it doesn't actually do anything about it for another 0.5 second. After the 0.5 second "cooling off" period passes, the ECU checks the sensor again. If the sensor still reports that the bike is down, then the ECU decides that it really must be down, so it disables the fuel pump and kills the engine.

From a nerd point of view, it is interesting to look at the processor instructions involved in shutting things down and hope that they never have to execute...

Defeating The Tip-Over Sensor

If your tip-over switch is flakey or broken and the computer always thinks you are on the ground, it would be possible to defeat it, should you so desire. In fact, it is quite easy: simply replace the entire tip-over switch with a 1/4 watt 62K ohm resistor. One side of the resistor should be connected to the wiring harness tip-over sensor wire input, and the other side of the resistor should be connected to ground. The wattage of the resistor is rather unimportant since almost no current flows through it. A 1/8 watt or 1/4 watt resistor would do just fine.