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Home >> GMC >> 1991 >> Value Van 7.4 N >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Diagnosis - A/T >> Diagnostic Flow Charts (Cars) >> Code 27, 28 Or 29 Gear Switch Circuits - 3.8L >> Diagnostic Aids

Diagnostic Aids

Check for poor connections at ECM pins. Inspect harness for incorrect routing (too close to high voltage wiring) or chafing. Monitor the voltage of each terminal while moving related harness connectors. If the failure is induced, the voltage reading will change.

Fig 1: Code 27, 28 Or 29 Troubleshooting Diagram
G91F07320Courtesy of GENERAL MOTORS CORP.
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