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Diesel Transmission Fluid 101

Posted by Scott Goldfarb on

Much like a regular gas engine, diesels use a transmission system to transfer the power generated by the engine to rotate the vehicle's wheels and move it forward. Transmissions, often referred to as 'trannies,' generally vary between manual and automatic operation. The former requires the driver to manipulate a clutch system to switch the vehicle into a different gear, and the latter automatically performs all of the gear functions.

Both systems have pros and cons, depending on your driving preferences. While an automatic tranny offers convenience, higher clutch life expectancy, and integrated brake and exhaust features, it also represents a more complex system than its manual counterpart. Translation - repairing or replacing an automatic transmission assembly will cost you more. Manual tranny repairs are less expensive, and you'll get more efficiency from a manual setup. Still, operation takes more driver involvement, and you miss out on the automatic brake and exhaust integration.

Regardless of which type you choose to drive, manual and automatic trannies require fluids not only for lubrication but also to create hydraulic pressure and friction. That's because your transmission contains many moving parts that can wear down quickly and fail under constant motion.

The Main Parts of a Diesel Transmission (Automatic and Manual)

First, let's talk about the parts you need to protect. Your tranny contains several sensitive components, including shafts, gears, and bearings. When these go bad, they can quickly damage other parts of your transmission and engine, so it's critical to address any potential issues immediately. If your car is under warranty, a great first step is to contact your manufacturer. Otherwise, you may want to refer to a wholesale diesel parts supplier to find some good deals on high-quality replacements.

1. Automatic Parts

  • Input and output shafts
  • Clutch packs
  • Planetary gear packs
  • Torque converter
  • Brake band

2. Manual Parts

  • Input and output shafts
  • Synchronizers
  • Counter shaft
  • Shift rod
  • Shift fork
  • Idler gear

How Transmission Fluid Works

Since automatic and manual transmissions function differently, they each need their own kind of fluid to work effectively. The fluid's primary function in manual systems is to lubricate essential moving parts, such as bearings, rods, and gear ratios, as you have a lot of metal-on-metal contact. The fluid also cools the intense heat generated by all that movement.

While automatic trannies also use the fluid for cooling and lubrication purposes, another main function is to provide hydraulic pressure through the torque converter. The converter has five essential parts - the housing, lock clutch, turbine, stator, and impeller (pump). It separates the engine from the transmission, allowing the motor to rotate without spinning the wheels, such as when you have your foot on the brake at a stop light.

When you start the motor, the impeller rotates with the same RPMs as the engine and swirls the transmission fluid outward toward the turbine through the stator. The force created by the swirling liquid begins to turn the turbine and the stator, which magnifies the torque to accelerate the vehicle. As the turbine spins, it swirls the fluid through its center back into the impeller. This cycle, known as fluid coupling, keeps the engine and transmission operating in sync while the vehicle is in motion.

When and How to Check Transmission Fluid

Your transmission fluid will degrade over time, so you must change it regularly. Consult your vehicle's manual for the manufacturer's suggested change times, as these will depend on your transmission type and the make and model of your truck.

Other factors are equally important in determining when to change the fluid, such as if you're using your truck for heavy towing or lifting, long road trips, or stop-and-go traffic. Gear fluid will degrade faster with continuous, heavy use. Your owner's manual may only suggest a fluid change once you're at 100,000 miles. However, if you're hauling hefty loads up and down a mountainside, consider checking and changing at 50,000 miles or less.

Transmission fluid should be relatively clear or slightly pink if it's in good condition. If it looks dark red or brown, it is likely old and dirty and needs to be changed. Other indicators that your fluid has gone bad - you hear grinding or whining noises, feel the gears slip, you can't put the car in reverse, or the engine is running overly hot.

In an automatic tranny, you will find the fluid dipstick under the hood clearly labeled. You can check it just as you do the oil - pull the dipstick out, clean it, and re-insert it. Then remove it again to check the level. If it's between the hash marks, then you are good. If the fluid is low, top off a little at a time, re-checking the level after each addition.

Checking the fluid in a manual is more complex, as you'll have to get under your car and remove the transmission fill plug. Once you unscrew the plug, you can use any dipstick-like device to determine the fluid level and how clean or dirty it is.

It's not terribly difficult to change your transmission fluid on your own. But before you start, contact a good diesel parts supplier to have spare fill and drain plugs and any other parts you might need on hand. You'd be surprised how easily small components can go missing. It’s also essential to use the best available fluid product designed for your vehicle. Don't skimp on quality, as it may result in expensive repairs down the road.


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