3–10 Copyright © 1996 General Motors Corp.
AT 500, 1500 SERIES AUTOMATIC TRANSMISSIONS
c. Checks Before Adjusting Shift Points
(1) When calibration is to be made during a
road test or on a test stand, certain preparations must
be made.
(2) Warm up the transmission or test stand
setup to normal operating temperature of 160–220°F
(71–104°C).
(3) Check the engine governor setting, and
adjust if required, to conform to the transmission’s en-
gine speed requirements.
(4) Check the engine for satisfactory perfor-
mance before checking shift points.
(5) For diesel applications, check the linkage
that controls the mechanical modulator valve actuator
for proper travel, routing, and operation. For most gas-
oline applications, check condition and routing of vac-
uum modulator lines, hoses, and connections.
(6) Check the selector linkage for proper
range selection.
(7) Provide accurate instrumentation for ob-
serving speeds, temperatures, pressures, vacuum, etc.
d. Calibration by Road Test Method
NOTE:
Before road test, determine the vehicle tachometer
error with a test tachometer. Make corrections for
error, as required, in subsequent tests.
(1) Note the governed speed of the engine.
This is the base speed from which checks and adjust-
ments are made.
(2) Drive the vehicle and check the engine
speed (at full throttle) at which each upshift occurs.
Each upshift should occur as follows:
NOTE:
If all full throttle upshift points are too low by ap-
proximately the same amount, check adjustment of
the modulator external linkage.
(3) If an upshift speed does not reach the
specified rpm, the shift point may be raised by adjust-
ing (increasing) the spring force on the 1–2, 2–3, or
3–4 shift signal valve. If the upshift speed exceeds the
specified rpm, or if upshift does not occur at all, the
spring force must be reduced. Adjust the force on only
the springs for valves that do not upshift at the proper
speed.
(4) Refer to Sections 5, 6, and 7 for proce-
dures covering removal and replacement of affected
components.
e. Calibration by Speedometer
Readings Method
(1) When a tachometer is not available for
checking shift points, the vehicle speedometer can be
used.
(2) Check the top speed of the vehicle in
each selector hold position (first, second, third ranges).
Record the top speed for each.
(3) For checking the shift points, place the
selector at
D
(Drive) so that all automatic shifts can
occur. Drive the vehicle at full throttle from a standing
start until the 3–4 upshift occurs, recording the mph at
which each upshift occurs.
(4) Compare the upshift speeds with the hold
speeds recorded in Paragraph 3–13
e
(2). The 2–3 up-
shift should occur at approximately two mph below
the top speed of second range. The 3–4 upshift should
occur at approximately two mph below the top speed
for third range. The 1–2 upshift is not to be adjusted
relative to hold speed. The 2–1 downshift at closed
throttle should occur at 3–5 mph (4–8 km/h).
f. Calibration by Test Stand Method
(1) Tables 3–4 through 3–9 provide detailed
information required for adjusting shift points.
(2) The adjustment procedures are as out-
lined in Paragraph 3–13
d
, except the base for checks
and adjustments is output shaft speed instead of engine
governed speed.
1–2 — within 400 rpm to engine governed speed.*
2–3 — within 300 rpm to engine governed speed.*
3–4 — within 200 rpm to engine governed speed.*
* Vehicle loaded
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