A 2006 Mazda 3 with a 2.3L engine using a front-wheel-drive five-speed automatic transmission (FS5A-EL known as the FNR5 in Ford) comes into Bebes Transmitech shop in Puerto Rico. The A/T warning lamp is illuminated so codes were immediately checked. Only P0791 was stored in memory. It was logged and cleared before going on a road test where the code set immediately after take off.
This transmission has an input/turbine speed sensor, which is a two-wire AC pulse generator while the intermediate and vehicle-speed sensors are 3-wire hall-effect sensors. P0791 is an intermediate sensor circuit malfunction code. Terminal A (red wire with an orange tracer) receive B+ voltage via the AT main relay. Terminal C (black wire with a yellow tracer) is the ground circuit. Terminal B (white wire with a red tracer) is the 5 volts signal wire from the computer to the sensor. Pedro Seda and Luis Rivera started checking each one of these circuits to determine if the sensor was bad or the wiring. They first verified that terminal C had a good ground. Then, with the vehicle running in park, they verified that terminal A had system voltage. This left terminal C. When it was checked, they saw 5 volts, which would suddenly disappear and then return again while in park.
The sensor is excited by the driven transfer gear on the intermediate shaft, which is not in rotation when in park. The 5 volts ought to be consistent during this time. It should not be coming and going as it was when checked in park. Only when the vehicle is moving forward in drive should this 5-volt signal be pulsed to ground. Each tooth of the driven transfer gear passes the tip of the sensor pulling the 5 volts to ground. A uniform digital pulse should be seen at this time. But with the 5 volts being erratic, the signal was erratic causing P0791 to set. The result of this test caused Pedro and Luis to inspect terminal C wire from the transmission to the computer. This was relatively easy to do since the TCM is in the engine compartment close to the transmission (Figure 1). The inspection, however, found nothing wrong with this wire. They discovered that in handling the TCM while checking the wire, they could see the voltage change more frequently. The circuit board inside the TCM is insulated with black silicone (Figure 2), making it very soft. When they squeezed the silicone in the area of where the 5-volt terminal was, the 5 volts would remain consistent. So they removed the silicone to get to the circuit board (Figure 3) and traced the circuit to two Surface Mount Devices (SMD’s) that are 1.5K ohm capacitors (152 = 15 plus 2 zeros).

Surface mount device or technology means that these capacitors are glued onto the circuit board. Pedro and Luis said they have fixed several of these by quickly dropping a little solder onto them (Figure 4). Quickly is the key as you can overheat the capacitor, which will cause it to fail. Since they have seen this occur on more than one vehicle, it seems to suggest that there may be an issue with the manufacturing of the circuit board. This means one may pop up in your shop with the same problem. Now you know how to fix it yourself, thanks to Pedro and Luis.
Wayne Colonna is president of the Automatic Transmission Service Group (ATSG).