GM Cadillac Northstar DIS to GM HEI SBC ECM Project

Project Status: Discontinued
Project Started: Fall 2005

1. Introduction

I did not originally think of this idea but have done a great deal of research on the operation and components in the first generation 1990’s (93-99) Cadillac northstar ignition setup.  I have put together a lot of useful information for anyone looking to complete this project or modify the Northstar DIS setup for use on there car.  These pages outline some of the needed information to run a Cadillac northstar DIS (distributorless ignition system) ignition module and coils on a SBC with almost any GM OBD-I HEI ECM \ ECU.  I planed to use this on a 1227730 90-92 f-body or 1227727 90-91 C4 Y-body speed density ECM.

The Cadillac northstar first generation ignition system was used on the following vehicles:

Brand	Vehicle	Year
CADILLAC	ALLANTE	1993
CADILLAC	DEVILLE	1996 – 1999
CADILLAC	DEVILLE CONCOURS	1994 – 1999
CADILLAC	DEVILLE DELEGANCE	1997
CADILLAC	ELDORADO	1994 – 1999
CADILLAC	ELDORADO ETC	1998
CADILLAC	ELDORADO SPORT	1993
CADILLAC	ELDORADO TOURING	1993 – 1997
CADILLAC	SEVILLE	1999
CADILLAC	SEVILLE SLS	1994 – 1998
CADILLAC	SEVILLE STS	1993 – 1998
OLDSMOBILE	AURORA	1995 – 1999

 

2. Cadillac Northstar DIS Theory & Operation

2.1) Overview

The Cadillac Northstar ignition system on a factory 1993-1999 Northstar engine is composed of an ignition module, four coils, a crankshaft trigger wheel, two variable reluctance crankshaft position sensors, and a cam sensor used for SFI operation by the PCM.  The two crankshaft position sensors are used to quickly determine crankshaft position for fast engine starting, know as “Fast-Start”.  Using two crankshaft sensors allows the ignition module to determine the engine position within 180 degrees of rotation of the crankshaft.  Another benefit of two crankshaft sensors is redundancy; in case one sensor fails, the module can operate off the remaining working sensors.  If you are running two crank sensors and a cam sensor any one sensor can fail, including one of the crank sensors and the module will continue to operate.  If you operate the module without a cam sensor both crank sensors are needed to maintain operation.

2.2) Crankshaft Trigger Wheel (reluctor ring)

The Northstar crankshaft has a trigger wheel (also called a reluctor ring) cast into the crankshaft between #3 and #4 main bearings.  Two crankshaft position sensors are installed into the side  of the engine block (facing front of car in FWD applications) between cylinders 4 and 6.  The trigger wheel has 32 notches, 16 large raised areas, and 16 small raised areas, as seen in Figure 1 and 2 below.

Figure 1: GM drawing of Northstar trigger wheel

Image from GM, Cadillac division.

 

Figure 2: Cadillac Northstar crank trigger wheel put into CAD

Note: Drawing not to scale, for accurate scaled version download the the CAD or PDF drawing below.

Each notch and small raised area is 1/3 the size of the large raised area.  In the GM drawing of the Northstart in Figure 1 you can see that they blacked out the notches between the small raised areas to create 24 even notches and large raised areas.  This is why GM considers this a 24x trigger wheel.  The eight uneven notches are used by the ignition module to determine crankshaft position to determine which cylinder is next in the firing order.

2.3) Northstar PCM & ICM wiring

Note: The firing order above is that of a GM northstar block and must be changed for SBC applications, see below

2.4) Crankshaft Position sensors

The two crankshaft position sensors used in the Northstar setup are variable reluctance sensors separated 27 degrees apart.  Each sensor is mounted within a tight tolerance of the trigger wheel (reluctor wheel), 0.05″ (1.3mm) on the factory engine.

2.5) Camshaft Position sensors

A camshaft position sensor is not needed on HEI ECM conversions because they are bank fire applications this is only needed for SFI PCM’s.
The camshaft position sensor is positioned over a dowel pin located on the exhaust camshaft cam gear.  One Off-On-Off signal pulse is created every camshaft rotation and every two crankshaft rotations.

2.6) Coils

The four coils used in the Northstar setup employ a waste spark firing method.  Waste spark firing means that two spark plugs are fired at one time thus lowering the number of coils needed and the cost.  When a coil fires of the two cylinders fired, one is on the compression stroke and one is on the exhaust stroke.

2.7) Ignition Control Module

The Northstar ignition control module is what processes the input from three sensors and the proper signal output to the ECM \ ECU \ PCM.
It performs the following functions and more:
– It monitors the On-Off pulses produced by the two crankshaft and one camshaft position sensors.
– It creates a 4X and 24X reference signal (4X REF HI and 24X Crank) sent to the PCM for ignition control.
– It creates a camshaft reference signal (CAM HI) sent to the PCM for fuel injection control.
– It provides a ground reference (REF LO, CAM LO) to the PCM.
– It provides a means for the PCM to control spark advance (BYPASS and IGNITION CONTROL) called IGNITION CONTROL MODE.
– It provides a limited means of controlling spark advance without PCM input called MODULE MODE.
– The IC module is not repairable. When a module is replaced the remaining components must be transferred to the new module.

 2.8) Ignition Synchronization

Ignition Control Module signal description
Single	Signal Name	Voltage Range	Description
A	Crankshaft position sensor ‘A’	+/- 100V AC	Voltage and frequency vary with engine speed
B	Crankshaft position sensor ‘B’	+/- 100V AC	Voltage and frequency vary with engine speed.  Signal lags crankshaft position sensor ‘A’ by 27 degrees
C	Camshaft position sensor	+/- 100V AC	Voltage and frequency vary with engine speedThis signal occurs every two crankshaft revolutions
D	4X Ref Hi	0 – 5V DC	Key on, Engine Off: Signal is high. High to low occurs at first sync, then follows firing order.
E	Bypass	0 – 5V DC	Key on, Engine Off: Signal is low. Low to High occurs with PCM ignition control.
F	Ignition control	0 – 5V DC	PCM sends ignition control signal to ICM.  The falling edge signals ICM to turn off coil primary current.
G	24X Crank	0 – 5V DC	This signal will remain high if the ICM identifies a fault with one of its three position sensorts
H	1/2x Cam Hi	0 – 5V DC	The PCM uses this signal for sequential (SFI) fuel control
I	Ground Ref.	0V DC	This is the ground reference between the PCM and ICM

1) The crankshaft reluctor ring has 24 evenly spaced notches plus eight additional notches (shaded) used for synchronization for a total of 32 notches.

2) As the crankshaft rotates, the notches pass the position sensors and create a voltage pulse signal in the sensor that is an input for the ignition control module (ICM).

3) Because of the physical location of the 2 crankshaft position sensors, the signal of B lags the signal of A by 27 degrees of crankshaft revolution.

4) To synchronize the ignition, the ICM first counts the number of B pulses between every 2 A pulses. There can be 0, 1, or 2 B pulses between A pulses.

5) When the ICM sees 0 B pulses between A pulses , it starts counting B pulses between A pulses. When the ICM counts exactly 4, it synchronizes the ignition on the very next A pulse. If the ICM counts over 4 (jumps from 3 to 5), it waits for another B pulse between A pulse to start counting again.

6) This process allows the ignition to synchronize and fire the first spark plug within 180 degrees (1/2 engine revolution).

7) The camshaft position (CMP) sensor provides the ICM with cylinder #1 firing order information, which the PCM uses for sequential fuel injection.

8) Using 3 sensors allows the ICM to maintain ignition synchronization even if one of the 3 sensors fails.

 

2.9) Firing Order:

To run the northstar ignition module on a SBC you will connect the wires to the coils as follows.

1’st Gen Northstar Firing order
1-2-7-3-4-5-6-8

1’st Gen SBC Firing Order:
1-8-4-3-6-5-7-2 looking at the front of the motor counting back: driver side (right) = 1-3-5-7 pass side (left) = 2-4-6-8

2.10) Ignition Control Module Pinouts:

I have created a column in the tables below to outline which pins would be used if using the northstar ignition module on a 1227730, 1227727, or pretty much any HEI distributor OBD-I GM car.

Connector 1 – Crankshaft Sensors
Pin	Used on HEI ECM	Function
A	Yes	Crankshaft sensor A high signal
B	Yes	Crankshaft sensor A low signal
C	No	Not Used
D	No	Not Used
E	No	Crankshaft sensor B low signal
F	No	Crankshaft sensor B high signal

 

Connector 2 – Power and Tachometer output
Pin	Used on HEI ECM	Function
A	Yes	Ground
B	Yes	Tachometer signal output
C	Yes	Ignition controlled battery voltage

 

Connector 3 – Cam sensor
Pin	Used on HEI ECM	Function
A	No	Camshaft sensor high output
B	No	Camshaft sensor low output
C	No	Not Used

 

Connector 4 – PCM \ ECM Output
Pin	Used on HEI ECM	Function
A	No	24x spark reference signal
B	No	1/2x cam signal
C	Yes	4x fuel control signal
D	Yes	Ignition bypass
E	Yes	Ignition control
F	Yes	Ignition reference low

 

3. Trigger Wheel Design & Fabrication

3.1) Crankshaft Trigger Wheel

I purchased a used Northstar crankshaft off eBay to get the measurements of the trigger wheel and create the wheel in CAD.  Here are the measurements of the Northstar trigger wheel:

Most measurements taken from the trigger wheel are giving in a range because measurements taken at different points of the wheel yielded different results, this is due to machining tolerances of the wheel.  The tolerances of the wheel do not need to be very close, the notch position and size is what is important so the module can figure out where the engine is in rotation.  Each of the 32 notches on the crank trigger wheel is about 15/64 (or 0.235 to be exact) of an inch in length and 3/16 – 13/64 of a inch deep. There are 16 small raised areas which are about the same length as the notches, they are about 7/32 (0.21 – 0.215) of a inch. There are 16 large raised areas which are about 3 times the length of the notches and measure in at 0.66 – 0.665 of a inch.  So calculating in a percent of error for machining tolerances, each notch and small raised area are the same length. The large raised area is three times the length of the notches.

The above broken down into degrees:
– 32 notches 3.75 degrees in length
– 16 small raised areas 3.75 degrees in length
– 16 large raised areas 11.25 degrees in length

The thickness of the trigger wheel is 9/32 – 5/16 of a inch thick, the thickness is different at different points around the wheel due to the fact that one side of the wheel is un-machined cast and the other is machined flat.. The diameter of the trigger wheel is about 6 & 15/16 inches.

3.2) Northstar Crankshaft Trigger Wheel CAD

Here is the AutoCAD file of the Northstar trigger wheel, this can be scaled in size as needed:
Northstar trigger wheel (AutoCAD File)
Northstar trigger wheel (PDF File, only for viewing purposes)

 

Page created: January 2006
Page content last updated: 06-13-2009