MTD sells a SHIFT-ON-THE-GO line of lawn tractors that have transmissions, which do not have real gears, but instead rely on a combination two drive belts and variable speed dual-grooved pulley assembly for the transmission. There are no gears per se, however, there is a lever that can be set to several notched positions, giving you the impression that you have gears. You will notice that your tractor has a foot pedal (as we will see this pedal is closely related to the so called “gear” lever). This pedal servers three functions: it is the brake, clutch, and the speed control. When pressed fully down it is the brake and the clutch. In this position even with the engine at full throttle, the tractor does not move (See note 1). Also, in this position it permits you to put the hand-operated “gear lever” in one of the several numbered notches that related to speed or you can put it in the notched parking brake position. Now how does this really work? Pushing down the foot pedal moves mechanisms that stretch a strong spring. The hand-operated “gear” lever only limits the upward movement of the foot pedal when you ease your foot pressure off the pedal. If you have an 8-speed “gear” lever, but have the lever in the position number one, the tractor is going move at the lowest speed when you release the foot pedal. This is because the foot pedal is limited from moving very far up by the “gear” lever’s setting.
So, how do the two drive belts fit into the picture? The front end of the front drive belt goes over the engine pulley thus getting propelled by the engine. The back end or the rear belt goes over the transaxle input pulley, thus being the driving force for the rear wheels. Each belt goes over a separate groove on the variable speed dual grooved pulley thus, joining the two belts. This pulley has the unique quality of being able to change diameters in a way that when one groove’s diameter increases, the other decreases and visa versa. This change occurs by one spring overcoming another spring as you release the foot pedal. Conversely when you press down on the foot pedal you make one spring stretch and increase its force, thus overcoming the other spring and the groove diameters change in the opposite way.
In my opinion this is a cheaply designed mechanism and at best may be suitable for very low powered tractors mowing on a level surface in an open area. For all other application, this is a dangerous design.
I have a 20HP tractor with a 46" mowing deck. I mow a very hilly grounds and have had many surprises from the unpredictable way this unit operates, including being thrown off the unit twice, once by the tractor making a complete back flip when it started riding up a utility pole’s guy wire and couldn’t be stopped. Sometimes this transmission will seem to pull like a mule, at other times it slips and slows to a halt (see note 3), until you push the clutch in and then release it (“popping the clutch”). Humidly, condition of the belts (slightest wear on the side edges), how long since the last use (pulley rust), temperature, all seem to be factors in how the transmission functions. You need to test how the machine going to functioning virtually every time you use it and then drive it accordingly.
Notes:
1. “Move” means the tractor is not propelled. Whether it rolls or is free to move on it’s own depends on the brake rod being correctly adjusted. Mine was returned from an authorized dealer repair with this rod incorrectly adjusted (no brakes). Even when properly adjusted the brakes are not great. You wind up relying greatly on the engine to hold you back going down a steep grade. If the transaxle forward-neutral-reverse lever slips into neutral or a drive belt breaks, you in for a thrilling uncontrolled down hill ride. (I had to make a sharp left at the bottom of the hill to avoid going into a row of razor sharp brush. The tractor attempted to turn over but the wideness of the 46’ deck acted as outrigger and I just got bucked off. Thank goodness for the kill switch under the seat or it might have turn around and tried to mow me.
2. Instructions tell you to mow with the engine throttle at full speed. This is done so that the deck blades turn at highest speed and you achieve a nice clean cut. Unlike an automobile accelerator pedal which is pushed down against a return spring to go faster, thus if your foot slip off, you slow down. The tractor foot pedal increases speed via the transmission as you release foot pressure, allowing the pedal to spring up. When your foot accidentally slips off this pedal you are accelerate to full speed. If you are on level ground out in the open, you can quickly regain control and not have a problem. However if you are mowing parallel to some thorny brush and an unobserved branch springs back and catches you in the foot or leg, you are suddenly at full speed being ripped by thorns or heading straight for that low hanging tree branch.
3. MTD’s factory service manual has a fix for this so called safety feature of the tractor crawling to a stop as you are going up a hill (nice safety feature, eh). It involves replacing the foot pedal mechanism’s spring with a stronger one. It’s not an easy job and did not improve the problem all that much.
4. MTD makes some great products at reasonable prices. They have great customer support and will sell you parts direct.. Go to their website at www.mtdproducts.com
Pat Sullivan, April 2006
