Here's a breakdown:
* Torque Converter Slip: At low speeds, a torque converter inherently slips. This means the input shaft (connected to the engine) and the output shaft (connected to the transmission) aren't directly coupled. There's a fluid coupling that allows for speed differences between the two, providing smoother starts and reduced shock loading on the drivetrain. This slip creates the feeling of a slightly less direct connection between the engine and wheels. This is normal.
* Diesel Engine Characteristics: Diesel engines, especially older ones, often have a significant amount of low-end torque. This torque can be amplified by the torque converter's multiplication effect at low speeds, leading to a feeling of more noticeable "slip" or less direct engagement.
* Transmission Wear: If your transmission is worn, the torque converter might be exhibiting more slip than usual, or there might be other issues causing a less direct feel. Worn clutches or internal components can exacerbate this sensation.
* It's NOT a Lock-Up Clutch: While many modern automatic transmissions have a lock-up clutch in their torque converter to directly couple the engine and transmission at higher speeds for improved fuel efficiency, this is less common in older trucks like your 1999 Dodge, and especially unlikely to be engaging and disengaging at low speeds. A lock-up clutch would generally engage at higher speeds and cruising conditions.
In short: What you're likely feeling is the normal behavior of a torque converter at low speeds, accentuated by the characteristics of your diesel engine. However, if you feel excessive slip, unusual noises, or other drivability issues, it might be worth having a mechanic inspect your transmission for potential wear or problems. They can diagnose if anything is truly amiss beyond normal operational characteristics.
