Fun With Transmissions - March - 2021

Ford’s 6F35 Auto Start-Stop Function and Repair

Auto start-stop systems have been around for a while now and they are coming out of the warranty period and showing up in our shops now.

In this issue of Fun With Transmissions, we are going to check out the 2017 & up Ford Escape with the auto start-stop system. We’ll look at the pump’s disassembly and let you know where you can get the parts to do the job right.


Simply put, when you come to a stop, like at a red light, the engine shuts down, but essential components like power steering, brakes, sound system, etc., continue to run. When you take your foot off the brake, the engine restarts, and you go on your merry way. In a vehicle with a manual transmission, the system operates from the clutch pedal instead of the brake. Ford says the auto start-stop system increases fuel mileage by about 6-8%.

There are a couple of reasons not to have the auto start-stop active. If you like the cool air from the AC systems blowing on you in the dead of summer, the engine needs to remain on. Conversely, if you like the warmth of the heater in the dead of winter, the engine needs to be on. As an owner of the vehicle, you can turn on or off the auto start-stop system manually by the on/off button on the dashboard.

One key to making the auto start-stop function operate smoothly is keeping the transmission pressurized during the start-stop event. This is necessary because the engine starts as soon as the brake pedal is lifted. If the transmission is not pressurized, there would be too long a lag time between the engine and the transmission engaging into drive.

There are different methods of pressurizing the transmission depending on the manufacturer. Some Dodge applications use a large accumulator to keep the transmission pressurized. Ford and many other manufacturers use auxiliary pumps to keep the transmission pressurized during the start-stop event.

Now let’s take a look at the line pressure hydraulic diagram for the 6F35 with the auxiliary pump (figure 1). We can easily see where pressure from the auxiliary pump enters the line pressure circuit during an auto start-stop event.

Because these units are starting to show up in the shops, there is a need for replacement parts for the auxiliary fluid pump. When ATRA first started talking to technicians about the rebuild, we noticed that the dealer was not supplying any parts to service the pump. This became a huge issue very quickly because there are seals and a filter that must be replaced during overhaul. The dealer’s solution is to replace the pump for around $400.00 (Ford part# DG9ZP-086J).

It became clear early on that Ford is not interested in producing replacement parts for these pumps. TransTec®, a brand of Freudenberg-NOK, disassembled the pump, took the seals and the filter to an aftermarket filter/seal manufacturer, and made the replacement seals and the replacement filter (figure 2). With this kit, we can reseal the pump and replace the filter.

The removal of the pump in the car is a simple process. First, unplug the pump. It’s a good idea to heat the three bolts before breaking them loose. The heat will soften the Loctite. If you skip this step, you may round out the Torx teeth on the bolt head. Then it becomes a real hassle. Once the three bolts are out, the pump comes off the case (figure 3). Don’t forget to remove the pick-up tube from the case (figure 4).

Once this pump is on the bench, the disassembly/assembly procedures are straight forward (with a few exceptions).

The first thing we’ll look at is the filter under the cover. This is the filter that must be changed on the rebuild and is only available from TransTec®, a brand of Freudenberg-NOK (figure 5).

The next noteworthy items are the six tiny #10 Torx bolts and the computer known as the Electric Transmission Fluid Pump Control Module. There is a molded seal that fits into the module body that needs to be changed (figure 6). Just like so many other electronic parts, DO NOT put this part in the cleaning machine! The module has six or seven DTCs as well. One example is a P0C28 Electric Transmission Fluid Pump Motor High. The diagnostics are covered in Pinpoint Tests N and M.

The tricky part of this operation was figuring out bolt torque for these tiny bolts. Remember, there is no factory manual for this pump. We went to the Engineering ToolBox website to input the numbers in the bolt torque calculator and came up with 15 inch-pounds as the spec for these small bolts.

Here are a couple of other notable items:

The dot on the Gerotor type pump gears faces up (Figure 7).

There is a small thrust spring under the pump drive shaft that will be easy to lose if you are not paying attention when disassembling the pump motor for cleaning and resealing (Figure 8). Once again, there is no factory manual, so we had to come up with a name for this spring. It is called the Pump Drive Shaft Thrust Spring.

The auto start-stop systems on the cars today are going to be with us for some time, and thanks to innovating companies like Freudenberg-NOK, we will be able to service these pumps for years to come. And that is having Fun With Transmissions!