EXPERTEC TSB (40-7)

CIRCUITRY DIFFERENCES HEI-EST vs DISTRIBUTORLESS IGNITION SYSTEMS (Section No. 6D)

When you are trying to cope with a new technology, it is often helpful to compare the similarities between the old and the new. This is especially true for distributorless ignitions. If you remember what carries over from distributor - type systems, then all you need to learn are the differences.

The standard Oldsmobile distributor - type system, HEI-EST, has a long history. The system has been used in Oldsmobiles for over a decade and continues to be used in 1990 Custom Cruisers and Silhouettes. You are probably already familiar with the ignition circuits that make up the four-wire interface between the ECM and ignition module:

• Reference low or "ground"
• Reference high or "reference"
• Bypass
• EST

With few exceptions, the operation of these circuits is the same in HEI-EST as in the distributorless ignition systems -- C3 1, DIS, and IDI. In fact, the wire colors, circuit names, and circuit numbers for these wires are also the same for most distributorless and HEI-EST systems. The C3I system on 3300 engines varies only slightly: The EST line may be white/black instead of plain white. Also, the C3I system used with the 3800 engine and the IDI system used with 1989 and 1990 Quad 4 engines have some circuits that you may not be familiar with.

In other words, you probably already understand most of the operation of the various distributorless ignition system if you remember what these circuits do in HEI-EST.

Reference Low or Ground (CKT 453 BLK/RED) Simply provides a common ground level between the ignition module and ECM.

This allows both components to accurately read the same voltages and times. Note that although this circuit is electrically connected to the ECM, it is not connected to ground through the ECM. The wire is connected to ground only through the ignition module. If this circuit is open or grounded, it may cause poor engine performance and possibly an SES light with no trouble code.

References High or Reference (CKT 430 PPL/WHT) Sends a digital voltage signal to the ECM from the ignition module. This reference is based on the signal received from the crankshaft sensor. The ECM used the ignition module reference signal to determine engine RPM and crankshaft position. It also used this signal to initiate fuel injection and to calculate dwell time and spark advance.

If the ECM receives no signal pulses on this circuit, no fuel injection pulses will occur. Thus, an open or ground in this circuit will stall a running engine and cause a "cranks but won't run" condition.

Bypass (424 TAN/BLK)

Provides a 5 volt ECM input to the ignition module to switch spark timing control from the ignition module to the ECM. When 5 volts are not present on this circuit, the ignition system operates in the "bypass" or "module" mode.

In this mode, the reference high signal is used by the 'ECM for fuel injection, but not for spark timing. EST signals are sent on CKT 423 from the ECM to the ignition module, but are grounded by the module without being used. Thus, the ignition module alone -- with no input from the ECM -- operates the coils and fires the spark plugs with a set degree of timing advance.

An open or grounded bypass control circuit will set an ECM code 42 and cause the ignition system to operate in the bypass or "module" mode. Bypass mode on all C3I systems causes ignition operation with fixed spark timing. As a result, customers may note an SES light and a lack of vehicle performance and fuel economy.

EST (CRT 423 WHT)

Carries the signal from the ECM to the ignition module to allow the ECM to control spark timing. This occurs once a certain RPM is reached -- 400 to 700, depending on the engine - - when the ECM switches ignition operation from bypass to EST mode, by raising voltage on the bypass circuit from zero to five volts. This causes the ignition module to stop grounding the EST signals and use them to time the energizing of the coils and the firing of the spark plugs. The ECM varies spark timing based on the reference high signal from the module and on other inputs from its various data sensors.

If the EST circuit is open or grounded when the engine is started, an ECM code 42 will set and the ignition system will stay in the module mode. If the circuit becomes open or grounded during EST mode operation, the engine may stall but will restart, an ECM code 42 will set, and the ignition system will operatc in the module mode. Again, customers may note an SES light and a lack of vehicle performance and fuel economy as a result of fixed timing in the module mode.

As for those exceptions, here are the circuits that may be unfamiliar: 1989 & 1990 IDI system, Quad 4 engines On these engines you will find three "reference" circuits. One of those circuits is the reference low and is the same as the reference low in HEI-EST and other distributorless systems. The second is the 2x reference. The 2x reference is the same as the reference high (CKT 430 PPL/WHT) circuit we have already looked at. It is called the 2x reference because its signal pulses two times per crankshaft revolution in response to two of the notches on the crankshaft reluctor (the first and fourth notches). A fault in this 2x reference will cause the same symptoms as a fault in the standard reference high circuit.

The third reference circuit is the 1x reference. The 1x reference (CKT 969 BLK/LT GRN) provides a signal to the ECM to tell it when to start Alternating Synchronous Double Fire (ASDF) fuel injection. Its signal, as you might have guessed, pulses once per crankshaft revolution in response to a single notch on the reluctor -- the sync notch. A fault in this circuit will cause the ECM to set a code 41 and revert to simultaneous fuel injection. Simultaneous injection may result in a loss of performance.

C3I (Type 1-Fast Start System), 3800 Engines

In this system, you will find high resolution, fuel control, and reference low circuits. The reference low is the same as all other reference low circuits. The high resolution or 18x signal (CKT 647 LT BLU/BLK) pulses 18 times per crankshaft revolution. Its voltage pulses are generated as the 18 evenly spaced blades in the outer crankshaft sensor interruptor ring pass the dual crank sensor. The 18x signal allows the ECM to measure engine RPM more accurately, particularly at low engine speeds.

The fuel control circuit (CKT 430 PPL/WHT) serves the same function as the standard reference high circuit, though the signal itself is generated differently. Instead of having a direct relationship to a number of blades or notches passing a sensor, the ignition module creates the fuel control signal by dividing the 18x signal by six and sending the three resulting pulses to the ECM to control the fuel injectors. The 18x signal is divided by six because only three fuel injectors are pulsed per crankshaft revolution. Any faults in this circuit will result in the same symptoms previously described for reference high circuit faults.

Also note these similarities between HEI-EST and distributorless systems:

Distributorless ignition systems do, of course, use different sensors from the ones used in HEI-EST systems, but they work the same way. The magnetic crank sensors function like the pickup coils used in some HEI-EST distributors, and the Hall effect crank and cam sensors function like the Hall effect switches used in other HEI- EST distributors.

This summary of how distributorless ignition systems compare with HEI-EST systems should give you a head start in understanding and diagnosing them.

Specific training on distributoriess ignition systems is available from Service ALERT and at the GM Training Centers. When working with a new technology, look for the similarities between the old and the new.