Hi, I have a running 2014 Diesel promaster van 438k miles. You should change your transmission oil periodically (every 30k)or near. Manual owner said nothing about it, and dealers said never need to be changed. Gears in your transmission make friction and generates metal debris. This transmission has not oil filter, not oil pump, only a little piece of magnet, not enough to trap all the iron debris inside. This transmission has about 6 little bearing with plastic holder cases (balls separators are plastics) on each gear wheels. Very fragile. Any debris is enough to mess it. Oil change is the way to flush and clean it. Drain plug is in the bottom of differential case on driver side. Refill plug is the other like drain, and you can find it in the front face of the transmission over the speed sensor. You are going to need a piece of hose and a funnel, Fill 3 liters of gear lubes 75w 85 or similar. NOT ATF, THIS IS NOT AUTOMATIC TRANSMISSION. BUT 6 SPEED AUTOMATED MANUAL TRANSMISSION. Easy and worth it ,About 15 dollar at Walmart. Do not work your van idle. Idle affect particle filter, engine, and transmission. While idle your transmission is running but is not enough lubricated because the oil do not splash the uppers shaft which are spinning and dry. Tires are not moving because your clutch is working to keep disengaging.