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.
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HOMESERVICE MANUALSPONTIAC1991TRANS SPORT SE, 3T40/MD9REPAIR AND DIAGNOSIS (SINGLE PAGE)HEATING, VENTILATION & A/C (HVAC)HVAC CONTROL SYSTEMSHEATER SYSTEMTROUBLE SHOOTINGTEMP LEVER DOES NOT CHANGE TEMP
1991 Pontiac Trans Sport SE, 3T40/MD9
Temp Lever Does Not Change Temp
1991 Pontiac Trans Sport SE, 3T40/MD9SECTION Temp Lever Does Not Change Temp
- Turn ignition switch to RUN. Using a DVOM, backprobe heater control assembly C1 connector terminal No. 11 and ground. See Figure . Move temperature lever from HOT to COLD then from COLD to HOT. If voltage varies approximately between 1-11 volts, go to next step. If voltage DOES NOT vary, check for poor connection at heater control assembly C1 connector terminal No. 11, or short to ground in Light Blue wire. If connection and wire are okay, replace heater control assembly.
- Using a DVOM, backprobe temperature door actuator terminal No. 10 (Pink/White wire) ground. If there is NO voltage, repair Pink/White wire to temperature door actuator. If battery voltage exists, go to next step.
- Using DVOM, backprobe temperature door actuator terminals No. 10 (Pink/White wire) and No. 7 (Black wire). If there is NO voltage, repair open in Black wire to temperature door actuator. If battery voltage exists, go to next step.
- Using a DVOM, backprobe temperature door actuator terminal No. 8 (Light Blue wire) and ground while moving temperature control lever. If voltage DOES NOT vary between approximately 1-11 volts, repair open in Light Blue wire to temperature door actuator.
- If voltage varies, check for poor connections at temperature door actuator, or binding air temperature valve. If there is no binding and connections are okay, replace temperature door actuator.
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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.