AX4N (4F50N) Harsh 1-2 Shift

Written by Jody Carnahan

I received a call from one of our wholesale customers which is a shop that does general repair and transmission work on occasion. He called looking for help on a 2004 Ford Freestar that he could not get to leave his shop. In fact, the vehicle has been to several other shops in the past. He advised me that the transmission had been worked on recently by another shop and that it was now back in his shop. I asked him what the previous failure was and he indicated that it was a problem with a 2^nd gear slip. They had removed the transmission and found the intermediate clutch piston bonded seal was damaged and that was all that was done to repair the unit. Now that they have it all back together, the trans bangs hard on the 1-2 shift. It was not setting any codes and from what the shop could tell everything with the vehicle and load sensors were working properly. I decided to take on this challenge and advised him to drop the vehicle off at our shop.

My diagnostic process started with our normal procedures; hooking up scanner and putting pressure gauge on EPC pressure tap, EPC pressure command on the scan tool and pressure on the pressure gauge appeared to be normal. I put the scanner into the graphing mode so that I could capture and record the pressure control readings during the up-shift through the gears. I did this so I could compare it with a graph that I had on a normal working 4F50N. . We found that on a normal working transmission, the computer would ramp up the epc pressure just before the 1-2 shifts and during the shift it would come back down (see figure 1).

Figure 1

We then graphed the epc pressure on our vehicle with the harsh 1-2 shift and it appeared that the PCM was trying to compensate for the aggressive shift. Because the computer never ramped up epc pressure before the 1-2 shift it stayed steady before and during the shift as you can see in figure 2.

Figure 2

 Based upon this information, we concluded that the PCM was working properly and that it was not the cause of the problem. We also ruled out the epc solenoid, because with a pressure gauge hooked to the epc pressure port we were able to confirm that it was following epc command.

Based upon the scanner and pressure gauge data, we needed to start looking for the problem inside the tranny. We needed to identify what component(s) we were going to look at first and also needed to decide whether to try fixing this one in the vehicle or remove it and put it on the bench. We decided to try a fix in the vehicle and went after the valve body, hoping we didn’t have a problem with the channel plate or with an accumulator piston. Prior to removing the side cover and pulling the valve body, I wanted to verify what valve body component could be causing our issue, such as a particular valve, possible check ball in the wrong location, etc. In doing research on the 4F50N, I knew there were several differences in model year valve bodies. Taking a look at the factory manual, I saw that the 2000-2003 model year valve body did not have a 1-2 capacity modulator valve and used a check ball (B10) to orifice intermediate clutch oil to control shift feel. If the B10 check ball was missing, un-orificed oil flow into the intermediate clutch would cause a harsh 1-2 shift. As stated earlier, we were working a 2004 Ford Freestar and this model year went through some changes in the valve body. The 2004-up 4F50N went back to using a 1-2 capacity modulator valve just like the earlier AXODE/AX4S models. Having this information in hand gave us a good direction to start looking and we removed the valve body for inspection. We first inspected the 1-2 capacity valve line up and everything appeared to look normal. The valve stroked fine in the bore and the spring appeared to be in the correct position against the bore plug. The problem was found when we removed valve. The valve had been installed backwards (see figure 3).

Figure 3

With the valve in backwards, the land of the valve blocks off the accumulator causing a harsh 1-2 shift. The valve was turned around and installed correctly (see figure 4), the valve body put back on the unit and we road tested it. The transmission shifted correctly. We also hooked up the scan tool so we could graph the epc command and see if it changed back to having the normal spike before the 1-2 shifts. It looked almost identical to the graph we had one with a good 1-2 shift.

Figure 4

In conclusion, please refer back to the graphs in figure one and figure two. When we ran the graph on a known good 4F50N, it was similar to the graph in figure one. The graph in figure two, harsh 1-2 shift, is the only graph that we have seen with a harsh shift complaint.

While this one test may be inconclusive as the weather we can in fact use this test as a way to determine a harsh shift condition, we will be graphing more vehicles with felt harsh shifts in the future in hopes that this data can be useful within our industry.

I would like to challenge all of the reader of this article to provide feedback to me regarding whether or not the PCM in this case can pick up the harsh shift and adjust the duty cycle to compensate for this condition.