CODE 27, 28, 29 GEAR SWITCH DIAGNOSIS

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steveX

CODE 27, 28, 29 GEAR SWITCH DIAGNOSIS

Post by steveX » Sun Apr 20, 2008 5:15 pm

CIRCUIT DESCRIPTION:The gear switch are located inside the transaxle. they are pressure operated switches, normally closed. The ECM supplies 12 volt through each selected circuit to the switch. As road speed increases, hydraulic pressure applies the specific gear clutches and the gear switch opens. At this time, the ECM monitors a high, 12 volt potential, and interprets this to indicate that gear is applied. The ECM uses the gear signal to control fuel delivery (and TCC).

TEST DESCRIPTION:Number(s) refer to circled number(s) on the diagnostic chart.
code 27 will set if:
* no code 29 present
* Circuit (orange wire) indicates ground or closed switch for 10 seconds when vehicle is in 4th gear operation.
* circuit (orange wire) indicates an open (drive) when the engine is first started.
* circuit (light blue wire) is open when the engine is first started.

code 28 will set if:
* circuit (green/white wire) indicates ground or close switch for 10 second when vehicle is in 4th gear operation.
* circuit (green/white wir) indicates an open (drive) when the engine is first stared.
* circuit (light blue wire) is open when the engine is first started.

code 29 will set if:
* circuit (light blue wire) indicates ground or closed switch for 10 seconds when vehicle is in 4th gear operation, TCC locked, brake not applied.
* circuit (light blue wire) is open when the engine is first started.

1. Must use a DVM (digital voltmeter). A test light will not light due to the ver low current being supplied by the ECM.

2. checks to see if circuit is ground through the switch.

3. Checks for a good, properly operating switch and checks circuit within transaxle for an improper ground.


DIAGNOSTIC AIDS:An intermittent may be caused by a poor connection, mis-routed harness, rubbed through wire insulation, or a wire broken inside the insulation.
Check for:
* Poor connection at ECM pins. inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminal to wire connection.
* mis-routed harness Inspect wiring harness to insure that it is not too close to high voltage wires, such as spark plug leads.
* Damaged harness Inspect harness for damage. if harness appears OK, "scan" while moving related connectors and wiring harness. A change in display would indicate the intermittent fault location.




SOURCE: GM SERVICE MANUAL.

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