Main Index of MS2-Extra Manuals

Layout of Components -- Suggested Supply points -- List of common components and part numbers

Distributor Pickups

Variable Reluctors -- Hall Effect -- Optical Sensors -- Points

Spark Control Circuit -- Settings

MS2-Extra can be triggered from the negative terminal of the coil, just like MS1-Extra , as long as MS-II is not being used to control the ignition timing. However, if you are controlling the ignition timing, then the tach signal must be fixed with respect to the crank position (not vary with the advance timing like the coil signal) .

Most modern electronic ignitions use one of three types of sensors in the distributor (or crank position sensor). They use either a variable reluctor (VR) sensor, a hall sensor, or an optical sensor. Older engines might use mechanical contact points.

Variable reluctor sensors are cheap and very rugged, Hall effect sensors are much smaller, more expensive, and nearly as rugged.

There are some differences in the signals these three types of sensors send to MegaSquirt-II. The Hall effect, optical, and points systems produces a similar signal - a square wave of constant amplitude, polarity, and duration (in engine degrees). However, the variable reluctor sensor produces a sinusoidal (alternating current) wave form (with both + and - components) whose amplitude varies with rpm. This signal can be processed by an ignition module to produce a square wave suitable for input to MegaSquirt-II.

Variable Reluctor Pick-up
Internal Wiring

The Variable Reluctor (VR) sensor is an induction type sensor, it is "passive", i.e. it does not require a power source, and has a small magnet built in.

The sensor uses a magnetic pickup to produce a signal. A core of steel is wrapped with hundreds of turns of fine wire at one end. A small magnet is attached to the other end, and this assembly is mounted in the distributor facing the distributor shaft. When a notch, pin, teeth or hole in a timing wheel (the reluctor) moves past the sensor, it causes a change in the magnetic flux field around the sensor. As the teeth of the reluctor approach the coil assembly, the flux from the magnet is pulled in close to the bar. The sudden field change induces an electrical current in the coil, which is then converted to a voltage signal by electronic circuitry in MegaSquirt-II. As the teeth move away, the flux springs back outward, inducing a voltage in the pickup coil. This induced current has reversed direction as the magnetic field returns to normal.

The result is an alternating current (AC) voltage that reverses polarity and crosses zero as it the pin aligns with the sensor. The output voltage of this sensor varies with the speed of the engine. This voltage is then chopped/filtered/amplified and used to drive a high voltage/high current transistor that switches the coil current. This signal is used by MegaSquirt-II for ignition timing as well as injector timing in engines with sequential fuel injection.

The variable reluctor sensor has been the most widely used in automotive ignitions. It has been used by virtually every auto manufacturer for many years and is still widely used. The GM high energy ignition (HEI) is one example. It is a rugged, reliable system that holds up well in a high temperature, high vibration environment. Because it generates a signal without requiring external power, it is very easy to implement. The magnetic variable reluctor sensor is gradually being phased out in modern automobiles, however, because it has limited ability to sense teeth that are very close together, which is necessary to gain the positional accuracy required by modern engine management systems.

At idle the output is approximately 0.6 volts, at mid-RPM it is close to 3 volts, and at very high RPM it can go as high as ~50 volts. This type of sensor produces an alternating current (AC) wave output. The pulse is positive when the "pole", is approaching, and negative when the pole is leaving (if you have the right polarity). The simplest way to see this is by hooking it up to a cheap analog voltmeter and using a wrench or other "non magnetic - soft iron", piece of metal. When you place the metal piece on the sensor the needle on the voltmeter will swing one way. When you quickly remove it the needle will swing the other way.

Magnetic sensors can be checked by unplugging the electrical connector and checking resistance between the appropriate terminals. On the Rover 820, for example, the sensor should read between 1200 and 1450 ohms. Check the manual for your car for the specified voltage.

A magnetic crank position sensor should also produce an alternating current when the engine is cranked so a voltage output check can be performed. With the sensor connected, the output voltage across the appropriate module terminals while cranking the engine should be at least 20 milliVolts (mV) on the AC scale of your meter. If this is the case, the sensor is probably good, meaning any fault is probably in the module.

A variable reluctor magnetic sensor typically has two wires, and possibly a shield wire.

VR Sensor internal wiring for MS ECU:

For a suitable coil driver circuit see the Spark Control Circuit HERE

Hall Effect Sensors
Internal Wiring

A Hall effect sensor is an "active", magnetic field presence sensor. It is based on the Hall effect. The Hall effect is the change of resistance in a semiconductor in a magnetic field. The Hall effect sensor consists of semiconductor material which will conduct current when the material is subject to a magnetic field. These types of sensors require a "flying magnet", wheel. Instead of teeth on the wheel, as in a variable reluctor sensor, you must have small magnet and a shutter wheel.

The Hall effect sensor consists of three parts:

The Hall sensor has electronic circuitry that provides a constant voltage pulse regardless of the speed. The square wave it produces is particularly suitable for use in digital electronic systems. The sensor is also sensitive to the polarity of the magnet. North pole will turn it on, South will not, or vice-versa, depending on the orientation of the sensor. The pulse produced is as long as there is a magnetic field of some strength present, and is always of the same polarity (positive with respect to ground).

There are many advantages to the Hall effect sensor. Since it is an integrated circuit, it can be made very small with a number of features at minimal cost. It exceeds all current automotive temperature specs. Its accuracy is unaffected even when covered in under hood muck. Hall-Effect triggering has been widely used on European vehicles with Bosch electronics since the late 1970s. It has been used in the U.S. as early as 1975. In the 1980s it became more common, mainly on Chrysler imports. Ford was the first domestic manufacturer to embrace the technology with the advent of the TFI (Thick Film Integrated) ignition.

Hall-Effect has become the most popular sensor in recent times as automotive manufactures migrate to Crank Angle Sensors. These typically are placed to read special crank wheels, or the starter gear teeth on the flywheel, providing a high degree of positional accuracy MegaSquirt-II's fuel and ignition computations.

Hall effect crankshaft position sensors typically have three terminals:

The sensor must have voltage and ground to produce a signal, so check these terminals first with an analog voltmeter if you suspect it is not working. Sensor output can be checked by disconnecting the coil and cranking the engine to see if the sensor produces a voltage signal. The voltmeter needle should jump each time a shutter blade passes through the Hall effect switch.

You can use the LED tester to check the signal. it should blink as the distributor is rotated:

On an oscilloscope, you should see a square wave form:

Hall Sensor internal wiring for MS ECU:

Wiring for V3.0 PCB -- Wiring for a V2.2 PCB

V3.0 PCB Wiring:

Hall or Optical input sensor wiring for V2.2 PCB

When hall sensor output is 5V use:

When hall sensor output is 12V use:

Settings

For correct operation it is critical that the Ignition Input Capture, Trigger Angle and Angle between main and return are specified correctly. For assistance in setting attach crank timing tape to your damper and disconnect the coil, then enable "middle LED indicator" on the More Ignition page. Now rotate the engine to 10 Deg BTDC (Assuming this is where you want your cranking advance). Place the vane so the Middle LED just turns off. (see above diagram). Now turn the engine backwards (this is so we keep on the same vane) the LED will come ON imediatley as it will see the vane. Continue to turn it untill the LED goes out again. From the timing tape you can determine the Trigger Angle.

Also make a note of the number of degrees the LED is lit. This is for the Angle between main and return.

Ignition Input Capture Setting:
All Hall sensors we have tested appear to ground the supply when switching. We need to know when it switches the supply down to ground, i.e. when it see's a tooth or when it see's the gap between teeth. To do this, turn the engine backwards until the edge is well out of the Hall sensor. Measure the output voltage from the Hall sensor Turn the engine forward until the edge has passed through the Hall sensor. Measure the output voltage.

If the sensor switches to ground when it sees a tooth and goes high (12 or 5V) when it see's a gap then set the Ignition Input Capture to "Rising Edge"

If the sensor switches to ground when it sees a gap and goes high (12 or 5V) when it see's a tooth then set the Ignition Input Capture to "Falling Edge"

Once the engine is running double check your timing using a strobe. (To fix the angle at say 10deg to make reading it easier set the "Fixed Advance" to Fixed and set the "Timing for Fixed Advance" to 10deg. Change the Fixed Advance to "Use Table" once youve finished!!) If it is out then alter your Trigger Angle untill it is correct.

For a suitable coil driver circuit see the Spark Control Circuit HERE

Optical triggering

Optical (a.k.a. photo-electric) sensors include:

Kettering Triggers (points)

The Kettering 'points' type of ignition used a distributor as a mechanical switch to turn the primary circuit on and off. An arm with a set of contacts was controlled by a small cam inside the distributor to control the current from the ignition coil’s primary circuit. One point contact is connected to ground, the other point contact is connected to the coil's negative terminal. The points ground the coil's negative terminal (allowing current to flow from the 12V source) to charge it and then open to fire.

The result is that the points produce an approximately square wave signal (on the point connected to the coil negative terminal), with an 'amplitude' of 12 volts when they are open), pulled to ground when they close (charging). By replacing the coil with a current limited pull-up circuit, we get a much cleaner signal that is not electrically demanding on the points (so they last much longer).

This type of switching has not been used in ignition system by manufacturers for many years. In these cases the distributor also mechanically adjusted the timing, though MegaSquirt-II could be used with a points type distributor by locking the mechanical and vacuum advance mechanisms.

To use a point type signal for MegaSquirt and advance timing control, you need to:

Spark Control Circuit

To control the coil (single coil driven through the distributor to the correct cylinder) you will need to build the VB921 circuit to your MS ECU. The V3.0 already has a VB921, so it is easy to do, the V2.2 doesnt have one so it is a little harder to do.

V3.0 PCB ONLY

Very Important: Set Spark Out - Going High (Inverted) and set the Dwell to around 6.0mS for cranking 3.5mS for Running and 0.1mS for the Minimum Time as a starting point! Also set D14 as the Spark A output pin.
See HERE for settings

V2.2 PCB ONLY

It is recommended that V2.2 users fit a 15 or 25 pin db connector onto the case of the MS ECU. The reason is that the spare connectors on the V2.2 are not really capable of driving the current required for a coil unless you double them up (e.g. X11 and X12 as the ground X13 and X14 as the source). This is only suitable for a single VB921. Drill a 3mm hole into the case and mount the VB921 securely using the hole. Use heatsink compound between the VB921 and the case.

Very Important: Set Spark Out - Going High (Inverted) and set the Dwell to around 6.0mS for cranking 3.5mS for Running and 0.1mS for the Minimum Time as a starting point! Also set D14 as the Spark A output pin.
See HERE for settings

Settings

Suggested points for Supplies inside the V3.0 ECU

Component Pinouts

Pull up circuit diagram

List of component part numbers used in the ignition section:

Please note: Above part numbers will need checking, some components will come with a minimum order in multiples of 5 and 10.

If you have a question, comment, or suggestion for this FAQ please post it on the forum.

No part of this manual may be reproduced or changed without written permission from James Murray, Ken Culver and Philip Ringwood.