Triggering is one of the most poorly
understood areas of ECU configuration, and it’s arguable the
most important one to get right. It’s certainly the one that
cause a lot of engine damage if it’s not right. One thing that I always say is to make sure we
understand the objective. What problem are we solving? The
whole point of triggering is for the ECU to know the current
engine angle as accurately as possible, and as quickly as
possible from when the engine starts to crank. The more
accurate the ECU’s knowledge of the engine angle, the more
stable the ignition timing will be and the closer it can run
to the ragged edge. The less rotation that the ECU requires
from the triggering system before it knows the angle, the
faster the engine will fire up from cranking. The ECU always tracks the engine angle over a 720
degree period; and this is represented as a range from -360 to
+360 degrees and then straight to -360 again. Zero degrees is
defined as cylinder 1 TDC ignition. For 2-stroke engines,
including rotaries, the ECU still calculates the engine over
the 720 degree range but the injection and ignition is fired
every 360 degrees. There are several preconfigured triggers for various
engines, so if your engine is listed then use it. Start with
the base angle of zero degrees, and then fine tune the
ignition timing using timing lock when the engine is running. But this article is mostly about the generic trigger
mode. In this mode, you will need to configure it yourself
based on what’s on the actual engine. There are several
variables that need to be known, which are the angle
increment, the base angle, and the reset types. First we’ll discuss sensor wiring and sensor types.
Firstly, we should have the highest tooth count input
connected to CAS1. This is the main trigger that the ECU will
use for working out the engine angle, which affects ignition
timing and so on. Using the trigger with the most teeth gives
the most accuracy. For example, if you have a Toyota crank
sensor engine with 12 teeth on the crank and 2 on one or two
camshafts, then the 12 tooth crank sensor will need to connect
to CAS1. If you have a 36-2 trigger on the crank, then that
must connect to CAS1. Another example is the Mitsubishi /
Mazda sensor with 4 teeth on the camshaft and either 1 or 2
for cylinder identification; the 4 tooth sensor needs to
connect to CAS1 whereas the 1 or 2 trigger sensor must connect
to CAS2. For the Nissan optical style triggers, they have
their own dedicated trigger modes and they will accept either
sensor output connected to either input, but following this
protocol, the 360 window trigger would connect to CAS1.
Sample Pictures Only Secondly, there are broadly speaking 2 types of
sensor output, as far as the ECU is concerned. The first is a
variable reluctance sensor; also called reluctor, VR, or
confusingly a “magnetic” sensor. These sensors have 2 pins on
the sensor only. The sensor is a coil of wire and has an
internal magnet. A tooth passing the sensor creates a change
in reluctance, which causes a change in the magnetic field
strength. This changing magnetic field creates a voltage in
the coil, just like in an alternator. The voltage that the
sensor generates is proportional to voltage; at 6000 RPM it
will generate double the voltage as at 3000 RPM. From this you
can extrapolate that the sensor generates no voltage when the
engine is stopped. So one downside of this type of sensor is
that with the engine stationary, you can’t tell from the
output from the sensor whether it’s currently facing the
trigger tooth or not. The other thing to be careful about with the reluctor
sensors is that the polarity is important. The voltage should
increase as the tooth approaches the sensor, and then when the
tooth is facing the sensor, the voltage drops. When the tooth
is in the middle of the sensor, the voltage is zero. Then the
voltage goes negative, and finally as the tooth goes away from
the sensor it goes back to zero. So the voltage goes positive,
then negative through zero, then back to zero again. With a
missing tooth trigger, you can tell the polarity during the
missing tooth gap; the voltage should rise slowly through the
gap because the ECU triggers off the negative zero crossing.
With a multitooth trigger, where there is no missing tooth
gap, the main thing to consider is the relative angle of CAS1
and the reset event. You can see both the polarity of the
waveform and the relative timing on the built-in scope in the
ECU. The other type of
sensor is a digital sensor. These can take the form of a an
optical sensor, or a Hall effect sensor. In some cases there
might also be reluctor sensors with built-in signal
conditioning to give a digital output. In this case the sensor
requires power, which may be 5V or 12V. When the sensor is
triggered, it pulls the output to ground, and otherwise it
doesn’t – therefore there must be a pull-up resistor in the
ECU. For both the reluctor and digital trigger types, you
can adjust the voltage at which the ECU triggers. If this
figure is too low, then the sensor will trigger off noise and
cause triggering problems. If the figure is too high, then the
sensor may not trigger at all, and this will cause triggering
problems (for example, difficulty in starting). This threshold
voltage can be adjusted against RPM. The ECU can also handle
the threshold automatically, which is what we recommend unless
there’s a problem. So in a generic trigger mode, you need to select that
you’re using a generic trigger mode. This will open the
trigger settings for each input. Next you can select each of
your trigger inputs, and select whether it’s a digital or a
reluctor input. Then select the automatic voltage threshold option. The next feature is the filtering. In general, you
probably won’t need to touch this. Normally about 20
microseconds is a good duration to deal with ignition noise,
but on very high tooth count engines like the Nissan optical
sensors this will need to reduce to about 4 microseconds at
high RPM. Now, we’ll look at how
the ECU interprets these trigger events. The first setting we
need to consider is the angle increment. This is the crank
angle, in degrees between each of the CAS1 trigger events. For
example if you have 12 teeth on the crankshaft, this value is
30 degrees. If you have 60-2 on the crank, then the value is 6
degrees. If the spacing is uneven (apart from missing teeth),
you will need to use a dedicated trigger mode, not the generic
trigger mode. The next thing to
consider is how the ECU will know when the engine is at TDC,
ie where to start counting from. There will need to be some
kind of reset event that occurs once per crank revolution, or
once per cam revolution, that the ECU can use to know what
angle the engine is at. If there is a missing tooth on the
crank, then you will need to use that. In that case, set the
reset type for Trigger 1 to be a crank type reset, and select
either 1 or 2 missing teeth as appropriate for your engine. In this case, the base
trigger angle will be the angle BTDC of the first tooth after
the gap. Now, being a 720 degree cycle and a crank sensor,
this will occur twice within the 720 degree cycle. So there
are two possible values; one value and the same value plus or
minus 360 degrees. At this stage, just select one of them. The other common case is where there is a single
tooth on the cam, and no missing teeth on the crank. In this
case, the reset type for trigger 1 must be “none”, and the
reset type for “trigger 2” must be “cam”, and set to “every
tooth” (rather than missing teeth). If it’s on the crank
instead, then you should select “crank” instead of “cam”, for
example the factory trigger on the RX7 FD engine. If you are using this
trigger type with a multitooth and a cam reset, then you will
need to check the relative trigger angles. You can do this
using the built-in scope, during cranking. The important
criterion is that the reset event (eg single tooth cam
trigger) happens as far as possible from CAS1 trigger events.
If you have a reluctor on CAS1, and its negative edge happens
at almost the same time as the CAS2 reset trigger, then you
may need to reverse the polarity of CAS1. Or similarly you may
need to select the rising or falling edges differently on a
digital trigger. If you so far have only selected a crank type reset,
for example if you have a missing tooth on the crank, and you
are running a 4 stroke engine, then best results will be
obtained by using a cam sensor as well so that the ECU can
achieve 720 degrees of crank angle information. If it’s a
single tooth, then select the trigger type as “first half
only”. If it’s a missing tooth
type sensor for example the Toyota 2ZZ or BEAMS which has 4-1
on the cam, you should select it as first half only, but
select it as “reset on 1 missing tooth” rather than “reset on
every tooth”. Note that in first half
only type reset, this only switches the ECU’s crank angle by
360 degrees or leaves it as is. There must be a separate
reset, usually a crank type reset, to tell the ECU where TDC
is. You can use the built-in scope to verify not only the
trigger inputs, but also the ECU’s understanding of the
current engine angle. As you can see here the crank angle
estimated by the ECU goes up in steps where each step
corresponds to another CAS1 trigger. The typical scope settings you’ll need to see during
cranking, are: Channel 1: CAS1 inst voltage, 1V/div Channel 2: CAS2 inst voltage, 1V/div Channel 3: Current engine angle, 100/div Timebase: 50ms per division Once you have the correct triggering, you can set the
base timing using the base angle setting. When cranking you
should now have stable RPM. The other setting that must be set correctly is the
cranking RPM threshold. This must be higher than the actual
cranking RPM with some margin. Once the RPM exceeds this
value, the ECU considers the engine to be running, switches
over to the main fuel map instead of the cranking map and so
on. The final thing we need to discuss with respect to
trigger inputs is VVT. The ECU always records the crank angle
of the CAS2 and CAS3 trigger events, and the CAS4 and CAS5
trigger events if you have the mini realtime expansion module.
For digital triggers you will need to select the correct edge
(positive or negative) to get stable readings. Reading the crank angle can be done with the factory
timing marks, or making your own as we describe in another
article, but how do you know whether it’s TDC ignition or TDC
overlap? If you can see the cam pulleys then that might tell
you, or you could put a hose into the spark plug hole and try
blowing into it to see if the valves are closed. Another way,
which I like because I don’t really like getting my hands too
dirty, that works if you have a coil per plug system, is to
start off the engine in wasted spark mode. Ie, set the
ignition output firing pattern to 360 degrees instead of 720.
With the engine running, change the ignition output firing to
720 and see if the engine stalls. If so, change the base
timing setting by 360 degrees and try again. Once it’s only
firing every 720 degrees, then you know it’s correct. Finally, if you have a
crank reset that occurs potentially a long time before the
reset on the camshaft, which would be a “first half only”
reset type, you can set the ignition output mode to fire every
360 degrees, until the cam sensor is triggered, and then
switch over to 720 degrees. This gives you the nyeh-nyeh-vroom
starting performance of a factory car. Thank you and happy
learning!