G 10 Van V8-379 6.2L DSL (1987)
Throttle Valve Cable/Linkage: Technical Service Bulletins
Throttle Valve System - Diagnosis
GMC NUMBER, 87-T-27
SECTION:
7A - Automatic Transmission
DATE:
February, 1987
SUBJECT:
SUMMARY OF THROTTLE VALVE DIAGNOSIS
MODELS:
ALL LIGHT DUTY MODELS WITH A
This bulletin contains background and diagnosis information on the Throttle Valve (T.V.) system and applies to all THM tranmissions, except the 400
model. Included are revised procedures using a pressure gage during adjustment/diagnosis, inspection of components and other items not yet covered in
the Service Manual. Throttle valve (T.V.) pressure controls the shift pattern and the amount of hydraulic line pressure used to apply clutches and bands.
If proper line pressure is not available, excess slippage may occur during the shift. If the T.V. cable is tailored in an effort to modify shift pattern/feel,
this misadjustment can result in tranmission failure.
IMPORTANT:
Do not attempt to correct a condition by changing the T.V. cable setting from its proper adjustment. Apparent improvements from T.V.
cable "tailoring" indicate a need for further diagnosis into the real cause of a condition (such as engine performance, the control valve,
governor or servo assemblies, etc.) Dealer personnel and customers should know that certain types of heavy duty vehicles have
transmissions calibrated to provide crisp, firm shifts for durability and/or performance. Once again, the T.V. cable must not be tailored
in an effort to modify shift pattern or feel on these applications. Make sure engine fuel, electrical and mechanical systems are
performing properly before attempting transmission diagnosis. For example, a fouled fuel injector, cracked spark plug, damaged spark
plug wire or vacuum leak can affect transmission performance. A T.V. system which is improperly adjusted, kinked, binding,
diconnected, broken or has a wrong part number may cause one or more of the following conditions:
-
Delayed or harsh upshifts
-
Early and slipping upshifts
-
High or low line pressure
-
No upshifts
-
Chatters on takeoff
-
1-2 shift at full throttle only -No full throttle or part throttle detent downshifts
-
Intermittent second gear starts
CAUTION: To avoid possible personal injury and/or damage to the vehicle, the following diagnosis procedure must be performed with the vehicle
parking brake properly applied.
DIAGNOSIS
Inspect-
^
T.V. cable for correct part number, listed in
the parts catalog.
^
T.V. cable for kinked, binding, disconnected or broken condition.
Figure No. 1T.V. Pressure Range Chart
Install or Connect -
^
Line pressure gage.
^
Engine tachometer.
^
Warm up engine to normal operating temperature.
^
Run engine at 1000 rpm.
^
With gear selector in "Park", note oil pressure.
(NOTE:
Remaining steps do not apply to the THM 400.)
