TranSynd 668 Full Synthetic ATF 3 Gallon for Allison 1000

Three gallons of TranSynd 668 is sized for one job: a complete pan drop and filter service on an Allison 1000 behind a Duramax or 8.1L in a 2001-2019 Chevrolet Silverado or GMC Sierra 2500/3500 HD. Owners shopping for it usually have a truck that is due by mileage, one that just came home from a hard summer of towing with dark fluid, or a used truck with no service history and a stick that smells cooked. This pack is three one-gallon bottles, 384 ounces, of Allison TES 668 approved full synthetic.

What the fluid does inside an Allison 1000

The 1000 Series applies its five clutches with fluid pressure that the TCM shapes through the trim solenoids, so the fluid's viscosity and friction behavior directly set shift feel. Old fluid that has oxidized and thickened varnishes the valve body, wears the valve bores and lets pressure fall off, and that is when clutch packs start to slip under load. TES 668 is Allison's current spec, recommended for the 1000 through 4000 Series, and built for stability under exactly the sustained heat a towing pickup produces.

Symptoms that say the service is overdue

• Dark, burnt-smelling fluid or fluid that has lost its red color
• Light-throttle shudder near lockup speed
• Harsh or late shifts, or a flare between gears when hot
• C1/C2 clutch slip under a trailer, which is the Allison 1000's most common heavy-towing wear
• A spin-on filter that has gone past 50,000 miles without a change

How to confirm before you buy

Pull the stick after a drive and look at it on a white rag. Dark fluid with no metal on the filter magnet is a service. Metal larger than dust on the magnet, or a slip that does not go away after the fluid is fresh, points to internal wear, and our Allison 1000 valve body testing guide walks through the pressure and solenoid checks. If the truck came in on old TES 389 fluid, plan for the fact that one pan drop leaves fluid in the converter and cooler, so the unit counts as a mixture on the short interval until a second change.

Fitment and how much you need

The fluid fits every Allison 1000 and 2000 Series unit, which covers the 2001-2019 GM HD pickups with the Duramax or 8.1L and the Kodiak and TopKick, and it is also approved for 3000 and 4000 Series medium-duty units. What decides the quantity is the pan, not the model year. Allison lists a standard-sump refill at about 10.6 quarts and a shallow-sump refill at about 7.4 quarts. Three gallons is 12 quarts, so it covers a standard or deep-pan refill with room to set the hot level, and leaves a spare gallon on a shallow pan. A dry initial fill after a rebuild or cooler replacement takes more, about 14.8 quarts plus the lines; for that, the five-gallon pail is the better buy. Look under the truck before you order anything else, because many of these trucks got an aftermarket deep pan at some point. More parts are on the Allison 1000 parts page.

Install notes

• Drain hot, at a sump temperature of 160-200 F, so more of the old fluid comes out.
• Replace the internal filter and pan gasket while the pan is down. For a factory-depth pan use the Allison 1000 filter and pan gasket kit; for a deep pan use the deep pan filter and gasket kit.
• Change the spin-on filter too: oil the gasket, hand tight to contact, then one full turn only.
• Drain plug gets a new sealing washer, 22-30 lb-ft.
• Refill short of the full amount, run the engine at 1,000-1,500 rpm for a minute, cycle D, N and R, then set the cold level. Finish on the hot check after a drive.
• On a Prognostics truck, reset the oil life and filter life monitors after the change using the procedure in the operator's manual or a capable scan tool, so the TRANS SERVICE light does not come back early.

The full interval table is in our Allison 1000 fluid and filter service specs.

Is 3 gallons enough for a deep-pan Allison 1000 service?

For a pan drop and refill, yes. Allison lists the standard-sump refill at about 10.6 quarts and three gallons is 12 quarts. A completely dry fill after a rebuild or cooler replacement needs more, about 14.8 quarts plus external lines.

How often should the fluid be changed on a Duramax Allison?

With 100 percent TES 668 or TES 295 fluid and Prognostics off, Allison lists 150,000 miles or 48 months on general duty and 75,000 miles or 36 months on severe duty. The spin-on filter is due sooner, at 50,000 miles or 24 months.

What filter should I buy with this?

Match the internal filter to the pan on the truck, deep or factory depth, and replace the spin-on control main filter at the same time. A pan gasket is usually included in the filter kits.

My truck had TES 389 fluid. Will one change with TranSynd 668 give me the long interval?

Not right away. A pan drop leaves old fluid in the converter, cooler and lines, so Allison treats it as a mixture on the short TES 389 schedule. A second change gets the unit to full TES 668.

Do I need to reset anything after the service?

On trucks with Prognostics enabled, reset the oil life and filter life monitors using the operator's manual procedure or a capable scan tool, so the TRANS SERVICE indicator tracks the new fluid and filter.

• Allison 1000 Fluid and Filter Service: Real Capacities, Intervals, and the TES 668 Change
• Allison 1000 Transmission Service Guide: Fluid, Filters, and Common Issues
• Allison 1000 Valve Body Problems: Symptoms, How to Test, and Replacement
• Allison 1000 Torque Converter Shudder: Diagnosis and Fix