CRUISE CONTROL DROPOUT (FAILS TO SET OR RESUME)

SUBJECT: CRUISE CONTROL INTERMITTENT DROP-OUT, AND/OR FAIL TO SET OR RESUME (NEW DIAGNOSTIC PROCEDURES)

VEHICLES AFFECTED: 1988-92 CARS/TRUCKS

B-car application is only 1988 to 1990.

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 16 - 48 KM (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 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 gauge at the hose end and start the engine. The gauge should read at least 10 in. Hg. (34 Kp/m2). 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/m2). 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/m2), 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 'Al 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 10-20 in. Hg. (34 - 68 Kp/m2). Replace the servo if either of the following conditions occur:

1. Diaphragm will not move in completely.

2. Diaphragm moves in completely. Gauge reads 10-15 in. Hg. (34 - 50 Kp/m2), but vacuum decreases quickly (In a good servo the vacuum may decrease slowly).

Vacuum Release Valve (On Brake Pedal) C

Disconnect the vacuum release hose from the servo (large hose). Apply vacuum (10-20 in. Hg. (34-68 Kp/m2)) 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 misadjusted vacuum release valve (at the brake pedal), or a leaking hose. A leaking vacuum release system will dramatically effect the operation and life of the servo.

Other items that could affect servo life and operation are:

- Improper servo cable or rod adjustment; - Binding throttle linkage; - Loose or bent servo bracket; or - Poor electrical connections.

If the previous diagnostic procedures fail to determine the cause of the cruise system intermittent condition, replace the servo.

For vehicles repaired under warranty, use Labour time .5 hrs., for diagnostic time only. Use existing Labour operations for replacement of parts.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.