Some two-wheel-drive F-Series pickups may have been equipped with 4×4 wiring harnesses. Should the 4×4-low signal wire become grounded in some way, the powertrain control module receives a false signal, causing the rapid upshift. Since the vehicle is two-wheel-drive, the 4×4 parameter may be overlooked.

On 1989 and some 1990 vehicles, using a scan tool will not reveal the cause of the problem, since data stream is not available. On vehicles with data-stream capabilities, you can observe the ON/OFF status of the 4×4 switch through the scan tool. With two-wheel-drive vehicles the status of the switch should be OFF at all times. If you observe an ON status, the wire has been grounded in some way or the PCM is malfunctioning.

A quick method for deciphering a wire or PCM concern is to locate the No. 12 wire in the 60-pin PCM connector (see Figure 4) or the No. 14 wire in the 104-pin PCM connector (see Figure 5). This is the 4×4 signal wire, and since it is not used on two-wheel-drive vehicles, cut it. The ON/OFF parameter should change to OFF if this wire was shorted to ground, and the vehicle should operate correctly. If the parameter remains ON, the PCM is faulty.

For the early 1989 and 1990 vehicles that do not provide data stream, cut the No. 12 wire (see Figure 4) and take the vehicle for a road test. If the problem remains, you will need to check parameters for engine load, throttle-position sensor and vehicle-speed sensor with a multimeter and verify proper operation before condemning the PCM.