Calais L4-138 2.3L VIN D FI (1990)
Cruise Control Servo: All Technical Service Bulletins
Cruise Control - Intermittent Drop-Out/Fail to Set
NUMBER: 92-T-48
CORP.REF.NO: 239001
DATE: February 1962
SECTION: 9B
SUBJECT:
CRUISE CONTROL INTERMITTENT DROP-OUT
AND/OR FAIL TO SET OR RESUME
- NEW DIAGNOSTIC PROCEDURES
MODELS/YEARS:
1988-92 OLDSMOBILES WITH VACUUM SERVO CRUISE CONTROL
CONDITION:
When driving a 1988-92 model equipped with Vacuum Servo Cruise Control, the cruise control will operate normally from 10 to 30 miles, then lose
speed and/or drop out of cruise. The cruise control may also, at times, fail to set or resume.
Note:
Vehicles using the Electro-Motor (Stepper Motor) Cruise Control System, such as the 1991-92 Custom Cruiser, are not affected.
CAUSE:
In a small percentage of servos, the vent valve, which normally cycles to control the, amount 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:
Preliminary Diagnostic Check: First verify the condition with a test drive. 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.
Next, evaluate the cruise system as specified in the Service Manual. Then make the following additional checks. The checks below may assist you in
detecting conditions in the vacuum portion of the cruise system.
Source Vacuum Check:
Disconnect the vacuum supply hose from the servo (small hose). Install a vacuum gauge at the hose end and start the engine. The gauge should read at
least 69 kPa (10 in.Hg.). 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 69-104 kPa (10-15 in.Hg.). Replace the servo if either of the following conditions occur:
1.
Vacuum valve will not hold any vacuum.
2.
Gauge reads 69-104 kPa (10-15 in.Hg.), 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 servo. Using a hand held vacuum pump connected to the large port, stroke the pump until the servo
diaphragm pulls in and the gauge reads 69-138 kPa (10-20 in.Hg.). Replace the servo if either of the following conditions occur:
1.
Diaphragm will not move in completely.
2.
Diaphragm moves in completely. Gauge reads 69-104 kPa (10-15 in.), but vacuum decreases quickly (in a good servo the vacuum may decrease
slowly).
Vacuum Release Valve (On Brake Pedal) Check: Disconnect the vacuum release hose from the servo (large hose). Apply vacuum 69-138 kPa (10-20
in.Hg.) to the hose using a hand vacuum pump and gauge. The vacuum reading on the gauge should remain steady. If gauge reading does not hold, or
leaks down slowly, inspect for a leaking or mis-adjusted vacuum release valve (at the brake pedal), or a leaking hose. A leaking vacuum release system
