Electra V6-231 3.8L VIN C SFI (1988)
Cruise Control Servo: Technical Service Bulletins
Cruise Control - Intermittent Operation
Bulletin 92-9-9
Date Feb. '92
Ref. No. 92-155-9
Corp. Ref. No. 239001
SUBJECT:
CRUISE CONTROL INTERMITTENT DROP-OUT, AND/OR FAIL TO SET OR RESUME (NEW DIAGNOSTIC PROCEDURES)
MODELS AFFECTED:
ALL 1988-92 VEHICLES WITH VACUUM CRUISE CONTROL
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 may 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 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 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 gage 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.
Vacuum Release Valve (On Brake Pedal) Check:
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 gage. The vacuum reading on the gage should remain steady. If gage 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 affect 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.