Oil Leaks; Basic Pressure Checks - Chevrolet 10 1971 Series Chassis Service Manual

CLUTCHES AND TRANSMISSIONS 7-18
rolling. The pointer on the indicator quadrant should line
up properly with the range indicators in all ranges.

OIL LEAKS

Before attempting to correct an oil leak, the actual
source of the leak must be determined. In many cases the
source of the leak can be deceiving due to " wind flow"
around the engine and transmission.
The suspected area should be wiped clean of all oil
before inspecting for the source of the leak. Red dye is
used in the transmission oil at the assembly plant and
will indicate if the oil leak is from the transmission.
The use of a " black light" * to identify the oil at the
source of leak is also helpful. Comparing the oil from the
leak to that on the engine or transmission dip stick (when
viewed by " black light" will determine the source of the
leak.
Oil leaks around the engine and transmission are
generally carried toward the rear of the car by the air
stream. For example, a transmission " oil filter tube to
case leak" will sometimes appear as a leak at the rear
of the transmission. In determining the source of an oil
leak it is most helpful to keep the engine running.
The mating surfaces of servo cover, converter hous­
ing, transmission case and transmission case extension
should be carefully examined for signs of leakage. The
vacuum modulator must also be checked to insure that
the diaphragm has not ruptured as this would allow trans­
mission oil to be drawn into the intake manifold. Usually,
the exhaust will be excessively smoky if the diaphragm
ruptures due to the transmission oil added to the combus­
tion. The transmission case extension rear oil seal
should also be checked. All test plugs should be checked
to make sure that they are tight and that there is no sign
of leakage at these points. The converter underpan should
also be removed. Any appreciable quantity of oil in this
area would indicate leakage at the oil pump square seal
ring, oil pump seal assembly, or oil pump bolt seating
washers.

BASIC PRESSURE CHECKS

Four basic pressure checks are used for diagnosis and
operational checks for the Aluminum Powerglide trans­
mission. All checks should be made only after thoroughly
warming up the transmission.
• Wide Open Throttle Upshift Pressure.
• Idle Pressure in " Drive" Range.
• Manual " Low" Range Pressure.
• ' 'Drive" Range Overrun (Coast) Pressure.
It is not recommended that stall tests be conducted
*A " Black Light" testing unit may be obtained from several different service tool suppliers.
POWERGLIDE BASIC PRESSURE (MAINLINE) CHECKS
Wide Open
Engines Throttle
Upshift
L-6 250
93-113
V-8 307 103-118
which would result in engine vacuum falling below 10" Hg.
Pressure gauge hose connections should be made at the
low servo apply (main line) test point (fig. 5PG). Run the
gauge line into the driving compartment by pushing aside
the mast jacket seal.
drivers feet and connect to Pressure Gauge J-21867.
•
Refer to the pressure check chart for upshift pressure
points as indicated on the low servo apply (main line)
gauge.
•
In addition to the oil pressure gauges, a vacuum gauge
is needed for this check.
With the parking brake applied and the shift selector
lever in " Drive", low servo apply (main line) pressure
should be as shown on the pressure check chart.
If pressures are not within these ranges, the following
items should be checked for oil circuit leakage:
1. Pressure regulator valve stuck.
2. Vacuum modulator valve stuck.
3. Hydraulic modulator valve stuck.
4. Leak at low servo piston ring (between ring and
5. Leak at low servo piston rod (between rod and bore).
6. Leak at valve body to case gasket.
7. Leak at valve body gaskets.
8. Oil pump clearances.
9. Check passages in transmission case for porosity.
•
Connect a tachometer, apply the parking brake, place
the selector lever in " Low" range, and adjust the engine
speed to 1000 rpm with the car stationary.
Low servo apply (main line) pressure should be as
shown on the pressure check chart.
Pressures not within this range can indicate the follow­
ing possibilities:
1. Partially plugged oil suction screen.
2. Broken or damaged ring low servo.
3. Pressure regulator valve stuck.
4. Leak at valve body to case gasket.
5. Leak between valve body gaskets.
6. Leak at servo center.
7. Oil pump clearances.
•
With the vehicle coasting in " Drive" range at 20-25
MPH with engine vecuum at approximately 20" Hg., low
servo apply (main line) pressure should be as shown on
the pressure check chart.
Idle Pressure
In Drive Range
10" Hg.
16" Hg.
53-75 83-102
53-74 81-100
Tie line out of the way of the
Wide Open Throttle Upshift Pressure Check
Idle Pressure in " Drive" Range
bore).
Manual " Low" Range Pressure Check
Drive Range Overrun (Coast) Pressure
Manual Low
Range @
1000 RPM
125-138
115-128
Drive Range Overrun
(Coast) @ 20-35 MPH
(Approx. 20" Hg.)
48-53
48-53
10-30 CHEVROLET TRUCK SERVICE MANUAL