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Home >> GMC >> 1987 >> Forward Control 4.8 T, Standard >> Repair and Diagnosis >> Transmission >> Automatic Trans >> Overhaul - THM 400 >> Testing >> Downshift Solenoid Check
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Downshift Solenoid Check

  1. Place selector lever in "Park". Turn ignition switch "ON", but do not start engine. From under hood, slowly move throttle linkage to wide open position. One click should be heard from transmission.
  2. Allow throttle to return to closed position. One click should be heard from transmission. If clicks are heard, downshift solenoid is operating properly. If solenoid does not operate as outlined, go to step 3).
  3. Use test light to check Brown wire at connector on side of transmission case. Test light should light with throttle wide open and go out when throttle is released.
  4. If test light operates as described, but solenoid did not click during tests, replace solenoid after first check to ensure internal wiring is operational.
  5. If test light fails to light with throttle open, solenoid circuit is open. If light lights with throttle closed, solenoid circuit is shorted. Check transmission control fuse in fuse panel. Check solenoid circuit and repair open or short (Orange wire with Black stripe).
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