ZF 6 Speed Transmission Fluid Change (S6-650, S6-750)

Contents
  1. Service Intervals
  2. Lubricant Specifications & Requirements
  3. Transmission Cooler/Line Flush
  4. Service Parts List
  5. PTO Cover Gaskets vs Silicon
  6. Fluid Replacement Procedures

Applicable Models:

2001 - 2006 GMC Sierra 2500 & 3500
2001 - 2006 Chevrolet Silverado 2500 & 3500
1999 - 2010 Ford F-250, F-350, F-450, F-550 Super Duty

Applicable Engine(s):

6.6 Duramax V-8 (S6-650)
GM 8.1 Vortec V-8 (S6-650)
7.3 Power Stroke V-8 (S6-650)
6.0 Power Stroke V-8 (S6-750)
6.4 Power Stroke V-8 (S6-750)
Ford 6.8 Triton V-10 (S6-650)

Transmission Fluid Capacity:

5.8 U.S. quarts for all S6-650 transmissions
6.34 U.S. quarts for all S6-750 transmission

Service Intervals:

Drain and refill transmission fluid every 50,000 to 60,000 miles

ZF 6 Speed Transmission Service Information

Ford requires that the transmission fluid be changed every 60,000 miles in all S6-650 transmissions and the early S6-750 transmissions mated to the 6.0 Power Stroke. The later S6-750 transmissions found behind the 6.4 Power Stroke require fluid changes at 100,000 mile intervals under normal conditions and every 60,000 miles when the vehicle usage falls under the severe/heavy duty service category; all vehicles will fall under the severe/heavy duty conditions at some point, thus we recommend adhering to the 60,000 mile interval in all applications. If a vehicle is primarily used for towing, consider more frequent fluid changes to maintain shift quality and prolong transmission life.

General Motors provides no service intervals for their version of the ZF S6-650 manual transmission, claiming instead that the transmission oil is not required to be changed. Be weary of such suggestions, as manufacturers have no fiduciary obligation to a vehicle owner after the warranty period has ended and thus do not always demand maintenance practices that result in the greatest longevity of a particular component. Additionally, the GM variant does not utilize a transmission oil cooler, thus fluid in these gearboxes may be more susceptible to thermal breakdown than their Ford counterparts. As a result, we recommend 50,000 to 60,000 mile transmission fluid changes for GM vehicles equipped with the S6-650 manual transmission.

Fluid Requirements & Capacity

S6-650 transmissions have a fluid capacity slightly below 6 quarts and S6-750 transmissions have a fluid capacity slightly below 6-1/2 quarts. Expect to use an extra 1-1/2 to 2 quarts of fluid if the transmission cooler and lines are being flushed on Ford vehicles.

GM specifies TranSynd synthetic manual transmission fluid (ACDelco 10-4029) for its ZF 6 speed manual transmissions. Ford initially specified MERCON V automatic transmission fluid in its vehicles, but changed the requirements to XT-M5 synthetic manual transmission fluid during the 2008 model year. Note that the properties of TranSynd are similar to those expected from an ATF. Ford's manual transmission fluid, however, is a significantly more viscous lubricant.

For the curious, the various fluid specifications and options are discussed in great detail here: ZF 6 speed transmission fluid. Note that our combined experiences with these transmissions have made us preferential to the Ford XT-M5 manual transmission fluid and aftermarket manual transmission lubricants with the same general properties, including Amsoil 75W-90 manual transmission lubricant. The more viscous oils seem to provide smooth, crisp gear engagements without negatively impacting shift effort.

Flushing the Transmission Cooler & Lines

Ford vehicles equipped with the ZF 6 speed transmission utilize a transmission oil cooler that is essentially similar to that you would find on trucks equipped with an automatic transmission. Does the transmission cooler need to be flushed? Not necessarily, but its highly advised. The transmission cooler and its lines do not actually retain a great amount of fluid, but it could harbor containments or even be unwittingly plugged. Detergents in the new transmission fluid could dislodge contaminants, or the transmission pump and cooler could be functioning at a diminished capacity. In any event, flushing the cooler eliminates these concerns and ensures the system is free of any harmful contaminants. Flushing of the transmission lines and cooler is outlined in the service procedures below.

ZF 6 Speed Fluid Change Parts List

Description Part Number
Drain/fill plug ZF 0501209879 [1]
Drain/fill plug o-ring Dieselply DP-010504
PTO cover gasket Mahle H26205 [2]
PTO cover GM 12547312
PTO cover sealant Motorcraft TA-31 RTV silicon
Transmission fluid 2001 - 2006 GM TranSynd manual transmission fluid
1999 - 2007 Ford MERCON V automatic transmission fluid
2008 - 2010 Ford Ford XT-M5 manual transmission fluid
Recommended (all vehicles) Amsoil 75W-90 manual transmission lubricant

[1] - drain and fill plugs are identical, interchangeable
[2] - gaskets are available in various thicknesses because they serve as shims when a PTO apparatus is installed

PTO Cover Gaskets vs Silicon Sealant

PTO covers should be removed and resealed as part of the routine service. S6-650 transmissions have a single PTO access point while S6-750 transmissions have provisions on both sides of the case. Whether the covers are sealed with a silicon sealant or gaskets, they will eventually start to seep fluid. Additionally, a small amount of oil tends to pool in the valley of the transmission case and can only be removed using a suction gun through a PTO access cover. This is also where wear particles can accumulate and they should be removed before putting fresh oil in the unit.

With regards to sealing techniques, RTV silicon and gaskets both have their pros and cons. Silicon sealants generally require a 24 hour cure time, so gaskets hold the clear advantage if the truck needs to return to duty promptly. Silicon tends to be more forgiving in instances where a PTO cover does not have a perfectly flat sealing area or there is deep scratches in any of the sealing surfaces. Motorcraft TA-31 is a highly oil and temperature resistant RTV silicon designed to secure, amongst other things, engine oil pans. While it is a little more expensive than some of the over-the-counter sealants common in auto parts stores, it is a far superior sealant that works excellent for differential and PTO covers.

How to Change the Fluid in a ZF 6 Speed Manual Transmission

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ZF 6 speed S6-650 drain and fill plug locations
  • Locate the transmission fluid fill plug on the driver side of the front housing, about half-way up the case just behind the PTO access cover.
  • Remove the fill plug with a 3/8 inch male square drive socket, or by simply using the square drive on a 3/8 inch ratchet [3].
ZF 6 speed drain plug location
  • Locate the transmission fluid drain plug at the bottom of the center housing.
  • Place a suitable vessel beneath the drain plug location (minimum 7 quart collection capacity).
Drain plug removed, oil draining from transmission
  • Remove the drain plug with a 3/8 inch male square drive socket/ratchet [3].
  • Let the fluid drain completely before attempting to remove the PTO cover.
PTO cover on driver side
  • Remove the (6) PTO access cover bolts with a 9/16 socket.
  • Gently pry the bottom corners/edges of the PTO cover until it releases from the mounting pad. It is common for some additional oil to drain through the PTO opening.
PTO cover removed
  • Use a suction gun to remove any oil pooled in the sump of the transmission, and inspect for pieces of damaged components.
  • For S6-750 transmissions, also remove the passenger side PTO cover.
PTO cover gasket surface cleaned
  • Thoroughly clean the PTO cover mounting pad while protecting the inside of the transmission from debris. The mounting surfaces should be perfectly clean and free of sealant or gasket material.
  • Inspect the mounting surface for deep nicks or scratches. If any damage or significant imperfections are found, you will want to seal the PTO cover with a silicon sealant. If the surface machining is defect free, a simple gasket will suffice to reseal the PTO cover.
PTO cover cleaned and ready to be reinstalled
  • Thoroughly clean the PTO cover and inspect it for damage. If the cover is bent, it should be replaced. If the gasket mating surface has imperfections, silicon sealant is a more viable option than a gasket.
  • Thoroughly clean the PTO cover mounting bolts.
  • Remove the o-ring seal on both the drain and fill plug (note that there are no differences between the drain and fill plugs; they are interchangeable).
  • Thoroughly clean the drain and fill plugs.
New drain and fill plug o-rings installed
  • Lightly coat the new drain/fill plug o-rings in clean transmission fluid (or motor oil, silicon grease, etc).
  • Install an o-ring into the groove on both the drain and fill plug.
PTO cover installed and sealed to transmission
  • Reinstall the PTO cover, snugging bolts in a criss-cross or alternating fashion.
  • Torque the PTO cover bolts to 30 - 35 ft-lbs in an alternating fashion.
  • If RTV sealant was used, follow manufacturer's recommendations for curing and torque procedure.
Transmission cooler line locations on passenger side
  • Note - oil cooler flush procedures apply to Ford vehicles only.
  • Remove the oil cooler supply and return lines at the transmission housing by holding the nut with a 21 mm wrench and loosening the corresponding fitting with a 16 mm flare nut wrench.
  • If fluid dribbles from one or both of the fittings, reposition the drain pan to collect it.
Flushing the transmission cooler and lines using a suction gun
  • Attach 1/2 inch ID chemical tubing to the oil cooler lines and secure the tubing with hose clamps.
  • Place the chemical tubing from one oil cooler line into a bottle of clean, new transmission fluid.
  • Connect the chemical tubing from the other oil cooler line to a fluid extraction tool (recommend MityVac MV6835).
Old, dirty transmission fluid pulled from cooler and lines
  • Begin the fluid extraction process by activating the tool. Monitor the clean transmission fluid container to ensure that it is not emptied, allowing air to infiltrate the cooler.
  • Inspect the fluid color and clarity after approximately half a quart has been extracted. Drain and clean the extraction device's collection container so that it remains obvious when the system has been flushed and is filled with clean fluid.
Clean transmission oil
  • Continue to flush the system, monitoring the level of the clean fluid being introduced into the system and the condition of the fluid being removed. It generally takes 1-1/2 to 2 quarts of fluid to properly flush and refill the oil cooler/lines.
  • Once it is noted that the system is filled with new, clean fluid, stop flushing and remove the chemical tubing from the oil cooler lines. Vacuum caps can be used to temporarily cap the lines, limiting fluid loss.
Reconnecting transmission cooler lines
  • Reconnect the oil cooler lines in reverse order; tighten snug.
Reinstalling transmission drain plug
  • Reinstall the transmission fluid drain plug; torque to 26 lb-ft.
Refilling transmission case with transmission fluid
  • Use a fluid pump (bottle mounted type used here) to slowly add fluid to the transmission through the fill plug hole.
  • Add transmission fluid until the fluid level reaches the lower height of the fill plug opening and begins to seep out the fill plug hole.
Reinstalling fill plug after transmission is full of new oil
  • Reinstall the fill plug; torque to 26 lb-ft.
  • Clean up any oil spills, then check for leaks.
  • Recheck and adjust fluid level after driving to operating temperature and then letting the vehicle cool. This is especially important on Ford vehicles as it is possible that the oil cooler/lines had some air in the system.
[3] - Ratchet drive shanks generally fit looser than an actual 3/8 square drive socket; socket recommended.
Summary
  • Transmission fluid in ZF S6-650 and S6-750 transmissions should be replaced at 60,000 mile intervals
  • Ford versions of the ZF S6-650 and S6-750 transmission utilize a transmission oil cooler, which should be flushed at fluid change intervals
  • Proper manual transmission maintenance prolongs component life, maintains shift quality, and prevents excessive shift force
  • The ZF 6 speed manual transmission is found in 2001 to 2006 Chevy/GMC HD and 1999 to 2010 Ford Super Duty trucks