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 MANUALSMERCURY1991TRACER BASE, 4D SEDAN, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 122 (A/C-HEATER SYSTEM - MANUAL)TESTINGVACUUM CONTROL SYSTEM
1991 Mercury Tracer Base, 4D Sedan, Automatic
Vacuum Control System
1991 Mercury Tracer Base, 4D Sedan, AutomaticSECTION Vacuum Control System
WARNING: This page is about a different car, the 1994 Mercury Sable and 1994 Ford Taurus. However, it is still accessible from the selected car via links, so may be relevant.
- Start engine. Rotate function selector knob from one position to another. A momentary hissing sound should be heard while knob is rotated, indicating vacuum is available at control panel.
- A continuous hissing indicates a major vacuum leak in system, although not necessarily at control panel. If momentary hissing cannot be heard while knob is rotated, check for good vacuum supply to control panel. Check for kinked, pinched or disconnected vacuum supply hose. Inspect check valve between intake manifold and vacuum tank. See Figure.
- If hissing is heard when knob is rotated, vacuum is available at control panel. With blower motor at high speed, rotate function selector knob from one position to another. Notice where air exits (panel, floor, defrost, etc.).
- Compare actual airflow with correct airflow. See CONTROL PANEL & DOOR ACTUATORS under OPERATION. See Figure. If airflow is incorrect, diagnose where leak exists or replace faulty door actuator as necessary.
- If airflow exits from proper ducts at idle but exits from defrost ducts during acceleration, a small vacuum leak exists in system. To locate leak, turn engine off. Using a gauge, check for vacuum delay while selectively blocking off vacuum hoses.
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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.