Ignition System - Hei-Est (calif) GMC Value Van P2500

DESCRIPTION

The Delco-Remy HEI-EST system is controlled by the Computer Command Control (CCC) system. It is designed to provide optimum performance through electronic control of spark timing, air/fuel ratios and idle speed.

The system consists of an Electronic Control Module (ECM), HEI-EST distributor, external ignition coil, and wiring harness. The distributor does not use vacuum or centrifugal advance. (Scheme 11)and (Scheme 12).

The distributor contains a 7-terminal HEI-EST electronic module, timer core, pole piece, pick-up coil, and a radio noise suppression capacitor.

HEI-EST Distributor With Sealed Connector Ignition coil is external (not shown). Scheme 11

The distributor is integrated to the EST system by a 4-wire connector, leading to the Electronic Control Module (ECM). The ECM receives voltage signals from various sensors. A typical system is provided signals from oxygen, engine coolant temperature, throttle position, barometric pressure and manifold absolute pressure sensors.

HEI-EST Distributor Without Sealed Connector Timer core and pole piece have one tooth per cylinder. Scheme 12

OPERATION

During cranking or EST failure, a by-pass signal from ECM terminal 11 to HEI module terminal "B" is either absent or low. (Scheme 13) This notifies the HEI module to ignore EST information coming from the ECM. During this period, poor engine performance may result but, the diagnostic "CHECK ENGINE" light will not come on.

The HEI module transmit RPM signals through the by-pass switch and terminal "C" directly to the negative "TACH" terminal of the ignition coil. These signals cause electrical pulses in the ignition coil primary circuit creating high voltage surge by the secondary windings to fire the spark plugs.

The Programmed Read Only Memory (PROM) portion of the ECM carries the basic spark advance curve based on engine RPM. Spark timing is calculated by the ECM whenever an ignition pulse is present. Spark advance information is sent to the distributor when the engine is running (not during cranking). Engine sensor values are used by the ECM to modify the PROM information, increasing or decreasing spark advance to achieve maximum performance with minimum emissions. The coolant temperature switch informs the system when engine is cold so that ignition timing can be advanced. Advance will be reduced as engine reaches normal operating temperature. As the engine reaches high temperature, timing is retarded to prevent detonation.

The ECM is continually computing sensor information to maintain efficient engine performance with low emission levels. The system must function under constantly changing engine conditions.

Circuit Diagram of HEI-EST Ignition System. Scheme 13

TESTING

Note. Diagnosis of EST systems requires thorough understanding of Computer Command Control (CCC) system. For further information, see GENERAL MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE CONTROLS section.

PRE-TESTING DIAGNOSIS

Normally, 5-15 seconds after starting a warm engine, the by-pass signal electronically operates a by-pass switch in the HEI module. The module's RPM-controlled timing signal will no longer be able to flow directly to the ignition coil. It is instead diverted to the ECM for modification by information from engine sensors.

If the reference or EST signals are interrupted due to open wires or a faulty ECM, the vehicle can still operate. The HEI module will provide a timing signal based on engine RPM. If the by-pass signal is lost, the ECM cannot control timing. The by-pass switch will permit direct flow of information to the ignition coil rather than to the ECM. (Scheme 17)

Loss of the EST signal with the by-pass signal activated will stop the engine, as the HEI module will no longer be able to send signals directly to the ignition coil, but to the ECM. A lost EST signal interrupts flow to the coil. If an attempt is made to restart the vehicle, the engine will run for a few seconds and then stop when the by-pass signal is activated.

HEI-EST System Component Location. Scheme 14

Primary Resistance

1. Remove coil connector. Using the low scale, connect ohmmeter leads to battery and "TACH" terminals. (Scheme 15) Resistance should be zero or nearly zero. If not, replace ignition coil.
2. Set ohmmeter on high scale and attach leads to battery terminal and coil ground strap. Reading should be infinity. If not, replace coil.

Secondary Resistance

With ohmmeter set on high range, connect leads to "TACH" terminal and to secondary terminal. Reading should be less than infinite (approximately 6000 to 30,000 ohms). If reading is infinite, replace coil.

Ignition Coil Resistance Test Points Remove all coil wires for testing. Scheme 15

DISTRIBUTOR PICK-UP COIL SHORT & RESISTANCE CHECKS

1. Disconnect pick-up coil leads from HEI module "N" and "P" terminals. Set ohmmeter in middle range. Connect ohmmeter leads to pick-up coil wire terminal and distributor housing. Reading should be infinite. If not, replace pick-up coil.
2. Connect ohmmeter leads to both pick-up coil leads. Flex wires and connectors to inspect for intermittent opens. (Scheme 16) Resistance should read a constant unchanging value between 500 and 1500 ohms. If not, replace pick-up coil.

Pick-Up Coil Resistance Checks. Scheme 16

HALL EFFECT SWITCH TEST

Note. Hall Effect switches cannot be checked with an ohmmeter.

Scheme 17

1. Disconnect 3-wire connector at Hall Effect switch. Connect a 12-volt battery and voltmeter to switch terminals. (Scheme 17) Carefully note polarity markings.
2. Insert knife blade between magnet and Hall Effect switch. (Scheme 17) Voltmeter should read within.5 volts of battery voltage. With knife blade removed voltmeter should read 0-.5 volts. If voltage readings are not as specified, replace Hall Effect switch.

IGNITION SYSTEM CHECK

Note. If tachometer is connected to ignition coil "TACH" terminal, remove before performing tests. Use a digital voltmeter with 10 megohms impedance or higher.

1. Remove spark plug wire and attach Test Plug (J-26792) to wire. Crank engine and check for spark. If spark occurs, ignition system is not at fault. Inspect fuel system or spark plugs.
2. If no spark occurred, separate 4-pin connector at distributor. Connect terminals "A" and "C". If engine starts, proceed to step 5). If engine does not start, reconnect 4-pin connector. With ignition switch in the "ON" position, measure voltage at terminals "F" and "K" of the 10-pin connector.
3. If reading is under 11.6 volts, locate and repair excessive resistance in system. If voltage is over 11.6 volts, set volt meter on 2-volt scale to probe circuit from pin "J" to "K" while cranking engine. If voltage is.75 volts or higher, system is okay.
4. If there is no voltage, attach voltmeter probes to pins "G" and "H". Set scale on 2-volt range and crank engine. If meter reads 200 millivolts or more, replace the ECM unit. If less than 200 millivolts, replace pole piece.
5. If engine started in step 2), stop engine and attach high resistance test lamp from tachometer terminal to ground. If lamp lights, crank engine. System is okay if lamp flickers. If not, attach test lamp from ignition coil positive terminal to pin "A" of distributor 4-pin connector.
6. Coil should "click" when tapped lightly. If not check pole piece connection for good contact. If pole piece connections are good and coil does not "click" when tapped, replace distributor module.
7. If module is not replaced, separate pick-up coil connector from module and turn ignition on. Check voltage at module terminal "C". Watch voltmeter as a test light is momentarily connected (not more than 5 seconds) between battery positive terminal and module terminal "P".
8. If no voltage drop occurs, check for open in wiring between coil and distributor and module ground. If okay, replace module. If voltage dropped, check for spark at tester gap (attached to coil secondary terminal) when test light is removed from module "P" terminal. If spark occurs, replace pick-up coil. NOTE: Perform DISTRIBUTOR PICK-UP COIL SHORT and RESISTANCE CHECKS if not done previously. (Scheme 16)
9. If no spark occurs, use module tester to check distributor module. Replace module if defective. If okay, check ignition coil ground and wiring to distributor. If okay, replace ignition coil.
10. If module tester is not available, replace module and repeat step 9). If spark results, system is okay. If spark does not result, original module is okay. Reinstall module and replace coil.

EST EMISSION SYSTEM CHECK

Note. When test terminal is grounded, the ignition timing will adjust to a fixed position. This will usually cause a timing adjustment that can be verified by a change in engine RPM.

1. With engine operating at fast idle, note timing change as test terminal is grounded. System is operating properly if timing changes. If timing does not change, proceed to step 2).
2. Disconnect park/neutral switch and recheck for timing change. If timing changes, problem is either an improperly adjusted or faulty park/neutral switch. If timing does not change, check for grounded wire from terminal "H" of ECM to park/neutral switch. If wire is not grounded, replace ECM.

Note. See COMPUTERIZED ENGINE CONTROLS section for additional information.

CCC EST or Bypass Test Procedure. Scheme 18

Disassembly

1. Remove coil connector and disconnect 4-wire EST connector. Remove distributor cap and rotor. Separate pick-up coil leads from HEI module. Mark distributor reassembly reference and remove. Drive out roll pin and remove distributor shaft from housing.
2. Remove retaining "C" washer, pick-up coil, magnet and pole piece. Remove 2 module attaching screws and capacitor screw. Lift module, capacitor and harness assembly from distributor housing. Disconnect wiring harness from module.

Reassembly

Assemble in reverse order of disassembly noting the following. Clean distributor housing and module. Apply silicone grease between module and housing. Spin shaft to confirm timer core external teeth do not touch pole piece internal teeth.

SPECIFICATIONS

DISTRIBUTOR - AUTOMATIC ADVANCE SPECIFICATIONS (1) (5)

DISTRIBUTOR - VACUUM ADVANCE SPECIFICATIONS (1) (5)