HIGH ENERGY IGNITION (HEI)
The High Energy Ignition (HEI) system uses an iron reluctor, pickup, and ignition module to replace the breaker points of yesteryear.
The iron reluctor ("star wheel") is mounted on the distributor shaft. The reluctor has one "tooth" per cylinder, hence a four cylinder engine has four teeth, a six cylinder engine has six teeth, and an eight cylinder engine has eight teeth.
When the rotating reluctor lines up with corresponding iron pieces on the stationary pickup, an alternating current is produced.
The ignition module senses the alternating current created in the pickup and releases the ground of the ignition coil, thereby causing the secondary to fire. After the firing event, the ignition module restores the ground to the coil to saturate it for the next firing event.
Timing is provided by the conventional vacuum and centrifugal advance tandem.
There is no four-wire connector on an HEI distributor.
HEI WITH ELECTRONIC SPARK TIMING (EST)
High Energy Ignition (HEI) with Electronic Spark Timing (EST) differs from plain HEI by discarding the traditional centrifugal and vacuum advance components in favor of more-precise ECM control.
The HEI-EST system uses a 4-wire link between the distributor and ECM to communicate control data.
The ECM monitors all relevant sensors to compute the best advance or retard based on current engine conditions. For instance, the Coolant Temperature Sensor input causes timing advance when the engine is cold, with progressive retardation as the engine warms up. If the engine overheats, spark is retarded to prevent detonation.
The Throttle Position Sensor causes timing advance during light throttle operations.
The Manifold Absolute Pressure sensor causes less spark advance when output voltage is low (high vacuum), and more advance when output voltage is high (low vacuum).
Note. On distributors with integral coil, the 7-digit distributor part number is stamped on the lower skirt of the distributor housing. On distributors with separate coil, the part number is located on a label on the side of the distributor cap.
HEI-EST WITH ELECTRONIC SPARK CONTROL (ESC)
Electronic Spark Control (ESC) adds detonation detection and correction functionality to the HEI-EST system. This allows maximum performance due to aggressive spark advance schedules.
No modifications are made to the distributor or ignition module. Rather, a detonation sensor or detonation sensor/detonation module combination is wired to the ECM and is treated like another sensor. When the ECM detects detonation, timing is retarded accordingly.
HEI WITH ELECTRONIC SPARK CONTROL (ESC)
Electronic Spark Control (ESC) adds detonation detection and correction functionality to the base HEI system. This allows maximum performance due to aggressive spark advance schedules from the vacuum and centrifigul advance devices.
The HEI ignition module is modified to communicate with an external detonation module via a four-wire connection. The external detonation module utilizes a detonation sensor and possibly a "tip-in" vacuum sensor. The latter, when used, provides extra spark advance during acceleration (tip-in) conditions.
The detonation module is an integral part of the ignition module triggering loop. If the four-wire connector is unplugged, a no-spark condition will result.
Beware of mistaking the four-wire connector for an EST connector. If unsure, look for a vacuum advance along with the four-wire connector. If one is found, it's an HEI-ESC system. Otherwise, it's an HEI-EST-ESC system.
Scheme 1
Scheme 2
HEI-EST (EXCEPT "S" & "T" TRUCK W/2.5L)
The following section describes the operation of the four circuits (four-wire connector) used to communicate data between the ECM and ignition module. It performs the following functions
Terminal "A" - REF LOW
Terminal "A" of the 4-wire connector is the reference ground low. It is grounded in the distributor and ensures ground circuit does not have a voltage drop. If circuit is open, engine may experience poor performance.
Terminal "B" - BYPASS
Terminal "B" of the 4-wire connector is the by-pass circuit. At about 400 RPM, the ECM applies 5 volts to this circuit to switch spark timing control from module to ECM. An open or grounded by-pass circuit will set a Code 42 and the engine will operate at base timing, plus a slight amount of advance built into the module.
Terminal "C" - REF
Terminal "C" is the distributor reference High circuit. This circuit provides the ECM with RPM and crankshaft position information.
Terminal "D" - EST
Terminal "D" is the EST circuit, which triggers the module. The ECM does not know what actual timing is, but does know when it receives the reference signal. It will advance or retard the spark from that point. If base timing is set incorrectly, engine spark curve will be incorrect.
HEI-EST (ONLY "S" & "T" TRUCK W/2.5L)
The following section describes the operation of the four circuits (four-wire connector) used to communicate data between the ECM and ignition module. It performs the following functions
Terminal "A" - EST
Terminal "A" of the 4-wire connector is the EST (Electronic Spark Timing) circuit. It carries ignition timing instructions from the ECM to the ignition module.
The EST waveform is a lower-amplitude, squared off copy of the REF waveform (terminal B) that is simply shifted forward or backwards in time to command timing advance or retard.
The ignition module ignores all EST data until the ECM sets the BYPASS high at 5 volts (terminal C). Once the BYPASS is locked at 5 volts, the ignition module relies exclusively on the EST to fire the ignition coil. The engine will stall if the EST signal fails/opens when the BYPASS is high. If the BYPASS is low, the engine will run without the EST circuit.
Terminal "B" - REF
Terminal "B" of the 4-wire connector is the REF (ignition REFerence) circuit. It carries engine speed/crankshaft position information directly from the hall effect switch to the ECM. The REF signal is not created by the ignition module nor passes through it.
The waveform is a typical hall effect pattern, with high-to-low switches mirroring the shutter wheel passing through the hall effect. Amplitude is under .5 volts during the low state, and near battery voltage during the high state.
The REF signal is the most important "sensor" on the engine. Without this input, the ECM would never know when the engine was rotating, hence no injector pulse nor RPM reading on a scan tool during engine cranking.
Terminal "C" - BYPASS
Terminal "C" of the 4-wire connector is the BYPASS circuit. The BYPASS signal originates from the ECM and is sent to the ignition module. When the BYPASS is low (0 volts), the ignition module uses the in-the-distributor reluctor wheel (not the hall effect switch) to set the timing/fire the coil. When it is high (5 volts), the ignition module is commanded to use the EST to set the timing/fire the coil.
Normally, BYPASS is fixed low during cranking and fixed high high while the engine is running. The ECM changes the state from low to high when the engine speed exceeds approximately 400 RPM.
On some models, the BYPASS wire contains an additional single-wire connector located someplace between the 4-wire connector and the ECM. This is disconnected when checking or adjusting base timing.
Terminal "D" - REF LOW
Terminal "D" of the 4-wire connector is the REF LOW (ignition REFerence LOW) circuit. It provides the ECM with the reference ground of the distributor. This allows the ECM to baseline the REF signal and correct for any minor voltage drop errors.
The waveform should be a nearly horizontal line at 0 volts with only minor voltage fluctuates (from ignition coil operation).
HEI-ESC SYSTEM
The detonation sensor translates mechanical vibration into an AC voltage. This voltage typically runs on top of an DC reference voltage provided by the external detonation module.
The detonation sensor translates every vibration into AC output voltage. The stronger the vibration, the higher the amplitude. Detonation creates the strongest output.
The detonation module ignores all signals below a certain threshold as none-detonation noise, but retards timing for all signals above the threshold (detonation).
Beware of clearance in the valve train, or a loose trailer on a hitch, as they may create sufficient vibration to be considered as detonation.
If a tip-in hesitation is occurring and a tip-in sensor is being used, be sure to check it for proper operation. It works by triggering the detonation module to provide extra advance when the engine vacuum falls below an unspecified range.
As noted earlier in the DESCRIPTION section of this article, the engine will not run without the four-connector and detonation module connected, and both modules working correctly.
For a no-start situation, the detonation module can be eliminated from the circuit by unplugging the four-wire connector at the distributor and jumping terminals "A" to "C" on the distributor side of the connection. If spark returns, the detonation module or related wiring, power, or ground is faulty.
HEI-EST-ESC SYSTEM
The detonation sensor translates mechanical vibration into an AC voltage. This voltage typically runs on top of an DC reference voltage provided by the external detonation module or ECM for diagnostic purposes.
The detonation sensor translates every vibration into AC output voltage. The stronger the vibration, the higher the amplitude. Detonation creates the strongest output.
As noted earlier in the DESCRIPTION section of this article, the detonation sensor either communicates directly to the ECM or runs through an intermediary "detonation module".
In either case, the detonation module or ECM ignore all signals below a certain threshold as none-detonation noise, but retards timing for all signals above the threshold (detonation).
The ECM can retard timing directly, but the external detonation module must signal the ECM through a single-wire communications circuit. The ignition module leaves nine volts on the circuit to communicate no detonation, and grounds it to communicate detonation. The ECM responds appropriately.
If a retarded timing-type driveability problem is occurring, be sure to unplug this circuit to see if the problem goes away. If so, either the detonation module is falsely triggering a detonation condition, or the circuit is shorted to ground.
Beware of clearance in the valve train, or a loose trailer on a hitch, as they may create sufficient vibration to be considered as detonation.
ALL HEI/HEI-EST (EXCEPT "S" & "T" TRUCK W/2.5L)
If EST signal is interrupted and the BYPASS is high, the engine will stall. If a restart is attempted, the engine will restart and run up to approximately 400 RPM and stall again.
If EST signal is interrupted and the BYPASS is low, the engine will run on base timing with only a slight advance provided via the ignition module. A lack-of-power is very apparent, as well as a likely engine overheat due to the retarded timing.
HEI/EST ON "S" & "T" TRUCK W/2.5L ONLY
The ignition system used on this vehicle is an unusual implementation of HEI/EST and can cause misdiagnosis if the differences are not understood.
This system has both a reluctor/pickup (e.g. PM generator) and a hall effect. The ONLY time the PM generator is used is during cranking. It is ignored after the ECM switches the BYPASS high at approximately 400 RPM. To re-emphasize, the PM generator is ignored after the BYPASS is switched high.
The hall effect switch communicates directly to the ECM to communicate engine speed/crankshaft position information. The ignition module has absolutely no part in this communication; the hall effect is hardwired only to the ECM.
If there is a failure in one of the sensors, interesting combinations can occur. For example, if the PM generator fails, spark will be absent during cranking though injector pulse is present. If the hall effect fails, spark will be present while injector pulse is absent (during cranking). If both fail, both spark and injector pulse will be absent.
HEI-ESC
Beware of clearance in the valve train, or a loose trailer on a hitch, as they may create sufficient vibration to be considered as detonation.
If a tip-in hesitation is occurring and a tip-in sensor is being used, be sure to check it for proper operation. It works by triggering the detonation module to provide extra advance when the engine vacuum falls below an unspecified range.
As noted earlier in the DESCRIPTION section of this article, the engine will not run without the four-connector and detonation module connected, and both modules working correctly.
For a no-start situation, the detonation module can be eliminated from the circuit by unplugging the four-wire connector at the distributor and jumping terminals "A" to "C" on the distributor side of the connection. If spark returns, the detonation module or related wiring, power, or ground is faulty.
The ECM can retard timing directly, but the external detonation module must signal the ECM through a single-wire communications circuit. The ignition module leaves nine volts on the circuit to communicate no detonation, and grounds it to communicate detonation. The ECM responds appropriately.
If a retarded timing-type driveability problem is occurring, be sure to unplug this circuit to see if the problem goes away. If so, either the detonation module is falsely triggering a detonation condition, or the circuit is shorted to ground.
Beware of clearance in the valve train, or a loose trailer on a hitch, as they may create sufficient vibration to be considered as detonation.
ELECTRONIC MODULE (HEI)
Note. Testing applies to HEI systems with mechanical weights and vacuum advance only.
- An approved electronic module tester must be used to test the module. Use Module Tester (J-24642-E). Follow manufacturer's instructions.
- When installing a new HEI control module, use silicone lubricant on module-to-distributor housing contact surface to assist heat dispersement.
IGNITION COIL (HEI)
- Connect an ohmmeter between the tach terminal and battery terminal. See Meter "A" in (Scheme 3). Use the high resistance scale. If ohmmeter does not indicate zero or nearly zero, replace coil.
- Connect ohmmeter between the cap button and negative terminal. See Meter "B" in (Scheme 3). Use the high resistance scale. Record reading. Connect ohmmeter between cap button and tach terminal. Record reading. If both readings are infinite, replace coil.
Scheme 3
PICK-UP COIL (HEI)
Note. Activation of the vacuum advance may align trigger wheel tooth and pick-up coil pole piece causing ohmmeter pointer to deflect. This deflection should not be diagnosed as a faulty pick-up coil.
- Isolate 2 pick-up coil lead wires. Remove pick-up coil connector from module. Connect ohmmeter to either terminal and ground. (Scheme 4) Connect pump and apply vacuum to test vacuum advance unit. Replace vacuum advance unit if inoperative. Connect ohmmeter to pick-up coil terminals. Operate vacuum pump and observe ohmmeter throughout the vacuum range.
- Attach ohmmeter to one pick-up coil terminal and distributor housing. Set ohmmeter to middle scale. Operate vacuum advance throughout vacuum range. Reading should be infinite at all times. If not, replace pick-up coil. See Meter "A" in (Scheme 4).
- Connect ohmmeter leads to pick-up coil connector terminals. Operate vacuum pump to ensure proper operation throughout vacuum range. Flex terminal wires by hand to check for possible intermittent defects in wiring or connectors. Pick-up resistance should be 500-1500 ohms. If resistance is incorrect, replace pick-up coil. See Meter "B" in (Scheme 4).
Scheme 4
CAPACITOR (HEI)
The capacitor is used for radio noise suppression. Set ohmmeter at x1000 scale. Disconnect capacitor. Touch ohmmeter leads to capacitor terminal and ground. Slight needle movement will occur rapidly and return to infinity. A continuous reading other than infinity indicates defective capacitor.
External Mounted Coil (Sealed Module Connector Distributor)
Remove coil connectors and secondary coil wire. In test "A", use high ohmmeter scale. (Scheme 5) If continuity is present, replace coil. In test "B", use low ohmmeter scale. Reading should be very low or near zero. If not, replace coil. In test "C", use high ohmmeter scale. If there is no continuity, replace coil.
Scheme 5
Integrally Mounted Ignition Coil
- Turn ignition off. Remove the distributor cap and coil assembly. Turn upside down. (Scheme 6) Set ohmmeter to low scale. Connect leads to coil "BAT" and "TACH" terminals. If resistance exceeds one ohm, replace ignition coil.
- Set ohmmeter on high scale. Connect one lead to coil secondary terminal and the other lead first to "TACH" terminal and then to ground terminal. If resistance reading in BOTH instances is infinity, replace ignition coil.
Scheme 6
PICK-UP COIL SHORT & RESISTANCE CHECKS (HEI-EST)
- Disconnect pick-up coil leads from HEI/EST module terminals "N" and "P". Set ohmmeter to middle scale and connect one lead to either pick-up coil lead and the other lead to distributor housing. (Scheme 7) Flex pick-up coil leads by hand to check for intermittent shorts to ground. Reading should be infinity at all times. If not, replace pick-up coil.
- Connect ohmmeter between both pick-up coil leads. Check for intermittent opens by flexing wires and connectors. Resistance should be 500-1500 ohms. If not, replace pick-up coil.
Hall Effect Switch
Connect a 12-volt battery and voltmeter. (Scheme 9) With a knife blade inserted and held against the magnet, the voltmeter should read within 0.5 volts of the battery voltage. If not, replace the switch. Remove the knife blade. The voltmeter should read less than 0.5 volts. If not, replace the switch.
Scheme 7
Scheme 8
Scheme 9
Note. Diagnosis of HEI systems with EST and EST/ESC require thorough understanding of Computer Command Control (CCC) system. For testing, see appropriate IGNITION SYSTEM CHECK flow chart in COMPUTERIZED ENGINE CONTROLS section. For diagnosis of HEI system, refer to following diagnostic chart. (Scheme 10)
Scheme 10
OVERHAUL
| CAUTION | Although similar in appearance, components of HEI/EST and HEI distributors are NOT interchangeable. |
Ensure pick-up assembly arm is correctly installed on pin. If not, arm can float and cause ignition timing to vary. To prevent corrosion, ensure module terminals are lubricated with petroleum jelly before installation. To prevent heat damage, coat bottom of module and module rest pad in housing with silicone grease. Before installation of roll pin in driven gear, ensure timing mark on roll pin and rotor tip align. See
- (Scheme 11) for HEI-EST w/Non-Integral Coil
- (Scheme 12) for HEI w/Integral Coil
- (Scheme 13) for HEI-EST w/Non-Integral Coil & Hall Effect
