Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1990 Chevrolet Blazer 5.7 K, StandardFACTORY OEM SERVICE PROCEDURE
Downshift Control Relay (THM 400)
- 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.
- 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.
Factory Service Tools
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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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