OutdoorKing Repair Forum Forums Repairs and Maintenance Questions On Ride On Mowers Questions On Ride On Mowers Frames MTD 688 final drive

Hi All, Thank you for the birthday wishes.
I have an MTD model 688 with a transmission/final drive problem. This happens in Forward gear only & the severity increases as the speed range is increased from 1 to 8 (the engine revs are constant).
The machine 'surges' forward & then slows momentarily & 'surges' forward once more. It becomes noticeable above speed range 3.
Consulting my spare parts listing, I see that there are 2 bevel gears, presumably forward & reverse, driven by the input shaft. Beside one bevel gear is listed a 'Flat washer' while beside the other bevel gear is listed several 'Flat washer'.
Questions. If one or more washers is worn/missing, could this be the cause of the problem. Would this allow the input shaft to partially disengage from the bevel gear. Is there an 'easy fix' I can do to prevent the problem.
Any help you may offer is greatly appreciated. Regards Kamel

Kamel, when the surging occurs, is the engine speed varying to match the mower's speed? If it is, I doubt the cause is the transmission - it is more likely to be that the engine hunts under load. Hunting by a governed engine has one of two causes: friction/jamming in the governor linkage, or lean mixture. Lean mixture is by far the more common cause.

I suggest you post pictures of your mower, and details of the engine. I am guessing it is a Briggs & Stratton, but we need the model, type and code numbers which are stamped on the cooling air cowl. Then we can get specific about how to deal with this.

Hi Grumpy, Thank you for your reply. The engine speed is pre-set by a hand throttle. It does not surge or vary, except under load, as is normal. Mower speed is determined by 'gear lever'/variable pulley belt drive in the range of ( park - 1 to 8). Drive belts have been replaced. The direction of the mower is selected by a mechanical shift lever for either Forward or Reverse.

The problem occurs ONLY in FORWARD gear & the engine speed is constant.

It DOES NOT occur in reverse gear.

The problem occurs whether or not the mower deck is engaged & is more noticeable the higher the speed 'gear' number is selected. This ranges 1-8.

If the problem lay with the engine as suggested, it is logical to think that the surging would occur in both directions. However, it only occurs in the forward direction. This makes me think that it may be an internal problem, but I don't want to pull the final drive apart for no apparent reason.

The motor is indeed a B&S, model 283H07 type 0197E1. It is well maintained & in good condition. Regards Kamel

If the mower speed varies without the engine speed varying, there has to be one of three things:
1. Belt slip.
2. Cycling of the variable ratio belt drive, perhaps due to a loose part in the speed selection linkage.
3. Slippage in the final drive unit, after the belt drive. The first stage of the final drive unit is the forward and reverse selection by choosing one of two bevel gears and selecting it with a dog clutch. The last stage of the final drive is by spur gears. I haven't yet figured out a way for it to slip (except terminally, if one of the gears broke or failed to engage).

Have you had a close look at the illustrated parts list? You can download it here:
http://lawnandgarden.manualsonline.com/manuals/mfg/mtd/688_1.html?idRes=11462237

Right at the moment I'm inclined to suspect Problem 1, rather than 2 or 3. Although you have just replaced the belts, if one or more of the pulleys are worn, belt slip is a plausible explanation. You may be able to check for belt slip by stopping the engine while speed cycling is occurring, then immediately checking for one or more hot pulleys. Cyclical slipping is quite possible, since the belt will heat up when slipping and eventually become sticky. When it stops slipping due to stickiness, it will cool off and slip some more. With a slipping-belt clutch system, which you have, wear to the belt pulleys happens quite quickly. Hence your problem could be due to the wheel-drive belt bottoming in a worn pulley, which reduces the amount of torque that can be transmitted. Careful inspection of the bottom of the belt, and the bottom of the pulleys, will usually show whether bottoming is occurring.

Do not be surprised that it only happens in forward drive: I doubt you run it long and hard enough in reverse for the belt to heat up.

While the dog clutch for forward and reverse could "slip" in and out of mesh, it would be accompanied by loud bangs and almost certain breakage of gears, and when it was out of mesh, there would be no drive at all, so this just does not match the symptoms you have described.

Thank you. I'll double check the pulley system as you suggest. It does make sense, although it does occur from start-up.
As far as a loose part (suggestion #2) goes, 2 very strong springs are in place & the pivot for the variable speed pulley shows little sign of wear. The bearings of the var.spd pulley are new.
The 3rd possibility may be what I don't like to think about.
It's pouring rain up here at the moment & the forecast doesn't look good for the next few days, so it may be a while before I get back to you. Many thanks for your help. Regards Kamel