Copyright © 1996 General Motors Corp. 2–5
DESCRIPTION AND OPERATION
2–12. SPEEDOMETER DRIVE/SPEED
SENSOR WHEEL
a. Description
(Foldout 12,B)
(1) The speedometer drive consists of drive
gear 2 and provision for mounting the driven gear as-
sembly in the transmission housing. Drive gear 2 is a
worm gear with a left-hand helix. It is concentric with
the transmission output shaft. The drive gear is
clamped between adjacent components for rotational
drive.
(2) Speed sensor wheel 9 is provided for
models that are equipped with an electronic speed-
ometer. The 16-tooth wheel provides a rotating pulse
that can be read by an electronic speed sensor.
b. Operation
(Foldout 12,B)
(1) When the transmission output shaft ro-
tates, drive gear 2 or speed sensor wheel 9 rotates.
(2) For mechanical speedometers, the driven
gear, supplied by the vehicle manufacturer, rotates
clockwise during forward operation (viewed at the
drive cable connection on the transmission).
(3) For electronic models, the speed sensor,
supplied by the vehicle manufacturer, reads the output
speed by “counting” the pulses produced by the teeth
of the speed sensor wheel.
2–13. GOVERNOR
a. Description
(Foldout 12,A).
Governor assem-
bly 19 is a centrifugal (flyweight) governor, driven by
drive gear 1 (Foldout 12,B) and mounted on the trans-
mission output shaft. It is retained by cover 22 (Fold-
out 12,A).
b. Operation
(Foldout 12,A).
When the governor
rotates, centrifugal force causes weights to move out-
ward which moves a valve to admit main pressure to
the governor circuit. Governor pressure is regulated
when the centrifugal force of the weights is balanced
by opposing hydraulic pressure. Governor pressure
varies with transmission output speed, increasing as
output speed increases. Governor pressure, in combi-
nation with modulator pressure, controls the automatic
shifting of the transmission.
2–14. MODULATOR
a. Vacuum Modulator
(Foldout 12,A)
(1) Vacuum modulator 26 is calibrated and
sealed at the factory.
(2) The position of the diaphragm in the
modulator varies with vehicle speed, load, and engine
throttle opening. When the diaphragm moves, the
modulator valve is affected, increasing or decreasing
modulation pressure. Modulator and governor pres-
sures control the automatic selection of transmission
ranges.
b. Mechanical Modulator
(1) The mechanical modulator is attached to
the transmission and activated by a cable.
(2) The cable is connected to the fuel control
lever at one end and the modulator valve actuator at
the other. The fuel control lever moves the cable, con-
trolling the modulator valve.
c. Electric Modulator
(1) An electric modulator can be used with
electronic or non-electronic engines — any engine
which can provide an On/Off signal based on throttle/
pedal position.
(2) The 12-volt modulator assembly and the
24-volt modulator assembly utilize an electronic sig-
nal to stroke the transmission modulator pin from the
closed-throttle position to the full-throttle position.
The result is step-modulation.
(3) Step-modulation is a two-phase modula-
tion schedule which utilizes a closed-throttle shift
schedule until the throttle position is greater than 80–85
percent, then, remains at a full-throttle shift schedule
until the throttle position is less than 60–65 percent.
(4) Electric modulators are compatible with
an electronically-controlled engine that can provide an
On/Off signal based on throttle position. Consult the re-
spective engine manufacturer to identify the wire that
provides this signal. Electronic modulation can also be
applicable in mechanically-controlled engines. The
modulating system will still require a 12- or 24-volt (dc)
signal to stroke the transmission modulator valve.
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