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Transmissions

 

Lawn mower transmissions for the past few years have mostly all used the hydrostatic drive. These transmissions are used in riding mowers that usually cost around $1800.00 to $2,500.00 dollars. They have a pedal to go forward and a pedal to go into reverse. Some of the newer mowers have a switch that allows you to mow in reverse after pressing it. That's a pain when you have to turn around and look back while depressing a button on the dash, makes the back ache! These are simply put there due to a law suite from a customer that got hurt bad or killed while mowing in reverse, the liability issue in product sales.

 

These buttons simply connect together two wires that energize a relay that when energized allows the pto clutch to operate while reversing the tractor. A mechanic in a John Deere shop would get fired immediately if he or she were to disconnect the safety circuit in any way. Asking one to "wire around" the switch will certainly net a big no. However you as a homeowner can do it yourself easily without using any other electrical parts in a few short minutes. This will allow the mower to reverse and mow at the same time like old days style.

 

The transmission is simply a small hydraulic pump and as you push on the pedal in either direction a metal rod goes back to a lever hooked to the pump output volume like a valve allowing more oil volume in and out of the pump for each revolution of the engine.

 

So by pressing the forward pedal down the pump output flow is routed into a small hydraulic motor which is connected to the rear drive wheels. So the transmissions we are referring to are sealed tight and have no oil filter a person can change like in a car or other equipment. If you have one of these transmissions in your riding lawn mower here are some important items to understand about them.

 

Inside the transmission case is a hollow area the oil fills up. If the transmission gets hotter than designed to run the oil will expand inside the transmission case and be forced under pressure out the air vent valve or the hydraulic pump drive shaft seal. Either way there will be an oily surface present and usually a dirty greasy looking transmission top case.

 

These transmissions are designed to last approximately 400 to 600 engine hours, I have seen them fail at 275 hours so here is why.

 

Inside the transmission there are gears that drive the rear wheels from the hydraulic motor shaft. These gears are metal but based on cost to produce, the metal actually starts out like a sand and this is called cintered. Cintered metal takes heat and pressure and it forms a hard gear that is lighter and way cheaper than a machined out gear like in a bigger transmission that costs more than a homeowner paid for the entire mower.

 

These gears have a thin coating of very hard metal for wear purposes and they rub together called gears meshing, as these gears mesh together there is some friction involved. The power from the engine rotating at that point reduced into the final rotation of the rear wheels is called the final drive or differential in a more technical point of view. These final drive gears are creating heat in the oil and in fact as well as other rotating parts within the transmission case.

 

 

So in the design of the transmission there is a specific rated load these gears will hold, however the more load on the transmission final drive gears the more heat dissipated into the transmission oil. The load is actually you plus the weight of the mower. On a flat smooth lawn the transmission creates heat and when going uphill or downhill creates more heat due to more weight load on the final drive gear faces. Important to understand the entire surface contacting to the load is smaller than the diameter of a regular Q-Tip. So each extra pound of load weight adds heat and we could say perhaps a 100 pound person would create a 100 degree temeperture gain in that specific amount of oil and that the transmission cooling fan keeps it running on a flat surface with 100lb. weight load at a 100f constant oil temperature and depending on the temperature of the air while operating the mower.

 

 

When you add 10 more pounds of load in the exact same operating situation let's say we gain 1 degree in temperature f. and then add 10 more degrees in angle and gain 10 degrees in temperature. You can easily figure out now that more weight equals more temperature at the final drive contacting gear faces. Realisticly the transmissions are really good for the money they cost when you buy one.

 

As heat builds up the oil has less ability to reduce friction and then more heat builds up. Also the more load weight and hillside angles put added stress onto the gear faces so the small particles come off the faces in normal wear and drop into the heated oil. There is no filter inside the transmission but there are two very weak magnets that get coated solid with gear face metal particles floating in the hot transmission oil. That contamination of the oil by floating particles causes the failure to begin.

 

Small sandlike particles float downward and are sucked into and towards the hydraulic pump powering the transmission. These particles are small enough to go through the transmission pump screen. This immediately destroys the hydraulic pump rotating member sealing face surface. The feeling is a transmission that goes fine and then seems to get sluggish to change direction and speed will reduce as transmission get's hot.

 

In conclusion these transmissions are severely overworked in this area and it is fact that "all" models with these transmissions fail within a range of the hours depending on how overloaded they were operated. There are a ways to extend the transmission life and also a fix for the transmission that is not out yet and also a repair kit available for after failure.

 

Either way it will cost you $1,000.00- $1,500.00 to get back mowing when you replace the transmission with a new one. To repair one it costs $100.00 for machine shop work, $250.00 to remove replace and disassemble old transmission. Cleaning transmission parts and modifying with filter system $200.00. The parts kit costs $112.00 and $18.00 shop glue. This is approximately $680 modified complete.

 

 

 

Bigger rear wheels seem to extend life by reducing revolutions of the transmission parts. A light operator under 150 lbs. going on hills in a way to reduce incline or decline angle while mowing. A temperature gauge so as transmission temperature raises operator can reduce speed or stop and let it cool. Keep transmission clean on top, grass on top keeps more heat in than a clean transmission. At first sign of oil on top of transmission it should be changed and have the modification filter kit installed so oil changes are easy and the hydraulic pump has a filter which is $14.00 and easy to get at any auto parts store.

After many years of refining and repairing these transmissions we have found that taking regular breaks every hour to let the transmission cool down extends transmission life. As soon as the transmission feels like it is not responding as quickly to the foot pedal movement from forward to reverse or hesitates to move remove the transmission and drain out the old oil and install new 0W 15 synthetic engine oil, and bleed the hydraulic system in the transmission. The mowers using this type of transmission are not intended to be run as a commercial unit working hours each day.