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Hydro-Gear 331-3000 Oil Change

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Many of us have the Hydro-Gear 331-3000 hydrostatic transaxle in various pieces of equipment and have received many years of dependable service, but, with any piece of equipment, maintenance is a must.

According to Hydro-Gear, this transaxle is a sealed, non-serviceable transaxle, and does not require an oil change. This unit is in fact serviceable and it is rather easy to do so.

The following are instructions on how to do an oil change on this transaxle, easily, cheaply, and most importantly, while in the tractor. These instructions serve as simply a guideline and are written based on a Craftsman GT-3000. Your equipment may have minor variances. I and cannot be held responsible for anything that may happen to your equipment while using these procedures. No warranties are implied or guaranteed. Proceed at your own risk.

Now that the legal stuff is out of the way, let's start with the required tools and materials.

You will need:

  • Drain Pan
  • Four quarts of your favorite 20W-50 engine oil (synthetic or non-synthetic)
  • 13mm wrenches and/or sockets
  • 11/16" socket or wrench
  • 5/8" wrench
  • 1/4" hex wrench
  • 5mm hex wrench
  • pocket magnet

First, verify the model of your transaxle by looking at the tag that should be on the back of the left axle housing as shown in the photo below. For clarity, the model number is circled.

The next step is to remove the drawbar on the rear of the tractor to gain access to the transaxle.

The drawbar is secured to the tractor using six bolts, three on each side of the tractor. The three on the left side are 7/16-14 x 1" placed through the frame and drawbar from the inside and secured with lock nuts. The head of the bolts take the 5/8" wrench and the nuts take the 11/16" wrench or socket.

The right side is slightly different due to the presence of the hand control lever for the transaxle. The location of the bolts are the same and two of them are the same 7/16-14 x 1 with lock nuts. The third bolt is a 7/16-14 x 1 1/2 that goes through a spacer and the bracket for the hand control lever. The control linkage that connects to the transaxle needs to be disconnected to gain clear access to the third, long bolt and to remove it. Remove the bolt and rotate the linkage straight up out of the way to gain access to the bolt.

Use the pocket magnet to retrieve the spacer so that it isn't lost.

Once all six bolts are removed, the drawbar can be pulled out to the rear of the tractor with a little effort. It is a tight fit so be patient. It will come out and expose the transaxle to view.

Take note of the gold colored plug on the right side of the back of the transaxle. This is the check plug for the transaxle fluid level and the 1/4" hex wrench fits the plug. Be sure to clear out any dirt and debris from the bottom of the hex portion of the plug before attempting to remove the plug. It is easily stripped if the wrench does not fully seat.

Now, let's move to the bottom of the transaxle. Bolted to the bottom of the pan is an oblong cover about a half inch thick. This is the cover to the gearator and what we will be removing to drain the fluid out of the transaxle. This is the lowest point in the system and works very well for this. It is held in place using two cap screws.

Take note of the orientation of the cover before attempting to remove it. It will bolt back up facing either direction but the pump will only function if the cover is replaced in the correct orientation. Mine is marked using a square marked in the cover and one of the bolt holes has an oblong opening and the other round.

Remove the gearator cover using the 5mm hex wrench with your drain pan under the transaxle before loosening the bolts. Oil will start to drain out as soon as the bolts are loosened.

Let the transaxle drain thoroughly. You will get anywhere from three to three and half quarts of oil out.

While the oil is draining, this is a great time to check a few items and do a little maintenance. First, inspect the cooling fan on top of the transaxle for cracks or breaks, and that it is clean. Also, make sure that it is still securely attached to the shaft.

Also, this is a good time to check that the bearings are still good and smooth. This is much easier to do while the drawbar is removed from the tractor.

One more maintenance item that can and should be done at this time is to adjust the brake on the side of the transaxle. To check and adjust the brake:

1. Depress clutch/brake pedal and engage parking brake.
2. Measure distance between brake operating arm and nut "A" on brake rod.
3. If distance is other than 1-1/2", loosen jam nut and turn nut "A" until the distance becomes 1-1/2". Retighten jam nut against nut "A".

Here is what the inside of the gearator looks like. It is a gear driven pump very similar to the oil pumps in many engines. Make certain that dirt does not get into the cover while it is off of the transaxle and make sure that the internal gear doesn't get turned while off the transaxle.

Once all of the oil drains out, carefully clean the O-ring on the gearator cover and the mounting surface on the pan with a clean, soft shop towel. Replace the gearator cover and tighten down the bolts firmly.

On the top of the transaxle, thoroughly clean the housing to ensure that no dirt gets into the transaxle. Remove the vent hose from the top and also unscrew the fitting from the top of the case. This is what we will be using to refill with oil.

Refill the transaxle with three quarts of oil.

Replace the fitting and vent tube and ensure that the check plug is in the transaxle and secure. The air must be purged from the transaxle to ensure proper fluid level and operation.

To purge the transaxle:

1. Place tractor safely on level surface with engine off and parking brake set.
2. Disengage transmission by placing freewheel control in disengaged position.
3. Sitting in the tractor seat, start engine. After the engine is running, move throttle control to slow position. Disengage parking brake.
4. Move motion control lever to full forward position and hold for five (5) seconds. Move lever to full reverse position and hold for five (5) seconds. Repeat this step three (3) times.

NOTE: During this step there will be no movement of drive wheels. The air is being removed from hydraulic drive system.

5. Move motion control lever to neutral (N) position. Shutoff engine and set parking brake.
6. Engage transmission by placing freewheel control in engaged position.
7. Sitting in the tractor seat, start engine. After the engine is running, move
throttle control to half (1/2) speed. Disengage parking brake.
8. Slowly move motion control lever forward, after the tractor moves approximately five (5) feet, slowly move motion control lever to reverse position. After the tractor moves approximately five (5) feet return the motion control lever to the neutral (N) position. Repeat this step with the motion control lever three (3) times.​
After the purging procedure has been completed, remove the gold colored check plug, vent hose, and adapter fitting and slowly add more oil to the transaxle until it starts to run out of the check plug. The total oil added will be between three and three and one half quarts. Mine took slightly more than three and one quarter quarts.

Replace the check plug, vent hose adapter fitting, and vent hose. I added a small hose clamp to my vent hose due to it fitting a little loosely and could possibly let dirt into the transaxle.

Reassemble the drawbar and motion control lever linkage in reverse order of disassembly. Use the pocket magnet to hold the spacer in place for the motion control lever while the longer bolt is being replaced.

You are now finished. Enjoy many more years of use from the transaxle now that you have fresh, clean oil in the transaxle.

Originally posted on July 2011.
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