Back on the Bench - September - 2017

Going Nowhere Fast… 5R55W Delayed Forward Engagement

Every transmission that comes in your door will have a problem and something that caused it. Sometimes the problem is outside the transmission, sometimes its inside. Knowing how things work will make it easier to locate the cause.

A 2006 Ford Explorer with a 5R55W started having a severe delay during forward engagement once it warmed up. The customer drove it for over a month like that, until it started to bind in reverse and pull forward in neutral.

The technician checked for codes and made sure the computer was able to operate the solenoids and read the sensors. Everything seemed fine, so he pulled the unit. Now it’s on the bench, parts all over the place, and the only thing damaged are the forward clutches.

Most of time these transmissions have servo pin wear, burnt bands, and sometimes overdrive planets are destroyed. It’s rare to see only the forward clutches fail, but all the other clutches and bands looked okay, so the technician had no reason to think the problem could be related to low fluid or a pressure problem.

He checked all possible issues, including:

  • cracks in the forward drum
  • worn sealing rings on the back of the forward drum
  • wear inside the center support where the forward sealing rings ride
  • cracks or damage on the center support

He even checked the case, separator plate, and gaskets. Everything looked good so he moved on to the valve body.

Before considering the valve body, it’s important to understand how Ford operates forward engagement for this transmission. When you shift into drive, the manual lever position sensor signals the PCM that the lever is in drive.

At the same time, the manual valve opens a path to several valves. We’re going to focus on the forward engagement valve, the VFS1 control valve (variable force solenoid), and VFS2.

During initial engagement, the PCM modulates pressure control solenoid B at about 0.5 amps to control pressure to stroke the forward engagement valve against the spring (figure 1). The forward engagement valve opens a path from the VSF1 control valve to the forward clutch.

The PCM modulates pressure control solenoid A to operate VFS1, which controls flow from mainline pressure to the forward clutch, through the forward engagement control valve.

Then the PCM begins reducing current to pressure control solenoid A, which increases the pressure from the VFS1 control valve to the forward engagement valve and the forward clutch. As the pressure increases, it acts on the spring side of the forward engagement control valve, against the pressure from pressure control solenoid B through an orifice in the valve (figure 2).

Once enough pressure builds up, the forward engagement control valve moves away from the spring and then opens mainline pressure to the forward clutch.

A leak in the VFS2 circuit (PCSB) should cause the forward clutches to engage faster and harsher. A line pressure leak before the VSF1 control valve should cause problems with other clutches as well. So the valve body issues that could be causing a delayed engagement are:

  • sticking VFS1 control valve
  • leak in the VFS1 end plug
  • leaking forward engagement control valve or its end plug
  • damaged spring for the forward engagement control valve

There were no sticking valves or damaged springs, so the next step was to vacuum test the valves. Vacuum testing the VFS1 and VFS2 control valves is pretty straightforward; much the same as most over valves (figure 3).

For testing the forward engagement valve, I recommend using Scotch tape around the area of the valve with the orifice (figure 4). Use an endplug clip to hold it in place while testing (figure 5).

The vacuum test revealed a leak between the end plug and the valve body. Fortunately, there are o-ringed end plugs for this valve body. After replacing the end plug, another vacuum test verified the fix.

With the test complete, it’s time to try to get the tape off the valve, which is sometimes easier said than done. From there it was time to build the transmission.

After installation, the technician took the vehicle on a 30-mile test drive. Forward engagements were clean and smooth, even after the transmission warmed up. He returned the vehicle to the owner, and, when he came back for his 10-day checkup, he acknowledged that the trans was working perfectly.

A problem like this could have been tough to pinpoint. But a thorough understanding of how the system operates made short work of isolating and repairing the problem. And that’s something you can count on, no matter what type of transmission or problem you’re facing.