Ignition System - Hei-Est (tbi Models) GMC Cab & Chassis R2500

DESCRIPTION

The Delco-Remy High Energy Ignition (HEI) system with Electronic Spark Timing (EST) 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 9)and (Scheme 10).

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

Scheme 9

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.

Scheme 10

OPERATION

EST system consist of distributor module, and connecting wires. (Scheme 11) The EST distributor circuits perform the following functions

1. Ground Circuit. This wire is grounded in distributor and makes sure ground circuit has no voltage drop which could affect performance. If it is open, it may cause poor performance.
2. By-Pass Circuit. At about 400 RPM, 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 run at base timing with a small amount of advance built into module.
3. Distributor Reference Circuit. This circuit provides ECM with RPM and crankshaft position information.
4. EST Circuit. This circuit triggers module. The ECM does not know what actual timing is, but it does know when it gets reference signal. It then advances or retards spark from that point. Therefore, if base timing is set incorrectly, engine spark curve will be incorrect.

Scheme 11

TROUBLE SHOOTING

Note. See the TROUBLE SHOOTING - BASIC PROCEDURES article in the GENERAL TROUBLE SHOOTING section.

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.

Scheme 12

Primary Resistance

1. Set ohmmeter on high scale, attach leads as shown in step one. (Scheme 13) Reading should be infinity. If not, replace coil.
2. Remove coil connector. Using low scale, connect ohmmeter leads as shown in Step 2. Resistance should be zero or nearly zero. If not, replace ignition coil.
3. With ohmmeter set on high range, connect leads as shown in step 2. (Scheme 13) Meter should not read infinite (open circuit). If reading is infinite, replace coil.

Ignition Coil Resistance Test Points Remove all coil wires for testing. Courtesy of General Motors Corp. Scheme 13

DISTRIBUTOR PICK-UP COIL SHORT & RESISTANCE CHECKS

1. Disconnect pick-up coil leads from distributor. Connect one ohmmeter lead to either pick-up coil wire terminal and the other lead to distributor housing. (Scheme 14) 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 14) Resistance should read a constant unchanging value between 500 and 1500 ohms. If not, replace pick-up coil.

Scheme 14

HALL EFFECT SWITCH TEST

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

Scheme 15

1. Disconnect 3-wire connector at Hall Effect switch. Connect a 12-volt battery and voltmeter to switch terminals. (Scheme 15) Carefully note polarity markings.
2. Insert knife blade between magnet and Hall Effect switch. (Scheme 15) 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

1. Disconnect a spark plug wire and connect Spark Tester (ST-125). Check for spark while cranking engine. If no spark occurs, check for spark at another wire. Intermittent sparks are considered no spark. If no spark occurs, proceed to step 2). If spark occurs, problem is in fuel system or spark plugs.
2. Unplug 4-terminal EST connector. Check for spark at tester. If spark occurs, replace pick-up coil. If no spark occurs, check for spark at coil wire with Spark Tester (ST-125) while cranking engine. NOTE: Leave Spark Tester (ST-125) connected during remainder of tests.
3. If spark is present while cranking, inspect cap for cracks or other defects. If okay, replace rotor. If no spark occurs while cranking, disconnect distributor 2-wire "C/+" (coil) connector. Turn ignition on and check voltage at "C" and "+" terminals of harness connector.
4. If there is less than 10 volts at terminal "C" only. Check for open or ground in circuit from "C" to ignition coil. If circuit is okay, fault is with ignition coil or connections. If both terminals read less than 10 volts, proceed to step 5). If both terminals read 10 volts or more, proceed to step 6).
5. Repair wire from module "+" terminal to "B" terminal of Black ignition coil connector or primary circuit to ignition switch.
6. Reconnect distributor 2-wire connector. Check voltage from "TACH" lead to ground with ignition on.
7. If voltage is less than one volt in step 6), repair open tachometer lead or connector and repeat step 6). If voltage reading is 1-10 volts in step 6), replace module and check for spark as described in step 12). If spark is present, system is good. If no spark occurs, replace ignition coil.
8. If voltage is more than 10 volts in step 6), connect test light from "TACH" lead to ground. Crank engine and observe light. If light blinks, replace ignition coil and recheck for spark with Spark Tester (ST-125). If no spark, install original coil and replace module.
9. If light remains steady in step 8), unplug distributor 4-wire connector. With ignition on, remove distributor cap and retaining screw in 4-wire end of module.
10. Loosen remaining module screw and pivot module out to gain access to pick-up coil terminals. Connect jumper from module retaining screw eyelet to ground (Scheme 16) Disconnect pick-up coil from module. Connect voltmeter between "TACH" terminal and ground.
11. Insulate test light probe to 1/4" from tip. Note voltage as test light is momentarily connected between module terminal "P" and 1.5 to 8 volt source. (Scheme 16) To ensure against module damage, DO NOT allow test light to be connected for more than 5 seconds.
12. If there is no voltage drop. Ensure module is properly grounded. If good ground is noted, replace module. If voltage drops, check for spark at coil wire with Spark Tester (ST-125) when test light is removed from module terminal "P".
13. If spark is present in step 12). On all engines except 2.5L S/T series. Ensure rotating pole piece is still magnetized, check pick-up coil resistance and connections (coil resistance should be 500-1500 ohms and not grounded).
14. If rotating pole piece is not magnetized, replace pole piece and shaft assembly. If spark is present in step 12) on 2.5L S/T series engines, check pick-up coil resistance and connections (coil resistance should be 500-1500 ohms and not grounded).
15. On all engines, if no spark occurs in step 12) check module with Module Tester (J24642). Replace module if defective. Check coil wire between cap and coil. If wire is okay, replace coil.
16. If Module Tester (J24642) was not available in step 14), replace ignition coil and repeat step 9). If spark is present, system is okay. If no spark is present, original ignition coil was okay. Reinstall original coil and check coil wire. If coil wire is okay, replace module.

Scheme 16

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.

With engine operating at 2000 RPM, note timing change as test terminal is grounded. System is operating properly if timing changes.

All Engines Except 2.5L & 7.4L

1. Check computer memory for code 43. If code 43 is present See COMPUTERIZED ENGINE CONTROLS section for proper diagnostic testing procedure, perform that test first. If code 43 is not in memory proceed with this test.
2. This test should be performed after other causes of spark knock have been checked, such as engine timing, EGR systems, engine temperature or excessive engine noise.
3. Connect "SCAN" tool to computer assembly line diagnostic link (ALDL) connector under dashboard. Set "SCAN" tool on knock signal. Run engine to 1500 RPM.
4. If there is a knock signal indicated in step 3), disconnect knock sensor and repeat test. If there now is no knock signal indicated, their is an internal engine knock or faulty sensor. If there is a knock signal indicated, check for routing of wire from knock sensor to ESC module for picking up false knock signals from an adjacent wire. Reroute as necessary. If routing is correct, replace ESC module.
5. If "SCAN" tool indicated knock signal in step 3), ESC system is working properly. If "SCAN" tool does not indicate knock signal, disconnect ESC module. Probe ground circuit at module connector (Brown wire) with a test light to 12 volts.
6. If test light does not glow, repair open ground circuit. If test light glows, reconnect ESC module. Disconnect knock sensor harness. Touch harness connector (Dk. Blue wire) with a test light to 12 volts. Each time test light contacts the circuit, a knock signal should be generated.
7. If knock signal is not indicated with "SCAN" tool, knock sensor harness (Dk. Blue wire) is open, shorted to ground or has a faulty ESC module. If knock signal is indicated with "SCAN" tool, connection to sensor or knock sensor is faulty.

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.

See also:
• TROUBLE SHOOTING - BASIC PROCEDURES