Firebird V8-305 5.0L VIN E (1992)
Cruise Control Servo: Customer Interest
Cruise Control - Drop Out/Fail to Set or Resume
Bulletin Number:
92-9-19
Reference Number:
239001
Publish Date:
1/93
Subject:
CRUISE CONTROL INTERMITTENT DROP-OUT, AND/OR FAIL TO SET OR RESUME (NEW DIAGNOSTIC PROCEDURES)
Models
Affected:
1988-92 ALL MODELS
THIS BULLETIN IS BEING REVISED TO CORRECT THE BULLETIN NUMBER. PLEASE DISCARD BULLETIN 92-9-6 ISSUED 1/93 AND
REPLACE IT WITH THIS BULLETIN.
CONDITION:
On 1988-92 vehicles using the vacuum servo cruise control system, the cruise control may intermittently lose speed and/or drop-out (release), while
using cruise control. It may also fail to initially set, or resume, at times. Vehicles using the Electro-Motor Cruise Control system (Stepper Motor Cruise)
are not affected.
When driving the vehicle, the cruise control will operate normally for 10 to 30 miles, then may gradually lose speed and/or drop out of cruise. If you
experience this condition, accelerate to a speed higher than the cruise set speed, using the accelerator pedal only. DO NOT use the set or resume
controls. Then allow the vehicle to coast down, and see whether the cruise catches and holds at the set speed. If the cruise catches and holds at the set
speed, the diagnostic procedure below should be used to test the servo and vacuum system. Because of the intermittent nature of this condition, you may
have to try several times to reproduce this condition.
CAUSE:
In a small percentage of servos, the vent valve, which normally cycles to control the amount of vacuum needed to maintain a set speed, may fail to close
at times. When this happens, the vacuum is being continually vented, so the servo cannot pull the throttle open to increase the vehicle speed. The valve
may close correctly the next time the cruise is cycled by setting or resuming.
Because this condition is intermittent, testing the vehicle following the Service Manual diagnostic procedures may lead to the replacement of good parts,
which will not correct the vehicle. This may lead to the customer having to bring the vehicle back for service again. It is recommended that the following
diagnostic procedure be performed to determine if the trouble can be found.
DIAGNOSTIC PROCEDURE:
First evaluate the cruise system as specified in the Service Manual. Then make the following additional checks that may detect conditions with the
vacuum portion of the cruise system.
Source Vacuum Check:
Disconnect the vacuum supply hose from the servo (small hose). Install a vacuum gage at the hose end and start the engine. The gauge should read at
least 10 in.Hg. (34 Kp/sq.m). If there is no vacuum, or very little vacuum, inspect for leaking or pinched hoses.
Servo Vacuum Valve Check:
With engine off, remove the vacuum supply hose (small hose) from the servo. Using a hand held vacuum pump connected to the vacuum port on the
servo, stroke the pump until the gauge reads 10-15 in.Hg. (34 - 50 Kp/sq.m). Replace the servo if either of the following conditions occur:
1.
Vacuum valve will not hold any vacuum.
2.
Gauge reads 10-15 in.Hg. (34 - 50 Kp/sq.m), but vacuum decreases quickly (less than 10 seconds). In a good servo, the vacuum may decrease
slowly (more than 30 seconds).
Servo Vent Valve Check:
Disconnect the servo from the throttle linkage.
Disconnect the electrical connector from the servo. Using a fused jumper wire, apply 12 volts to terminal "A" on the servo. Connect a jumper wire from
terminal "C" on the servo to ground. You should hear a click noise as the vent valve closes. Remove the vacuum release valve hose (large hose) from the