Cause of Leaking Lubricants
Now let us review some of the reasons why the
lubricant is likely to leak at any one or several of these
locations. First of all, a transmission (or almost any other
gear case) will usually start leaking if the oil level is too
high. To stiffen the oil used in gear cases, some
manufacturers use soap and soda in the oil. As the gears
operate, the oil is splashed all over the inside of the gear
case. Because of the soap and partly because of the
splashing and the heat, the oil starts to foam or fill with
air bubbles. Thus the oil expands and takes up more
room. This action creates excessive pressure inside the
gear case. If the oil level is too high to start with, the
pressure created inside the transmission many be more
than the seals and gaskets can resist and the oil will start
leaking out. Leaking can occur at any one or several
The transmission oil level should only be checked
after the vehicle has been parked for several hours or
overnight. During this time, the bubbles or foam will
cool and settle as a liquid in the bottom of the
l With the transmission cold, remove the fill plug.
The oil level should be at, or just below, the
bottom of the till plughole.
l If the oil level is too high, allow the excess oil to
run out the fill plughole.
Even if the oil level is correct, it is possible that the
foaming action of the oil will cause the pressure inside
the transmission to become too high. To permit the
excess pressure to escape, a vent valve is used. This
valve contains a seat and spring-loaded ball, and has a
dust cap over the valve assembly.
To check the vent valve, first make sure the area
around it is free of dust and dirt.
. Then try turning the dust cap with your fingers.
It should turn freely in either direction. If it does
not turn freely, replace it.
Gaskets or oil seals will always leak if the bolts
securing the plates, covers, or retainers are loose. All of
the bolts should be tightened uniformly with a torque
The bolts that secure the input shaft retainer, the
gearshift housing cover, and the retainer seals should be
tightened with a torque wrench to the manufacturers
specifications. If tightening the bolts fails to stop the
leak at this point, the transmission should be
disassembled and the source of the leak repaired.
Leaks around the threads of the fill plug, the drain
plugs, or any of the bolts can usually be stopped by
coating the threads of the plugs or bolts with a
A loose gearshift retainer will also allow the
lubricant to escape.
All of the seals need to be lubricated; otherwise,
they will be ruined. Therefore, a little seepage around
any seal is normal. A seal is not considered as leaking
unless enough oil is escaping by the seal to drip on the
ground and cause a small puddle.
With the power plant in the vehicle, you can inspect
all seals except the input shaft retainer seal. If this seal
is leaking, oil will drip out through the plughole in the
bottom of the pan under the flywheel housing when the
plug is removed.
If oil does drip out at the flywheel housing drain
plug, examine the oil closely. It may be engine oil
leaking from the engine crankshaft rear oil seal. The
engine oil is much thinner (has less viscosity) than the
transmission oil, so you should be able to tell which seal
An oil leak, either from the engine or transmission
input shaft seals, is serious, because the oil can ruin the
clutch. An oil-soaked clutch disk will almost always slip
TESTING TRANSMISSIONS FOR
In addition to the leakage problems, there are other
problems that can develop in the standard transmissions
used in almost all trucks. We can classify these as
The best way to locate mechanical problems in the
transmission is to road test the vehicle. Before road
testing, however, check for missing or loose bolts and
be sure the oil is at the proper level in the transmission
case. Check the parking brake mechanism for proper
mounting and correct adjustment. Check all moisture
seals or boots. Check the action of the gearshift levers.
The transmission is often blamed for problems that
are elsewhere. For example, with the engine running and
the vehicle standing still, disengage the clutch and move
the gearshift lever into first or reverse. You should be
able to shift into either of these gear positions without
any gear clashing or without the vehicle moving. If the