Throttle Control System Honda Prelude II

DESCRIPTION

Throttle control system is used to reduce hydrocarbon (HC) emissions during gear shifting and deceleration. Throttle opener diaphragm holds throttle open slightly as necessary to improve combustion.

On all carbureted models, system consists of a throttle controller, cranking opener solenoid valve, check valve, and dashpot check valve.

DASHPOT SYSTEM

This system slows closing of throttle. During periods of deceleration, ported vacuum in carburetor acts on the throttle control valve through dashpot check valve to hold throttle open slightly. Fixed orifice in dashpot check valve gradually decreases vacuum until the throttle closes completely.

Speed at which throttle closes is determined by size of fixed orifice in dashpot check valve, throttle return spring tension, and amount of vacuum generated at carburetor port above preset level in dashpot check valve.

THROTTLE CONTROL VALVE

This system aids engine starting. As starter is engaged, cranking solenoid valve is activated through starter relay to allow intake manifold vacuum into diaphragm. This helps to maintain correct throttle opening angle.

CRANKING SOLENOID VALVE

Note. Perform this test on hot engine.

1. Ground coil secondary wire to prevent engine from starting. Turn ignition to "START" position. Throttle controller arm should retract when engine is cranked. If controller operates, cranking solenoid valve system is okay.
2. If controller does not operate, check for voltage at cranking solenoid valve. Using voltmeter positive test lead, probe control unit connector Black/White wire. Ground voltmeter negative test lead. If no voltage is present, check wiring and fuse.
3. If voltage is present, disconnect hose between cranking solenoid valve and check valve at solenoid valve. Check for vacuum when cranking engine.
4. There should be vacuum with ignition switch turned to "START" position. If there is vacuum, replace cranking solenoid valve and retest. If there is no vacuum, check vacuum line for leaks or blockage. If they are okay, replace check valve and retest.

Scheme 14

1. Connect tachometer to engine. Warm engine to normal operating temperature (cooling fan on). Disconnect vacuum hose from throttle control valve.
2. Connect hand vacuum pump to throttle control valve and apply 8 in. Hg vacuum. Check that engine speed rises (within 1 minute) to 1500-2500 RPM.
3. If speed was too low, widen adjusting slot in throttle control valve lever with a screwdriver. (Scheme 14) If speed was too high, narrow slot with pliers.
4. If speed cannot be adjusted, or diaphragm will not hold vacuum, replace throttle controller and retest. (Scheme 14): Control Lever Adjusting Slot Location
5. If speed was okay, disconnect vacuum pump and reconnect hose. Raise engine speed to 3500 RPM and maintain for 2 to 3 seconds.
6. Release throttle suddenly and note length of time throttle opener arm takes to fully extend. Return time should be 1-4 seconds. If engine speed returns to idle in 1-4 seconds with arm fully extended, system is okay.
7. If return takes more than 4 seconds, check throttle controller linkage for free movement, replace dashpot check valve and retest. If return takes less than 1 second, disconnect hose from throttle opener and connect a vacuum gauge to disconnected hose. Start and run engine at 4000 RPM.
8. Vacuum should be at least 1.2 in. Hg. If so, replace dashpot check valve and retest. If vacuum is less than 1.2 in. Hg, check for vacuum at carburetor port. If there is no vacuum, clean carburetor port and retest. If vacuum is present, check vacuum line for leaks, blockage or disconnected hoses and retest.