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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 28 Or 29, Gear Switch Circuits - 3.3L A Body With 4T60 >> 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 28 Or 29 Troubleshooting Diagram (3.3L A Body W/4T60)
G91D07319Courtesy of GENERAL MOTORS CORP.
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