Building most six-speed units has become routine. Your diagnostician hands you a work order with the symptoms, findings, and recommendations. Then, you develop a parts list in your mind before the unit is even removed. I’m sure for most of you, the 6F35 falls in this category now. However, these units have not only aged, but many are on their second and third rebuild once they’ve reached you. Sometimes, symptoms may be subtle and intermittent; other times, they are obvious. As with other units, the 6F35 is showing signs of patterned failures that extend beyond typical rebuild needs. In this article, we will share some pointers that may keep you from facing the wrong direction to correct a transmission-related issue.
BEFORE YOU CONDEMN THE UNIT
There are some critical items to address when diagnosing a customer complaint. Always be sure to get a thorough understanding of what the exact problem is from the customer. Most complaints fall into the category of shift quality issues. They may be intermittent or constant. Of course, verifying the basics is imperative. Always check the following:
- Scan and record all codes in all modules and address them first
- Check the battery and charging system health and repair as needed
- Check the overall maintenance condition of the car (engine oil level and condition, tire condition, size and pressure, engine idle quality and performance, etc.)
- Transmission fluid level and condition
Perform a diagnostic test drive with a scan tool to verify shift and TCC commands relative to the customer’s complaint. Use graphing and movie mode to record and analyze data when you get back to the shop.
Keeping in mind the basics, there are some items that are outside the unit that can cause drivability issues with these units. Let’s look at a few.
DIRTY AIR FILTER
Vehicles that are properly maintained can have an air filter that is choked with dirt and debris. With advanced fuel management systems adapting to numerous changes, the effects of a restricted air filter can be completely masked on the air/fuel management side. However, the 6F35 transmission will respond with intermittent harsh shifts and firm garage shift engagements. The mass airflow (MAF) sensor is also in line with this complaint. The sensor can become contaminated, which compromises the signal. Catching a failing MAF requires a digital storage oscilloscope (DSO) to observe the signal. Usually, the signal response will be slow relative to the actual rate of change in airflow. If your diagnosis leads you to address the MAF, replace it with a new OEM part. Cleaning a high-mileage MAF may remedy issues temporarily. However, it is not a long-term fix. Also, remember to perform a Keep Alive Memory (KAM) reset after servicing air/fuel-related components.
UNIT REPAIR ISSUES
A good diagnostic test drive usually shows issues inside this unit. A few complaints that stand out as becoming more common include the following:
- Bind on the 1-2 shift
- Harsh 2-3 and 3-2 shift
These complaints may be caused by valve body issues such as wear, sticking valves, or malfunctioning solenoids. If this unit is undergoing an inspection, there are some specific items that must be addressed while the unit is out.
First, the 1-2 bind issue is something that does not seem possible based on the way this unit is designed. Looking at a clutch apply chart, you notice that the low one-way sprag holds until the 2-4 clutch applies for the 1-2 shift to happen (Figure 1). This is the age-old way of ensuring a smooth, high-torque gear shift since the shift is non-synchronous. Everything should work seamlessly if all is well inside the unit. However, the low one-way element becomes compromised with time and use, causing it to lock on momentarily during the 1-2 shift. Inspect the one-way by hand to see if the unit is failing. If the transmission suffered a catastrophic failure with a lot of metal contamination, the one-way is most susceptible to failure. New one-way elements are available through the OEM or your soft parts supplier.
Next, harsh 2-3 and 3-2 shifts are common complaints with high mileage units. Here is where building basics come into play. The bushings on these units take a beating, especially the reaction sun gear bushing (Figure 2). When this bushing wears excessively, the oil that is assigned for the balance circuit leaks, causing a lower pressure than required. If balance oil pressure is too low, the shifts will be harsh.
Continuing on the same subject, bushings inside this unit are critical. Any bushing with excessive clearance will cause shift quality issues that may be intermittent or worse when the unit is hot. Other important bushings in this unit are as follows:
- Stator
- 4-5-6 Drum bushings
- Axle
Again, it is important to check ALL bushings for proper clearance to ensure this unit will work correctly.
CRANKSHAFT PILOT WEAR
A curious problem affecting the 6F35 in vehicles equipped with the 2.0L motor is crankshaft pilot wear (Figure 3). The origins may be traced back to an engine swap or previous transmission repair. Always check for both engine-to-transmission alignment dowels either on the bell housing or the engine. If the engine was swapped out at the dealership, there is a good chance they could be missing. The new replacement motors did not come with dowels. Of course, installing and running a transmission without the alignment dowels will leave the torque converter pilot off-centered, which can cause crankshaft pilot wear and flexplate failure.
Always check for crankshaft pilot wear upon removal of the unit. Replacing the crankshaft or the entire motor is the only solution to fix this issue so far. Installing another unit or torque converter will result in repeat flexplate or transmission failure.
VALVE BODY WEAR
Aluminum cast valve bodies are subject to wear no matter what application you are working on. The higher the mileage, the greater the chances that the wear you find may require replacing the entire valve body. 6F35 units have valves that are more prone to wear than others. Here is a list in the order of most probable to least (Figure 4):
- Solenoid Pressure Regulator
- TCC Regulator
- Control Pressure Regulator
- Clutch Pressure Regulator valve plugs
Note that wear is not limited to these valves only. Assessing valve bore wear is important during a unit teardown inspection. There are several test procedures and repair solutions to restore a valve body to good working condition. Always consider the time factor involved when comparing repair versus replacement. Also, when repairing the valve body, make sure to order the correct kit based on the generation of the valve body.
One more area of concern that is worth mentioning in case you haven’t worked on many of these is the differential end play. Time and use cause wear on the main case bearing support land (Figure 5). The wear almost looks like there was intentional machining. There is no published specification for differential endplay. However, observing measurements from factory new units shows a range of .008” to .020”. The only OEM fix is to replace the case half. However, there are shim and bearing kits available through the aftermarket that allow you to restore a worn case to factory specifications.
As six-speed units age, more care must be taken to be careful critical items are not overlooked when assessing necessary repairs to ensure the transmission will be fixed correctly the first time. This article was meant to raise awareness of some items that you may not have considered before when working on a 6F35. Now, you have a reference point to help you deliver the goods to your customers with confidence!
