A few obvious signs that your transmission has gone bad are – whining and noise when on the move, transmission working only in forward or only in reverse mode, leaking oil, when it performs good after a cold start and loses power as it gets warmed up and when the problem was slowly building up over time (a sudden loss of power is usually a sign that the problem is not transmission related).
If you’ve noticed these symptoms, this article is for you.
What is an integrated hydrostatic transmission (IHT)?
Hydrostatic transmissions are a type of CVT (continuously variable transmission) that acts as a hydro-transformer (in the way). Hydro-transformer uses liquid (typically oil) as a means of transmitting and varying the power.
However, when a traditional hydro-transformer in an automatic hydro-glide gearbox use fans as means of transmitting, receiving and altering power in an enclosed oil bath.
Hydrostatic transmissions utilize swash plates (variable and static) and oil pistons that are enclosed in the oil chamber as means of transmitting and altering the power.
Hydrostatic transmission has two sets of swash plates that are paired to accordion piston assemblies, of which there are also two pairs.
The swash plate—piston assembly that sits on the input shaft(5) and receives power from the engine is called a pump(1) as it converts the rotational energy to liquid flaw energy.
The swash plate—piston assembly that sits on the output (drive) shaft(8) and receives power from pressurized oil flaw is called a motor(2) as it converts the liquid flaw energy to rotational energy.
The pump(1) and motor(2) have pistons(4) lined up in the circle as they ensure a constant contact patch with the swash plate and allow the dynamic swash plate angle to change for altering the oil pressure.
The gear ratio in this system as well as rotational direction is changed by altering the swash plates’ angle. Typically, only the pump (input shaft) swash plate(3) angle is altered by the travel lever or travel pedal, but there are also systems where the angle of both swash plates or only the motor swash plate angle is altered.
The difference in the gearing is achieved by the difference in the oil pressure within the pump and motor pistons. A higher pressure represents a higher or bigger physical gear.
The pressure is altered by the angle of the pump swash plate that presses the pistons in-words raising the oil pressure.
What can cause problems with hydrostatic transmissions?
Typically, timely maintenance is enough to prevent any problems with your transmission, but the problem is that a lot of tractor manufacturers claim that “their transmissions do not need maintenance,” while the real transmission manufacturers: Tuff Torq and others along with common sense are of a different opinion.
But a lot of people are misled by their claims of “unserviceable” transmissions which increases the number of premature wear cases.
Also, if you operate your tractor under harsh conditions, like hot temperatures, complicated terrain, dust, etc., you should be changing your transmission oil more often.
Oil is critical to transmission, it ensures the health of every component in it by providing adequate lubrication and cleanup of the services.
But as the oil in your transmission gets old, it starts to lose its viscous abilities, because it accumulates metal fillings, it loses its chemical properties with time as it interacts with metals, seals, and goes through friction and hot temperatures. Thus, old oil leads to premature wear of transmission components.
While the metal filings are a natural byproduct of the transmission operation process; when the amount of these filings in the oil surpasses a critical point, the oil becomes a dangerous abrasive substance that brings destruction, instead of lubrication and rejuvenation it is supposed to bring.
Typically, the oil filter and catch magnets protect the transmission from fillings but when they clog up with metal filings and other debris, they stop protecting the transmission. And they have a limited capacity.
Also, when a manufacturer uses a bad “for life” oil, it has a negative chemical effect on metal parts like gears, and it destroys their molecular structure.
Of Course, if the transmission is pretty old and has passed its resource, it can start developing problems even with proper maintenance (although it will happen much later).
Common problems with hydrostatic transmission
First, let’s start with the easy problem. It can be a worn-out input shaft pulley that has worn-out splints. Typical symptoms are loss of power and immobility.
As we’ve established earlier, bad oil is the usual suspect with the vast majority of transmission problems.
Bad oil can damage the drive shaft and input shaft seals which could cause oil leakage. The insufficient oil level will fail to provide adequate oil pressure inside the hydrostatic system, and it won’t be able to transmit the energy. The typical symptom is winning, complete immobility or severe lose of power.
Differential and reduction gear-set gears as well as bearings can get damaged by excessive metal filling in the system. The process rapidly spirals as chipped pieces of gears cause even more destruction. Typical symptoms are insatiable mobility, slipping, and grinding noises.
But the most common problem is, worn-out pump and motor pistons and their ports. Again, the premature wear of these is typically caused by bad oil that is full of metal fillings. The typical symptoms are loss of power, slipping, and whining noises; usually, the symptoms get worse as the transmission and the oil inside warm up.
What are the fixes?
First, take a look at the exact transmission model on the barcode that’s located on the axle part of the transmission and use it when ordering any new parts.
You will have to remove and disassemble your transmission to inspect it thoroughly.
Before disassembling the transmission, when you’ve removed it, inspect your drive pulley (it could be the reason for your problems) and check the oil level through the fill port (using a metal ruler). If the oil level is significantly lower than a user manual norm, you should take a look at your drive shafts and input shaft seals and probably change them (according to manufacturer’s guide).
Then, drain the oil through the filler port, unscrew all the bolts that hold uppercase and lowercase together and pry them open.
If you’ve noticed that both catch magnets(9) (differential and fill port one) are extremely dirty and the oil filter(10) is completely cluttered, it could be a bad sign.
Inspect all the gears in your differential(11) and reduction gear set (12), they should sit tightly and have a consistent surface on their teeth. You can spin the gear train with your hand to test the fit men. For a more thorough inspection, remove it altogether, Change any gears that could be damaged and clean the remaining gears in a degreaser breath.
to inspect the hydrostatic system(13) (or the center case), you will have to remove it. Be careful not to lose the motor and pump pistons, bearings(14), the disc brake(15) and brake shoes(16) washers, IDS valves, a jerky plate(17), etc.
Now, you could try to inspect the pump and oil pistons along with their ports and IDS yourself. However, they have extremely tight tolerances combined with complex rounded shapes, and having a measuring instrument alone won’t guarantee a proper measurement; so it would be best if an experienced professional would do the measuring.
I highly recommend swapping your center case(13) for a brand-new one or sending yours for a rebuild to a professional.
Assemble the transmission according to its repair manual and don’t forget to squirt some fresh oil on new and cleaned gears to prevent them from damaging during the assembly process.
Excluding problems with similar symptoms as faulty transmission:
Shutting off, losing power when under load and not being able to move the tractor on its own power. These symptoms could come from other problems with your tractor.
The engine. Take a listen to how it is working when the transmission is not engaged. If it runs and responds to throttle input smoothly, if it doesn’t misfire and doesn’t choke under load; you should look for a problem elsewhere, otherwise a thorough inspection of your engine and ignition as well as fuel systems is due.
If your loan mower shuts off as soon as you engage your transmission, you may want to check the seat safety switch which may be broken and unable to detect a driver in the seat causing the engine to shut off.
To test your seat safety switch, lift up your seat, unplug the lead wire terminal from the safety switch and test the switch for continuity with an Ohmmeter in both off and on positions by testing corresponding switch pairs of contacts (there are four contacts that make two pairs, ON and OFF). Change the switch if it’s bad.
If your tractor mower fails to move when you engage the forward travel pedal or loses power when under load, you should also check the drive belt. To check the drive belt, start your engine up and put it on full throttle, then engage the electric PTO switch.
If you hear your blades kick in (spin) immediately after engaging the PTO (in under half a second), your belts are fine; otherwise, when blades kick in with a delay or don’t kick in at all, you have A belt problem, but it can be just a deck belt problem.
To check the drive belt, you’ll have to remove the mower deck first. You need to check the tension (keep in mind that when the brake is engaged, the drive pulley is loose causing the belt to loosen a bit as well). You should change the drive belt if it’s too old/slack and has visible wearing marks on it.
Belt problems may also lie in bad or stuck pulleys. But to inspect all the pulleys, you’ll have to remove the drive belt altogether.
Also, this might be comical but check if the bypass valve lever at the back of your transmission (that’s meant to use when towing your tractor) is disengaged.
Otherwise, it’s probably your transmission.
To prevent these problems in the future, change your transmission oil along with the oil filter (that’s inside) every 250 moto hours or one in two seasons (whichever is sooner). And don’t forget to clean the catch magnets.
If you operate your tractor under harsh conditions: hot temperatures, complicated terrain, dust, etc., you should be changing your transmission oil more often.