Home and Main trigger signals explained for aftermarket engine management system
by Anthony Hyde, Australia. - Published April 2004. Updated July 2014
This article gives applied hall
sensor information, and a useful 'terminology' read for anyone
seeking to understand the function of a hall sensor / multitooth
trigger wheel relationship for crank based timing. The events
detailed here are a typical automotive use of hall-effect sensors and
their trigger inputs to engine computers / control units. Accurate
timing information from sensors (and a multi-tooth setup) will give
smoother running right through the rev range.
- We can thank American physicist Edwin Hall for discovering the Hall effect.
Description of Components for HOME (Sync) and MAIN
Section 2) Description of Events and Bench testing
Section 3) Project to create a Home signal
Section 1) - Description of
1A) HOME or Sync signal (Sync=synchronisation) - Bosch Hall-effect ignition distributors are found on many car models (from about 1985), in the era before individual coil packs per cylinder were available.
A single vane trigger is used to give a HOME signal. This occurs once in the 360° rotation of the ignition distributor (or camshaft) OR from the crankshaft point of view at 720° point being equal to 2 rotations of the crankshaft (to complete one 4-stroke engine cycle).
This Home / Sync signal lets the computer know a significant event is coming up (see next section 1B) .
And that next significant event is another set of hall sensor pulses from a eg 12 tooth trigger wheel or secondary hall sensor in the ign distributor or camshaft. The trigger signals go to the engine management system to ultimately trigger the coil and fire a spark plug at the correct time interval.
Ref signal (Ref =
Reference Tooth or Index tooth) - Two hardware combinations are
required - for example a Hall effect gear TOOTH sensor ( popular
Honeywell 1GT101DC) commonly
known as GT 101 combined
with e.g. a 12 tooth metal trigger wheel (=30 degree spacing)
mounted on the crankshaft. (Trigger wheel info down page).
The Main or Ref trigger signal occurs after the Home signal. Triggering occurs each time a metal tooth on the crankshaft mounted trigger wheel passes the tooth sensor. The signal voltage (square wave digital style) is fed into the ECU together with your software pre entered data such as trigger angle (eg 60°) or CriP, number of tooth in the trigger wheel (eg 12), and offset. These inputs enable the computer to calculate the crank angle position and engine rpm for each cylinder.
I used a Honeywell Hall effect 'Gear TOOTH Sensor' # 1GT101DC (works on input voltage between 4.5-24 Vdc). The GT 101 is a quality unit using a magnetically biased Hall-effect IC and readily available. Source example RS Components stock No. 235-5706.
the difference in Hall sensors
types: The following is a
very good example of differences between hall sensors and is rarely
talked about : The Siemens HKZ101 Hall sensor operates in an
opposite manner to the Honeywell GT
101. In operation both
hall-effect sensors use a change in magnetic field to produce a
voltage signal, but the difference to note is they have opposite
outputs when positioned in air or when passing metal.
In the examples below I will use 12V DC as the input and output, and 0 V means close to, or near 0.
- Siemens HZK101 is 0 V in air, (as it sees and responds to a magnet field on the other side) it rises instantly to 12 V when the rotating metal vane blocks the magnetic field. Therefore this is a rising edge signal OR specifically a positive pulse. As the metal vane passes it falls back to near 0 V in air on the trailing / falling edge.
A Mitsubishi Magna model TR-TS 1992-95 uses a HZK101 style hall sensor x2 - one for Home and the other for the Main trigger x4 for a 4 cyl engine, both vanes are on the metal wheel on two different diameters. Hall sensor spares are available in Australia through nationwideautoparts.com.au
- Honeywell GT101 is the opposite - its 12 V in air and pulls down to 0 V when a metal tooth / vane passes thru, therefore a falling edge or dropping pulse, being magnetically biased to conduct with metal.
As the metal vane passes it rises up again to 12 V in air on the trailing edge of the tooth, the signal is rising edge at this end of tooth point.
Crank Reference Index
Position (CRiP) (Source: MoTec definitions) The
following is a definitive explanation and has been included for
The Crank index Position (CRiP) is perhaps the most important timing value in the ECU. The CRiP tells the ECU where the engine is in relation to TDC Cylinder #1. The CRiP is defined as the distance in crankshaft degrees, between the reference tooth when it is aligned with the crankshaft position sensor, and Top Dead Center Compression Number 1.
For example, if the reference tooth is aligned with the crankshaft sensor when the crankshaft is 55 degrees before TDC Compression Number 1, then the CRiP is 55. An easy way to determine the CRiP before startup is to rotate the crankshaft in the direction of rotation until the reference tooth is aligned with the crankshaft position sensor. Then measure the number of degrees, required to turn the crankshaft in the direction of rotation until the number 1 cylinder is at Top Dead Center of the Compression stroke. Once you determine this value, you may start the engine and enter the CRiP set screen under the Ignition menu. Use a non dial-back timing light to check the CRiP. The timing advance displayed in the CRiP set screen should match the measured value using the timing light. If they do not match, move the CRiP value until the timing does match.
MoTec supplied 12 Tooth trigger wheel
360° / 12 teeth = 30° between triggers.
Watercut edge finish, quite heavy in weight.
WHEEL - useful on the Crankshaft (or at wheels, axles, diff
for speed sensing)
WHEEL MATERIAL - (Source: Honeywell article - Micro
Switch Sensing and Control)
If you have access to a guass meter, you place the meter probe in a metal shield/container, reading should be around zero. Now place your metal wheel inside of shield and take a reading and check for <26 gauss magnetic flux density.
2B) ROTOR PHASING - Typically for a 4 cylinder engine, the rotor button is aligned with pole 1 on the distributor cap - and should be around 22° BTDC at this point. For optimal phasing, the distributor Hall signal should occur on a falling edge at the end of a modified vane (from metal 5 V to air 0 V). The vane trailing edge = falling edge (in my case approx 78° at crank). Summary: HOME / Sync signal occurs well before rotor fires to cylinder 1.
After the HOME signal, the next trigger received by the ECU is the
MAIN / Ref from the GT101 sensor via the multi-tooth crank wheel.
This gives the start of an injection or ignition firing sequence The
trigger occurs on the compression stroke and the angle BTDC at the
crankshaft gives the ECU the all important reference position to TDC.
I used a falling edge (5 V to 0 V).
For the GT101, the Hall goes from 5 V air to ~0 V over the metal tooth, hence the electrical signal is falling edge (detected on the leading edge of the tooth), and rising edge again (as it leaves the tooth's trailing edge). In your computer software you need to enter the edge you decide, bearing in mind the Honeywell sensor response time is faster on a Falling edge 1 micro sec max, vs Rising edge 15 micro sec max.
For the trigger angle, the ECU needs to know exactly how many degrees are between the index tooth triggering and #1 cylinder TDC CriP, ..deg, Tooth offset =.....The ECU then fires each ignition / or fuel output channel sequentially until it gets to the last output channel, expecting to receive another HOME signal before the sequence repeates.
In-Situ Bench testing -
Step 1 is to find out the connector pin function. For bench testing of a hall effect sensor, the 3 wires are : Voltage input +Vdc, Return wire -V, and a Signal output wire. On the distributor is 3 terminals with marks + 0 - . To test, use an adjustable dc power supply (use around 5-12Vdc) - Before connecting, a 1 K ohm resistor load must be placed between the voltage Input wire and the Signal output wire. Connect the signal wire to a lead on your multimeter (thats set to VDC), and use the other multimeter lead to connect to the - negative terminal on the power supply.
As the rotor vane passes through the hall sensor a change in voltage will occur, either high or low. You can compare the signal to see which of the 2 types of hall sensor (previously mentioned) is fitted.
ORDER and sequence: Engine example 4 cylinder, 4 stroke
cycle, typical firing order 1-3-4-2 .To complete a 4-stroke
cycle, the crankshaft rotates 720 degrees, or two turns. The ignition
distributor, or cam, rotates at half-speed or 1 turn for 360°. A
four cylinder has 720/4=180 degrees between firing.
The firing order is the order the ignition system sends a spark to each of the cylinders via a distributor cap and rotor, or direct from ECU controlled coil packs.
Specs: Trigger wheel eg 12 teeth at 30 deg intervals. On an oscilloscope a Hall effect sensor signal is a square wave. The falling edge at the distributor must not occur at the same time as the trigger wheel, this will confuse the computer. Correct phasing is essential.
Pic shows HALL effect Ign Distributor (1986-88 years) Bosch 0 237 520 004 with Hall-effect sensor HKZ101 (on left) inside distributor. Found on Volvos with Chrysler Ign modules & LH2, LH2.2
3 pin Housing / Plug connector (on right) - Volvo #1389492 (Bosch 1 230 329 025) Connector assembly replacement is still available new from Volvo in 2013.
Pic shows GT101 Hall-effect tooth sensor aligned to the crank positioned 12T wheel. Note adjustment slot in hall mount. to obtain optimum trigger pick up position.
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Updated July 2014 AH