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.
Dwell Fixed Between 10 & 50 Degrees
This indicates an "open loop" condition caused by one of the following:
- Open O2 sensor circuit or bad sensor.
- Open coolant temperature sensor circuit.
- Open circuit No. 413 from ECM terminal No. 14 to ground.
- Run engine at 2000 RPM for one minute to ensure normal operating temperature. Idle engine and check mixture control solenoid dwell. Disconnect oxygen sensor and jumper ECM harness terminal (not sensor) to ground. Grounding O2 sensor input checks ECM response to a "lean" signal. Normal response is dwell decreasing to full rich command. If dwell does not change as indicated, go to step 4).
- If dwell decreases with oxygen sensor ECM harness terminal grounded, check for open in oxygen sensor ground circuit (ECM terminal No. 14). On some ECMs, an open circuit to terminal No. 14 can cause "open loop". Repair as necessary. If circuit is not open, leave oxygen sensor lead to ECM grounded. Connect a DVOM (minimum 10-megohm input impedance) set on the 2-volt scale to the oxygen sensor Purple lead. This checks O2 sensor output with full rich command from ECM caused by grounding O2 sensor input circuit.
- Normal response is O2 sensor voltage reading greater than .8 volt. If DVOM does not read greater than .8 volt, replace oxygen sensor. If DVOM does read greater than .8 volt, repair faulty connections at oxygen sensor.
- If dwell did not change when oxygen sensor lead to ECM was grounded, jumper ECM terminal No. 9 to terminal No. 14. This grounds O2 sensor circuit at ECM to check for open in wiring to ECM terminal No. 9. Normal response to "lean" signal is a decrease in dwell. If dwell decreases, check for open in circuit No. 412 (Purple wire of oxygen sensor).
- If there is no change in dwell when terminal No. 9 is jumpered to terminal No. 14, turn ignition off. Turn ignition on with engine off. Using a "Scan" tester, check coolant temperature sensor parameter. This checks coolant sensor input. Normal reading on a warm engine is greater than 167°F (75°C). An open coolant sensor signal or ground circuit would cause a reading of approximately 77°F (25°C). If this is the indicated reading, check coolant sensor harness for opens. Also, check coolant sensor for an being out of calibration. See SYSTEM/COMPONENT TESTS article.
- If coolant temperature scans greater than 167°F (75°C), check for faulty connections to ECM terminals No. 9 or No. 14. If connections are okay, replace ECM.
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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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