1987 Honda Prelude Si, StandardFACTORY OEM SERVICE PROCEDURE

Component Testing: THERMOSENSOR

  1. Thermosensor resistance must be checked both cold and hot. Coolant temperature must be less than 104°F (40°C) to perform cold resistance check. Disconnect vacuum hose No. 19 from charcoal canister. If there is no vacuum at idle, thermovalve B is open and thermosensor resistance check can be performed.
  2. Disconnect connector at control unit. Check for resistance between Blue/Yellow and Brown/Black terminals. Resistance should be above 1000 ohms. Run engine until cooling fan comes on and check resistance again. Resistance should be about 200 ohms.
  3. If resistance is not within specifications, replace thermosensor and retest. If ohmmeter shows no continuity, check wires. If there are no defects, replace thermosensor and retest.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.

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