Sometimes the simplest things in the world can lead to the biggest of headaches. I’ve known of many people, including myself, that will build ZFs, Mercedes, Hondas, and various other transmissions throughout the week, and the one that gives them a problem is an old THM350. Like the THM350, the FNR5 does not have an excessive amount of moving parts in it, but it can still create more problems than some transmissions with twice as many.
One of the most common problems with these transmissions is the 3-4 shift being harsh or binding during the shift. This problem can be caused by a few different things. Forward clutch control, band adjustment, servo pin bore wear, valve body wear, solenoids, and the torque converter clutch being stuck on during the shift. Like most things in life, understanding how things work is the key to fixing them.
Let’s look at what is going on during the 3-4 shift. As we can see in the component application charts (figures 1a and 1b), shift solenoid A and D are coming on, the forward clutch is coming off and the 2-4 band is coming on. In 3rd gear, the servo has apply oil on one side, with release oil and the spring on the other.
The oil pressure is the same on both sides, so the spring assists with forcing the piston off. During the 3-4 shift, the release oil is cut off by the 3-4 shift valve, and the apply oil over comes the spring, strokes the piston and applies the band.
Leaks around the servo pin can allow lower pressure on the release side, and when the fluid is cut off, the servo may apply faster than it should resulting in a harsh 3-4, or bind up on the shift. This problem can be verified by air checking the release side of the servo (figure 2) and checking for leaks around the pin in the case.
The forward clutch is shut off during the 3-4 shift, and looking at the hydraulics can be slightly confusing. Though most manufacturers might simply cut off the servo release oil and the forward clutch oil with the 3-4 shift valve (figures 3a and 3b), Mazda made it a bit more complicated. The forward clutch has two sources, the main source is from shift solenoid A, which is routed through the torque converter clutch control valve, to the neutral drive accumulator and the forward clutches, while the torque converter control valve is in the off position.
When the torque converter control valve is stroked, oil comes from the manual valve, to the 3-4 shift valve, back to the torque converter control valve, then to the neutral drive accumulator and forward clutches.
During the 3-4 shift, shift solenoid A is controlling the forward clutch release (figure 4), while the 3-4 shift valve is being controlled by shift solenoid D to cut off oil to the release side of the servo. And any problems with command to shift solenoid A, or mechanical problems with it, can delay the release of the forward clutch, also resulting in a harsh shift or bind. The cheapest and most effective way to test for a solenoid mechanical malfunction, is to swap solenoid A with B or C. If the problem relocates, you know the solenoid has failed.
The torque converter clutch can be applied in 2nd, 3rd, 4th and 5th on this transmission. It’s important to watch torque converter clutch slippage during the shift. Like any other transmission, if there is no slip, all the torque from the engine goes directly to the transmission, and if the torque converter clutch is stuck on during the shift, it will result in a very firm 3-4. Some of the possible causes for this can be computer software or hardware issues commanding the solenoid to stay on during the shift, a solenoid mechanical failure, or the torque converter control valve sticking in the valve body. If there is no command on the scan tool for the torque converter clutch, but there is slippage, start with an amp clamp on the command wire to verify if the solenoid is being commanded or not. If it’s not, the problem is either the valve or solenoid.
Like just about every other transmission out there, various problems can lead to the same result. By looking at all the information, and some experimenting in the car, we can help keep this transmission from coming back to the bench.
