testing & diagnosis diagnostic charts > code 42 ......1. est circuit no. 423 shorted to...

TESTING & DIAGNOSIS

Question: code 44 Lean Exhaust

Tip: LOSS OF POWER WITH TBI ENGINES

DIAGNOSTIC CHARTS > CODE 42 - ELECTRONIC SPARK TIMING > CIRCUIT

DESCRIPTION

When system is running on ignition module (no voltage on by-pass line), the ignition module

grounds the EST signal line. The ECM expects to see no voltage on the EST signal line during

this condition. If the ECM sees voltage on this line, Code 42 will be set and the system will not go

into "EST" mode.

When engine RPM for EST is reached (about 400 RPM), and by-pass voltage is applied, EST

should no longer be grounded in ignition module, so EST voltage should be varying.

If the by-pass line is open or grounded, the ignition module will not switch to "EST" mode. EST

voltage will be low and Code 42 will be set.

If the EST line is grounded, the ignition module will switch to EST, but because the line is

grounded, no EST signal will be present. Code 42 will be set.

Code 42 sets if there is an open, or a short to ground, in the EST or by-pass circuit.

DIAGNOSTIC CHARTS > CODE 42 - ELECTRONIC SPARK TIMING > TEST

DESCRIPTION

NOTE: Test numbers refer to test numbers on diagnostic chart.

1. Code 42 means the ECM has seen an open, or short to ground, in EST or by-pass

circuit. This confirms Code 42, and confirms that fault causing the code is also present.

2. Checks for normal EST ground path through ignition module. An EST circuit No. 423

shorted to ground will also read less than 500 ohms (will be checked later).

3. As test light voltage touches circuit No. 424, module should switch, causing ohmmeter

to "overrange" if meter is in the 100-200 ohms position. Selecting 10-20,000 ohms

position will indicate above 5000 ohms. The important thing is that the module

"switched".

4. The module did not switch and this step checks for:

Service Manual: CCC TBI TESTS W/CODES Print Date: 10/4/2017

1988 Chevrolet Pickup 7.4L Eng VIN N V30

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10/4/2017http://www2.prodemand.com/Print/Index?content=tabs&module=true&tab=true&terms=tr...

1. EST circuit No. 423 shorted to ground.

2. By-pass circuit No. 424 open.

3. Faulty ignition module connection or module.

5. Confirms that Code 42 is a faulty ECM and not an intermittent in circuit No. 423 or circuit

No. 424.

DIAGNOSTIC CHARTS > CODE 42 - ELECTRONIC SPARK TIMING >

DIAGNOSTIC AIDS

"SCAN" tester does not have any ability to help diagnose Code 42 problem.

See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this section.



Fig 1: Code 42, Electronic Spark Timing Schematic



Fig 2: Code 42, Electronic Spark Timing Flow Chart



Fig 3: Code 42, Electronic Spark Timing

WIRING DIAGRAMS



Fig 1: Astro & Safari (2.5L) Wiring Diagram



Fig 2: "S/T" Series (2.5L) Wiring Diagram



Fig 3: "S/T" Series (2.8L & 4.3L) Wiring Diagram



Fig 4: "M" Series: Astro & Safari (4.3L) Wiring Diagram



Fig 5: "G" Series Van (4.3L, 5.0L & 5.7L & 7.4L) Wiring Diagram



Fig 6: "C/K" Series (4.3L, 5.0L, 5.7L & 7.4L) Wiring Diagram



Fig 7: "R/V" Series (5.7L & 7.4L) Wiring Diagram



Fig 8: "P" Series Vans (5.7L) Wiring Diagram



testing & diagnosis diagnostic charts > code 42 ......1. est circuit no. 423 shorted to...

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