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Home >> GMC >> 1990 >> Magnavan 7.4 N, 475/M40 >> Repair and Diagnosis >> Engine Performance >> Testing & Diagnosis >> Engine Controls - System/Component Tests >> Relays, Solenoids, Motors & Modules >> Downshift Control Relay (THM 400)
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Downshift Control Relay (THM 400)

  1. Disconnect wiring harness connector to downshift control relay. Measure resistance between relay terminals "D" and "F". See Fig 1. Resistance should be 20 ohms. If resistance is not as specified, replace downshift control relay.
  2. Connect ohmmeter between relay terminals "A" and "E". See Fig 1. No continuity should be present. If continuity is present, replace shorted downshift control relay.
Fig 1: Downshift Control Relay Circuit
G90D15738Courtesy of GENERAL MOTORS CORP.
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