NOTE: A/C loss under Wide Open Throttle is a NORMAL condition.
APPLICATIONS: Vehicles produced by FLM from 70's to 90's prior
to Electronic control blend doors.
SYMPTOM: Airflow changes from current setting
to full defrost/heat upon medium to heavy acceleration/load
conditions.
PROBABLE CAUSES: Low vacuum condition to control
circuit due to insufficient vacuum supply.
POSSIBLE FAILURES: Hose, Check valve, Vacuum
tank, Control Assy, TBL switch, and others.
LOCATIONS:
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VACUUM HOSES - all over
CHECK VALVE:
- On vacuum tank
- Inline between tank and Manifold Tree
- On top of Manifold Tree
VACUUM TANK - Usually on the inner fender under the hood.
CONTROL ASSY - In dash
TBL SWITCH - Heater hose pass side.
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Testing the VACUUM CHECK VALVE
Check A/C vacuum check valve for correct installation by removing the hose from
the vacuum supply tank (coffee can) and the hose going through the firewall to
the control assembly (one at a time) from the check valve and check for vacuum
using a vacuum gauge. Engine must be running for this test.
Is vacuum available at both check valve ports?
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- Remove the check valve from the vehicle. Connect two hoses and a
Tee fitting to outlet ports of the check valve. Connect a vacuum source
to the Tee fitting as shown below.
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Pump up to 15 inches of vacuum. Observe the gauge reading. If the
vacuum loss exceeds 1 inch per minute, replace the check valve.
NOTE: On some vehicles (in particular 90's TC's) the check valve is part
of the Manifold Tree and must be replaced as a unit.
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Testing the VACUUM RESERVOIR (SUPPLY TANK, COFFEE CAN)
Supply a minimum of 15 inches of vacuum to the reservoir tank and check for vacuum
loss. Any decrease indicates tank should be replaced.
NOTE: Tanks that have two hoses connected will more than likely have the check
valve built into the tank. |
