Intermittent Blower and Erratic Sensor Readings After a Harness Repair
After a collision repair, a transit bus returns with intermittent HVAC complaints: the blower sometimes runs on high only, the driver's vent temperature reads erratically on the scan tool (jumping between 40 F and 90 F), and the compressor clutch occasionally drops out for no apparent reason. The body shop spliced several wires in the HVAC harness near the firewall. The HVAC control module stores an intermittent code for the evaporator temperature sensor circuit. Battery voltage is 12.6 V at rest and 14.1 V running. Grounds at the HVAC module measure 0.05 V drop under load. The evaporator temperature sensor is a two-wire thermistor. The blower has a PWM controller. The technician suspects the control module and wants to replace it.
1. Explain why the control module is an unlikely root cause and what the shared symptoms point to.
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The symptoms span three separate circuits - blower PWM control, evaporator temperature sensor signal, and compressor clutch command - that share the HVAC harness and its grounds/connectors. A single module failure could cause multiple faults, but the timing (immediately after a harness splice) and the intermittent, erratic nature point to a wiring/connection fault rather than a module.
Key evidence:
- The evap temp sensor reading jumps between 40 F and 90 F - a thermistor signal that jumps like that is classic high-resistance or intermittent connection, not a failed module. A module fault usually gives a fixed default or a hard code.
- Blower 'high only' suggests the PWM control signal is being lost and the blower defaults to full speed - again a wiring symptom.
- Clutch dropouts with no pattern can be the module reacting to a bad sensor signal (it may disengage on implausible evap temp) - a downstream effect of the sensor circuit fault, not a separate module failure.
- Grounds check good (0.05 V drop), so the module has a solid reference.
Root cause is most likely a poor splice, pin drag, or chafed wire in the repaired harness section, affecting the shared sensor/control circuits.
2. Describe the diagnostic approach to localize the fault in the spliced harness.
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- Inspect the splice area first - look for poor crimps, solder joints, tape-only repairs, pinched wires at the firewall grommet, and missing strain relief. The repair is the newest variable.
- Wiggle test - with the scan tool live-data on evap temp and blower command, flex the harness at the splice and at the firewall. A reading that jumps confirms an intermittent connection.
- Measure sensor circuit resistance/voltage - backprobe the evap temp sensor signal at the module and at the sensor. Compare. A large difference indicates harness resistance. Check the 5 V reference and ground at the sensor.
- Load-test the blower PWM signal - use a scope or a load (test light) on the control wire; a signal that drops under load indicates a high-resistance splice.
- Check for shared splices - if the evap sensor ground and blower control share a splice, a bad joint there explains both symptoms.
- Verify clutch command at the module output during a dropout to confirm the module is reacting to the bad sensor signal rather than failing.
Repair the splice properly (crimp and seal, or solder and heat-shrink), then re-test all three functions.
3. The technician replaced the module and the symptoms persist. What does that tell you, and what is the next step?
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Persisting symptoms after module replacement confirms the fault is in the harness, not the module - the original diagnosis was correct and the module was replaced without proof. This is a costly lesson: the module should only be condemned after the inputs and wiring are proven good.
Next steps:
- Do not replace anything else until the circuit is proven. Re-run the wiggle test and backprobe measurements with the new module installed.
- Focus on the splice - cut out and remake the splice properly, or replace the affected harness section. Verify pin drag and terminal condition at both ends.
- Check for a shared ground or reference that was disturbed during the collision repair.
- Re-test all three symptoms - evap temp stability, blower speed control, and clutch engagement - under vibration and temperature swing.
- Document the original module as a good part (retain it) and note the actual repair so the warranty and future diagnostics are accurate.
The takeaway for the exam: intermittent multi-circuit symptoms after a harness repair are a wiring fault until proven otherwise, and a module should never be replaced to 'see if it fixes it.'