Specifications

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VST Troubleshooting
The information on this page has been provided to help understand the internal operation of the VST. Do
not use this information to attempt any internal repairs. Tecumseh's current policy on hydrostatic transaxles
that have internal failures is to replace the complete unit. This has not changed. However, Tecumseh would
like to provide a failure checklist to assist in making an accurate evaluation of the complete tractor to
eliminate any unnecessary replacements. Here is a list of items to check and corrective actions to take.
To properly test the unit for power loss.
1. Allow the unit to cool before trying the following steps.
2. Put the shift lever in a position that is 1/2 of the travel distance from neutral to forward.
3. Place the tractor on a 17 degree grade.
4. Drive the tractor up the grade (without the mower deck engaged). The loss of power experienced
should be approximately 20%. This is considered normal. If the loss of power is approximately 50%,
this would be considered excessive.
5. Bring the unit to neutral, shift into forward and note the response. Care should be taken to move the
lever slowly to avoid an abrupt wheel lift.
To determine if the problem is with the hydro unit, all external problem possibilities must be eliminated. Here
are some potential problem areas.
1. Overheating: Heat can cause a breakdown in the viscosity of the oil which reduces the pressure
used to move the motor. Remove any grass, debris, or dirt buildup on the transaxle cover and / or
between the cooling fins and fan. Buildup of material will reduce the cooling efficiency.
2. Belt slippage: A belt that is worn, stretched, or the wrong belt (too large or wide) can cause belt
slippage. This condition may have the same loss of power symptom as overheating. Typically, the
unit which has a slipping belt will exhibit a pulsating type motion of the mower. This can be verified
visually by watching the belt and pulley relationship. If the belt is slipping, the belt will chatter or jump
on the pulley. If the belt is good, a smooth rotation will be seen. Replace the belt and inspect the
pulley for damage.
3. Leakage: The VST and 1800 Series have two oil reservoirs which can be checked for diagnostic
purposes. The first is the pump and motor expansion bellows, with a small diameter blunt or round
nose probe, check the bellows depth through the center vent hole. Proper depth from the edge of
that hole is 3-1/4 - 3-1/2 inches (8.25 - 8.9 cm).
The second chamber is for the output gears including the differential. FIRST make sure the tractor
is level, then remove the drain/fill plug. NOTE: Some units that do not have differential disconnect
will have two plugs. We recommend using only the primary plug. With a small pocket rule insert
until you touch bottom of case. You can then remove it and check for 1/4 - 3/8 inches (6.5 - 9.5 mm)
contact, this is full at its 8 oz. capacity.
4. Low ground speed: If the linkage is not synchronized to absolute neutral, or the shift lever is not
properly fastened to the tapered control shaft, full forward travel may not be achieved. This may
cause a false reading and be misdiagnosed as a low power condition. This also could be caused
by the brake not releasing.
To determine absolute neutral, the hole in the tapered control shaft must face straight up and down,
at this point make sure the OEM linkage is in neutral. To properly fasten the control lever to the shaft,
torque the nut to 25-35 ft. lbs. (34 - 48.3 Nm) of torque with the shaft and the lever in neutral.
When attaching the shifter arm to the shaft you must prevent any rotation during torquing. This can
be done by placing a long 5/16 bolt in the hole of the shaft. Hold the bolt until the tapers are locked
and the nut torque is correct.
To make sure that the brake is not binding, drive the unit up a slight grade.. Position the speed control
lever into neutral. The unit should coast backwards. If the unit does not coast back slowly, the brake
is not released from the brake disk. Adjust the brake linkage to release the brake completely when
the foot pedal is released.
5. Hard to shift: Typically hard to shift symptoms are not caused by the hydrostatic unit. The shift
arm should move with relative ease. Approximately 40-50 inch lbs. (4.48 - 5.6 Nm) at the transaxle
for foot pedal units or 150-200 inch lbs. (16.8 -22.4 Nm) for hand operated units. This varies
depending on the type of linkage. Binding may occur in the linkage connections due to rust or
moisture. Lubricating these connections and checking for bent or damaged parts should resolve
hard shifting.