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MR2 SMT, NEED HELP

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#repair diagnostic help!
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1.1K views 3 replies 3 participants last post by  shinobiworrior  
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#1 ·
Hello everyone,
I'm a professional auto technician seeking assistance with a client's 2002 Toyota MR2 equipped with the Sequential Manual Transmission (SMT). Despite extensive research and diagnostics, I've encountered persistent issues that I hope the community can help me resolve.
Background:
  • Initial Problem: The vehicle presented with diagnostic trouble codes (DTCs) P0860 (Gear Shift Module Communication Circuit) and P1646 (Transmission Control ECU Malfunction).
  • Safety Shutdown: Since the onset, these codes have caused the vehicle to stall shortly after starting, likely as a safety protocol.
  • Research and Initial Recommendations: Given my limited experience with the SMT system, I suggested consulting a specialist. However, the client prefers that I continue the work. I proposed a manual transmission swap for cost-effectiveness, but the client wishes to retain the SMT.
Actions Taken:
  1. Component Replacements: At the client's request, the following components have been replaced:
    • Hydraulic Power Unit (HPU)
    • Gear Shift Actuator (GSA)
    • Transmission Control Unit (TCU)
    • Engine Control Unit (ECU)
  2. Current Issue: After replacing the GSA, the vehicle now also displays DTC P1870 (Shift Stroke Sensor Malfunction/Low Voltage). The car starts and runs but shuts off shortly after, likely as a safety measure due to these codes.
  3. Intermittent Functionality: At one point, the initial codes cleared, and the vehicle operated but was limited to first and second gears. Following the GSA replacement, the original codes reappeared, and the stalling issue resumed.
Request for Assistance:
I'm reaching out to see if anyone has encountered similar issues or can offer guidance on the following:
  • Potential causes for the persistent DTCs P0860, P1646, and now P1870 after component replacements.
  • Recommendations for further diagnostics or specific procedures, such as TCM reprogramming, considering the vehicle's current inability to remain running.
  • Any insights into whether additional components should be inspected or replaced to resolve these issues.
I appreciate any advice or shared experiences that could assist in diagnosing and resolving these SMT-related problems.
Thank you in advance for your help.
 
S
#4 ·
You're likely dealing with a wiring, sensor, or software issue rather than just hardware failure. Start by checking the TCM to GSA harness for continuity, shorts, and corroded pins, ensuring good grounds and proper voltage to all SMT components. Since P1870 appeared after replacing the GSA, verify if it’s properly calibrated using Techstream, as incorrect initialization can cause issues. The TCM and ECU must be properly programmed and paired, as mismatched firmware can trigger P1646. A full hydraulic bleed procedure via Techstream is necessary after replacing the HPU or GSA—air in the system can cause shifting failures. Monitor live data for actuator function, and check the clutch actuator for proper operation, as a malfunctioning actuator could force the system into shutdown mode. If everything checks out and the codes persist, perform a CAN bus check to ensure stable communication between the ECU, TCM, and SMT components, and consider reflashing the TCM if necessary.





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