RPM Pickup Wizard

Overview

A wizard is available for GWv4 that simplifies configuring the crankshaft / camshaft sensors and the computation of the engine cycle position and RPM.

If you have an unusual tooth pattern or sensor configuration, you may need to manually configure these settings. Contact GEMS for assistance if you are unsure.

There are also existing Calibration and Aspect files available for many common configurations.

• Start Maps and Setups
• Calibration Aspects

Crank Sensor

Overview

The crank sensor is used to determine the position of the crankshaft and the speed of the engine. The crank sensor is usually mounted on the engine block and reads a toothed wheel mounted on the crankshaft. It may be used in conjunction with a camshaft sensor to determine the position of the engine cycle.

Sensor Type

The crank sensor is usually a Hall effect sensor or a VR (Variable Reluctance) sensor. The sensor is connected to ECU timer T1 which has a software selectable pull-up resistor.

Sensor Type	T1 Pull-up
Hall Effect / Optical	Yes
VR / Inductive	No

Edge Selection

The ECU man be configured to select which edge of the signal is used to interrupt the ECU for engine position related processing, these options may be named differently on some ECUs:

• Crank Rising Edge or T1 Rising Edge
• Crank Falling Edge or T1 Falling Edge

VR sensors have one direction edge that is sharply defined. In the other direction, there are 2 edges and they are slower. Selecting the correct edge here is crucial to ensure accurate engine position detection.

    xychart-beta
    title "VR Sensor Pulse"
    x-axis [0, 1, 2, 3, 4, 5, 6, 7, 8]
    y-axis "Voltage" -5 --> 5
    line [0, 0, -0.3, -3, 3, 0.3, 0, 0, 0]

An oscilloscope can be used to determine the best edge to use. The sensor output should be monitored while running / turning over the engine to determine which edge is the most sharply defined.

To verify that the correct edge has been selected, it is recommended to record an ECU internal data log with the crankshaft moving at a constant rate. It is expected that the synchronization strategy and Tooth Control table have been configured before this test.

Channel	Description
A Tooth	Actual teeth feed into to Tooth Control table.
Wheel Tooth	Decoded internal tooth number, increments up to the value of Wheel Teeth.
Fuel Tooth No	Decoded internal teeth for engine cycle, decoded by Tooth Control table used for fuelling, increments up to the value of Fuel Teeth..
Ign Tooth No	Decoded internal teeth for engine cycle, decoded by Tooth Control table used for sparking, increments up to the value of Spark Teeth..
MX Tooth Time	Time interval between actual teeth; i.e. time separation between crank interrupts.
Tooth Time	Time interval between internal teeth decoded by Tooth Control table. The most important measurement in ECU.

The following images show the difference between the correct and incorrect edges for a VR sensor:

Incorrect Edge

See the long tooth interval detected in MX Tooth Time when A Tooth = 0

The long tooth appears to be in 2 parts of 3.296 and 1.482 ms, with the regular teeth spaced at 1.185 ms.

This is from a 30-2 wheel so the missing tooth interval should be approximately 3x the regular teeth interval.

Notice that the Tooth Time channel has major discontinuities; it is expected that the internal teeth (decoded by the tooth control table) are regularly spaced and so a more constant value is expected here.

Correct Edge

With the Crank Rising Edge and Crank Falling Edge swapped we get just a single long MX Tooth Time 3.754 ms, equal to 3 regular teeth (on a 30-2 wheel) as expected. The regular teeth are spaced at ~1.243 ms:

3.754ms / 1.243ms = 3.02 teeth

Also notice that the Tooth Time and A Tooth channels are smoother, meaning that the edges are interrupting the ECU more reliably.

If you are unable to start the engine and idle, it is possible to get the timing wheel running at a steady speed by removing the spark plugs (to avoid compression) and using the starter motor to turn the engine over. This will allow the ECU to log the crankshaft position data without the engine running. For safety, any injection and spark outputs can be disabled in the ECU calibration for this test.

Crankshaft Position Sensor Filtering

Hardware Filtering

• T1 Filter Below - RPM value below which the T1 filter is active.
• T1 Low Sens Above - Reduces the sensitivity of the T1 input above this RPM value, can be useful for VR sensors.
• Crank Filter - Time window, in microseconds, for filtering edges on the crank sensor input.

Software Filtering

• Tooth Short Ignore - Enables software filtering of short tooth intervals. This is useful for filtering out noise on the crank sensor input.
• Tooth Short Running - This is a percentage of the last valid actual tooth interval, edges detected that are less than this percentage are rejected.
• Tooth Short Start - When cranking (starting) the engine, before synchronization is achieved, Stat Sync'd == OFF, software filtering uses a fixed time interval for rejecting edges - edges detected that are less than this interval will be rejected.

When running, Tooth Time x Tooth Short running is used to determine the minimum tooth interval that will be considered a valid tooth event.

The calculated value for the short tooth test is stored in A Tooth Short Test.

If a short tooth is detected, it will record this information in:

• A Tooth Short [ms] - captures captures MX Tooth Time
• A Tooth Short No - captures A Tooth
• A Tooth Short Rev - captures Cam count MX

This information will be recorded regardless of whether Tooth Short Ignore is set.

Cam Sensor

Overview

A cam sensor is used to determine the position of the camshaft, typically relative to the Crank Sensor.

Camshaft Position Sensor

If a camshaft position sensor is required, similarly to the crank sensor, it is usually a Hall effect sensor or a VR (Variable Reluctance) sensor. The sensor is typically connected to ECU timer T2 which has a software selectable pull-up resistor. It is possible to use a different timer input if required but T2 is the most common choice.

Sensor Type	T2 Pull-up
Hall Effect / Optical	Yes
VR / Inductive	No

As with the crank sensor the cam synchronization edge should be configured.

If you are planning on using the VVC (Variable Valve Control) features then T2 should be configured for both edges, with the appropriate edge for Cam or 4 stroke cycle synchronization selected by the Sync channel T option. VVC control would require configuration with the other direction edge where appropriate (VVC1 source T, VVC2 source T, VVCX1 source T, or VVCX2 source T).

For example:

Option	Value
T2 Rising Edge	ON
T2 Falling Edge	ON
Sync channel T	“T2 Rise”

If not using T3-T12 timing inputs they can be switched off with T3 - T12 Disable to avoid any additional CPU overhead.

Camshaft Sensor Filtering

Like the crank sensor, the cam sensor can have hardware filtering applied to it:

• T2 Filter Below - RPM value below which the T2 filter is active.
• T2 Low Sens Above - Reduces the sensitivity of the T2 input above this RPM value, can be useful for VR sensors.

Engine Cycle Sync (Camshaft)

Overview

On 4 stroke engines where sequential injection and/or ignition is required, the ECU must synchronize to the correct revolution of the engine cycle so that the correct fuel and spark events are generated.

The crank wheel typically rotates once per revolution and therefore does not provide the ECU with enough information on its own.

However, the camshaft rotates once per cycle (2 revolutions of the crankshaft) and a sync signal from the camshaft can be used to determine the engine cycle.

Camshaft Sync

The ECU counts the occurrence of rising or falling (or both) edges from the cam sensor.

The counter, typically T2R counter (rising) or T2F counter (falling), is captured at an actual crank tooth Sync Test A Tooth and is reset to zero to begin counting again; the counter will be reset every crank revolution at the specified tooth.

Example:

In this example with a 8-1 crank wheel, there are 2 rising edges in the 1st crank revolution and 1 rising edge in the 2nd crank revolution.

The synchronization test point may be adjusted using the Sync Test A Tooth option.

Sync Test A Tooth is the actual crank tooth number (i.e. not an internal tooth number) at which the camshaft counter is captured and reset to zero on every crank revolution. The value is captured into the Cam Tooth Count channel.

Sync Teeth is the number of Cam teeth expected at Sync Test A Tooth when testing for synchronization if the engine is within the expected half of the 4-stroke cycle.

If Sync Test A Tooth were set to 6 in this example (corresponding to the 7th tooth of the crank wheel, counting from 0), then the counter would be set to 2 in the first revolution and 1 in the second revolution.

Setting Sync Teeth to 2 would result in synchronization to be achieved on the 1st crank revolution. If set to 1 then synchronization would be achieved on the 2nd crank revolution.

During the ‘synchronization’ portion of the tooth control table, the captured counter value is tested against the Sync Teeth value.

Variable Valve Control Systems

On variable cam control systems, the timing of the camshaft signal varies with respect to the crankshaft signal.

Considering the example diagram again, if the camshaft was in its most retarded position at that point then the ‘Cam’ signal could shift to the left by up to 80 degrees (almost 2 Actual Teeth in this example).

The original example with Sync Test A Tooth set to 6 would still be correct, when counting rising edges, for either cycle 1 or cycle 2 if the cam signal were shifted to the left by as far as 2 teeth.

However, suppose Sync Test A Tooth was set to 4. There would be multiple issues:

• In the first revolution, the ECU would detect either 1 or 2 teeth, depending on whether the camshaft position was advanced by VVC.
• In the second revolution, Sync Test A Tooth closely coincides with the cam signal, and there is a race between the crank signal and cam signal - small timing variations could cause unreliable detection of the cam signal.

In both cases, the cycle detection would be highly unreliable.

Edge Selection

Depending upon the cam sensor configuration, the ECU could be counting rising edges, falling edges or both.

The cam sensor edge direction may be selected by a combination of T2 Rising Edge, T2 Falling Edge and Sync Channel T.

It is possible to use timers other than T2 for cam synchronization, but T2 is the most common.

Engines with Variable Valve Control (VVC / VVT) would typically require both edges to be enabled with Sync Channel T set to either “T2 Rise” or “T2 Fall” so that only the rising or falling edge of the cam signal is used for engine cycle synchronization.

The edge selection may require adjustment to improve engine cycle detection.

If you are using a VR (Variable Reluctance) sensor for the Cam sensor then there may only be 1 valid edge due to the nature of signals from VR sensors - see Crank Sensor.

Sync 2 (Reliably Synchronize Within 1 Revolution)

An optional feature is available to support synchronization within a single crank revolution.

If Sync 2 Active is set to ON, then the cycle shall also be matched using Sync 2 Teeth.

If Cam Tooth Count matches Sync 2 Teeth then synchronization will be achieved (Stat Sync'd will be set to ON) and the value of Sync 2 Fuel Tooth will be placed into Fuel Tooth and the value of Sync 2 Spark Tooth will be placed into Spark Tooth to ‘jump’ these important clocks to the next half of the cycle.

Considering the example again:

For sequential injection + ignition with 4 internal teeth per revolution, this example ECU could be configured with:

Option	Value
Sync Teeth	2
Sync Test A Tooth	6
Sync 2 Active	ON
Sync 2 Teeth	1
Sync 2 Fuel Tooth	4
Sync 2 Spark Tooth	4
Fuel Teeth	8
Spark Teeth	8

Options / Channels

Option	Description
Start No Cam Sync	The ECU will not attempt to synchronize to the camshaft while cranking (starting) the engine.
Sync Channel T	Selects Cam synchronization input timer (e.g T2 Rise)
Sync Teeth	Number of teeth expected to unambiguously identify 1 revolution in the engine cycle. Must be set to 0 if not using a Cam Sync input.
Sync Test A Tooth	The actual crank tooth number at which the camshaft counter is captured and reset to zero. This is the actual crank tooth (A Tooth) number, not the internal tooth number. Care should be taken, particularly with systems using VVC, to ensure reliable detection across the range from most retarded to most advanced camshaft positions.
Cam Tooth Count	Value of the sync channel (e.g. T2R counter) when A Tooth reaches Sync Test A Tooth
Cam Tooth MX	Value of Cam Tooth Count when ‘MX’ (Missing / Extra tooth) synchronization tested.
Sync 2 Active	Enabled Sync 2 feature.
Sync 2 Teeth	Number of teeth to identify 2nd revolution in the engine cycle.
Sync 2 Fuel Tooth	Value to set in Fuel Tooth if Sync 2 is achieved. Jumps the injection ‘clock’ to the next half of the cycle. Should be set to the number of internal teeth per revolution, if sequential injection is required.
Sync 2 Spark Tooth	Value to set in Spark Tooth if Sync 2 is achieved. Jumps the ignition ‘clock’ to the next half of the cycle. Should be set to the number of internal teeth per revolution, if sequential ignition is required.

Actual Teeth vs Internal Teeth

Overview

There is a difference between actual teeth on the crankshaft wheel and internal teeth used by the ECU for triggering ignition or injection events.

Internal teeth always coincide with actual teeth but they are arranged such that they are at evenly spaced crank angles.

Actual Teeth vs Internal Teeth

Events from ‘actual teeth’ on the crankshaft wheel cause the ECU’s CPU to be interrupted. The ECU decides whether these events are ‘active edges’ aka ‘internal teeth’ that may be used for triggering ignition or injection events.

If using a missing / extra tooth strategy, the actual tooth events are not evenly spaced. The ECU is able to detect this un-even spacing to identify missing / extra teeth.

If the engine cycle can be determined from a camshaft sensor, and sequential injection or ignition is required, then the Fuel Teeth and Spark Teeth options should wither be set to the same value as the number of internal teeth (e.g. wasted spark) or to double the value (sequential).

Synchronization

Overview

Once the crank / cam sensors are configured, the synchronization strategy and tooth pattern must be set up to be able to synchronize the ECU with the engine.

When cranking / starting the engine, the ECU will initially be in an unsynchronized state. The ECU will not be able to determine the engine cycle position until synchronization is achieved.

When the ECU reaches the synchronized state, the Stat Sync'd channel will be set to ON.

During cranking, the ECU uses a different control strategy to normal running.

Synchronization Strategy

The most common synchronization strategy is to use a Missing tooth (or eXtra tooth) on the crankshaft wheel, GEMS ECUs refer to this as the Sync MX strategy.

There are 4 synchronization strategies available, controlled by 3 options. These options must be set in a mutually exclusive way:

• Sync MX - Missing / extra tooth/teeth on the crankshaft wheel.
• Sync Cam Count - Simplest strategy, uses the camshaft sensor to determine the engine cycle position, prefer Sync Crank S count if possible.
• Sync Crank S Count - Similar to Sync Cam Count, counts the number of crankshaft teeth between camshaft teeth.
• Sync Cam - when the above 3 options are OFF. Ignores the Tooth Control table and used camshaft signal to subdivide the engine cycle.

Tooth Control Table

Overview

The main goal of the Tooth Control table is to subdivide the physical timing wheel teeth (actual teeth) into regularly spaced internal teeth.

RPM Pickup Wizard

The RPM Pickup wizard can be used to calculate the Tooth Control table values, given some common timing wheel types.

Tooth Control Table

The Tooth Control table is a low-level ECU feature that is not obvious but essential to understand if needing to accommodate unusual tooth patterns.

Each site in the Tooth Control table corresponds to the sequence of ECU interrupts (events) from actual teeth passing the crank sensor and are therefore not necessarily evenly spaced in time or angle; there will not be any events in the missing section of the wheel or there may be an extra tooth. Some timing wheels have more than one missing teeth section.

The aim is to select from these events an evenly spaced real-world pattern of internal teeth that can be used to schedule fuelling and ignition events. The internal teeth are used to subdivide the engine cycle into equal parts, and the ECU will increment internal tooth counters for each internal tooth detected, with separate counters for fuelling and ignition.

If using a wheel with 2 missing teeth in a row, for example, then the actual teeth will need to be divided by at least 3 (possibly more) to ensure that no internal teeth fall within the missing section and are also evenly spaced in terms of crank angles. Each internal tooth must coincide with an actual tooth passing the sensor; i.e. an interrupt/event occurs in the ECU for each internal tooth.

It is common to subdivide the timing wheel into 12 internal teeth. For example, with a 36-2 wheel that has 2 missing teeth in a row this can be divided by 3 into 12 equally spaced internal teeth that all coincide with actual teeth.

When actual teeth are detected, the ECU will increment A Tooth and use the value to look up the corresponding site in the Tooth Control table.

Each value in the table is composed of 3 digital ‘bits’ (1/0):

Value	Bits	Function
0	000	Do nothing.
1	001	Active Edge (Internal Tooth).
2	010	Alternate Edge (Used for Crank Alt Fire mode).
3	011	Reset A Tooth, if loss of sync. Indicates the end of the table.
4	100	Test for synchronization.
5	101	Active Edge (Internal Tooth) + test for synchronization.
6	101	Alternate Edge + test for synchronization.
7	111	Reserved (do not use this value).

Unless all sites in the table are required, the value ‘3’ should be placed at the end of the table so that loss of synchronization can be detected; if A Tooth overruns the valid section of the table then the ‘3’ shall be encountered and the ECU will reset A Tooth to 0. The channels Sync Error and Timing Error will be incremented if this event occurs.

It is expected that the synchronization event(s) will be detected before the ‘3’ value is encountered, which would also result in A Tooth being reset to 0.

Active edges (internal teeth) are used to schedule fuelling and ignition events and are indicated by either ‘1’ or ‘5’ in the table. The ECU increments internal tooth counters Fuel Tooth No and Ign Tooth No when an active edge (internal tooth) is encountered in the Tooth Control table.

The Synchronization Test bit (100), when set, will cause the ECU to test for synchronization. This allows for some reduction of CPU load when running at higher RPMs by skipping synchronization checks on all tooth interrupts (events) (which would be occurring much more frequently). This bit is normally set in the tooth control values near the end of the table where the missing / extra tooth is approaching, though for some other patterns there could be multiple synchronization points.

When not synchronized (Stat Sync'd == OFF), the ECU can be configured to test for synchronization on all actual teeth by setting Test Not Sync'd to ON (recommended).

Tachometer Output

Overview

The ECU may be configured to output a tachometer signal to drive an external tachometer. The signal is a square wave with a frequency proportional to the engine speed.

Tacho Edge Table

Tachometer state change events are inserted into the Tacho Edge table. They should be evenly spaced and typically there should be 4 state changes per revolution (i.e. 2 pulses per rev), depending upon the tachometer input requirements.

It is generally best, therefore, to place the state changes at the same A Tooth numbers as equidistant ‘active edges’ in the tooth control table, assuming that it is possible to do so.

The Tacho Pin option specifies which ECU pin the tachometer signal is output from. Check the ECU pinout to determine which pin number this should be set to.

If using the RPM Pickup wizard, the Tachometer output can be calculated for you given a pulse-per-rev value. The RPM Pickup wizard assumes that Wheel Teeth is equal to the number of internal teeth.