S15/T15 2WD Jimmy V6-262 4.3L (1991)
Cruise Control Servo: Technical Service Bulletins
Cruise Control - Intermittently Loose Speed/Drop-Out
GMC NUMBER:
92-9-54
GROUP:
9 Accessories
DATE:
February 1992
CORPORATE
NUMBER:
239001
SUBJECT:
CRUISE CONTROL INTERMITTENT DROP-OUT, AND/OR FAIL TO SET OR RESUME (NEW DIAGNOSTIC PROCEDURES)
MODELS:
1988-92 S/T, M/L, R/V/G/P
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 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 gage 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 gage 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.
Gage 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 servo. Using a hand held vacuum pump connected to the large port, stroke the pump until the servo
diaphragm pulls in and the gage reads 10-20 in.Hg (34 - 68 Kp/sq.m). Replace the servo if either of the following conditions occur:
1.
Diaphragm will not move in completely.
2.
Diaphragm moves in completely. Gage reads 10-15 in.Hg. (34 - 50 Kp/sq.m), but vacuum decreases quickly (in a good servo the vacuum may
decrease slowly).
Vacuum Release Valve (On Brake Pedal) Check:
