Working on the LA175 deck belts
The La175 John Deere Belt Diagram is something you need when either the mower deck belt is shredded or the transmission drive belt gives up. I replaced both on a customer's unit last fall. The thing that trips people up isn't the routing itself — it's knowing which belt is which before you pull the old ones off. John Deere makes this harder than it needs to be because they don't label the pulleys clearly. You'll find two separate belt systems: the deck drive belt and the transmission drive belt. They look similar at a glance but run completely different paths and serve different purposes.
La175 John Deere Belt Diagram basics
The deck belt is a 4L350 (sometimes listed as 4L355) V-belt. It runs from the spindle pulley on the left side of the deck, up and around the idler pulleys, across to the right spindle pulley, then back through the tensioner assembly under the deck. When you engage the PTO, a spring-loaded tensioner pulley presses against the flat side of this belt to drive the spindles. If your deck isn't spinning when the PTO is engaged, this is the belt to check first. Most of the time it's either glazed and slipping or snapped cleanly between two pulleys. The transmission belt is a different story. It's typically a 4L280 or 4L300 V-belt depending on the production year. This one connects the engine crankshaft pulley down to the transaxle input pulley. It's under much higher tension because it's actually moving the tractor. You'll find it running along the right side of the frame, behind the foot pedal assembly. Removing it requires backing off the tensioner spring, which is coil-spring loaded and can pop free if you're not careful. I once watched a spring launch across a workshop and nick a windshield. Always wear gloves and eye protection when touching that tensioner. Here's the practical part: before you remove anything, take a phone picture of the existing belt routing. John Deere's official diagrams are okay but they sometimes leave out the smaller idler pulleys that route the belt in a specific arc. I learned that the hard way on a 2003 LA175 where the factory diagram showed seven pulleys and the actual belt went around eight. The extra idler was a wear idler that isn't called out in the manual but changes the effective belt path significantly. My workaround was tracing the belt with a piece of string, marking each pulley contact point with chalk, and then laying the string flat on paper to recreate the route. It took about ten minutes and saved me from guessing.
Common routing mistakes
The most frequent mistake I see is routing the deck belt backwards through the tensioner. The belt needs to sit flat against the tensioner pulley face, not twisted or riding on the edge. A twisted deck belt will throw heat fast and melt the rubber within twenty minutes of operation. You'll smell it before it fails. Also make sure the belt is seated fully in every pulley groove. These are narrow V-belts and they love to pop out of shallow pulleys if you nudge them during installation. Another pitfall is mixing up belt orientation between the left and right sides of the deck. The pulleys aren't always mirror images. On some LA175 models the right-side spindle pulley has a slight crown that the left one lacks. Running the belt backward throws off the tracking and you'll have the belt jumping off the pulley within a day. Check that the belt follows the natural sweep of each pulley without crowding against the inner or outer flange.
Get the Full Details
Where to find the official diagram
John Deere's own technical documentation is available through their parts lookup system. Go to jdparts.com, enter your LA175 serial number, and pull up the engine section and the deck section separately. The engine section will show you the transmission belt routing with part numbers. The deck section has the spindle belt diagram. Don't try to mash both into one image — they're in different sections of the manual for a reason and combining them tends to create confusion. If you want a single reference sheet, the Sears Parts Direct website has free PDFs for many John Deere models that combine both diagrams on one page. The quality varies by model year but the LA175 ones are generally readable. When you're doing belt work, replace the idler pulleys if they have any flat spots or rough rotation. A $4 pulley will destroy a $15 belt in a week. Also inspect the tensioner spring for elongation. These springs stretch over time and lose clamping force. A weak tensioner spring means the belt slips under load even when it looks properly routed. I test this by pressing the tensioner arm by hand — it should snap back sharply. If it moves slowly or feels mushy, the spring is gone and the belt won't stay tight. The deck belt tensioner pulley itself is a free-spinning idler that rides against the flat side of the belt. Make sure it spins freely. I've seen these seized solid from grass buildup and moisture, and the belt just burns through because the tensioner can't follow the natural movement of the deck. Clean the pulley axle with penetrating oil and spin it by hand. If it doesn't rotate smoothly, replace it before installing the new belt.
Reassembly notes
Thread the new belt last. Get all the pulleys positioned correctly with the belt loose, then stretch it over the final pulley — usually the tensioner or the hardest-to-reach spindle — while applying steady pressure. Don't force it. If you're fighting the belt, something is probably out of place. Double-check your reference photo. The whole process usually takes about twenty minutes for the deck belt and thirty to forty-five for the transmission belt, depending on how tight the workspace is around the transaxle pulley. Once installed, engage the PTO by hand a few times and watch the belt track. It should run centered on every pulley. If it drifts toward one flange, shut it down and adjust the pulley alignment before running the engine.
