Ranger Biodiesel Swap - Automatic Transmission Control Wiring Bypass

In addition to the engine, I also planned to swap the original Ford 4R44E automatic transmission for a 4- or 5-speed manual, such as an M50D. Besides the better fuel economy, this would greatly simplify the conversion by eliminating the need for integrating the shift controls into the VW engine.

However, the automatic transmission cannot simply be unplugged without making additional modifications to the wiring harness. Otherwise, the truck's ECU would throw multiple OBD2 codes, and several crucial functions would be disabled. The common recommendations to avoid this are to simply convert a manual truck instead of an automatic, swap in a manual truck's wiring harness and ECU, or reprogram the ECU itself.

Instead, after studying the wiring diagrams, I designed a bypass box that tricks the original ECU into thinking that an automatic transmission is still installed, and that it is permanently locked into the Neutral position. I chose Neutral because:

1. The starter is normally enabled in both Neutral and Park.

2. The fewest internal shift solenoids are operated in Neutral.

3. The ECU won't compare the input and output rpm in Neutral, so I don't have to mimic those sensors.

Pages 29-3 through 29-5 of the Ford Wiring Diagram Manual provided most of the necessary information. The transmission is equipped with a "Digital Transmission Range (DTR) Sensor, which is a multiple-pole rotary switch attached to the gear selector lever. The diagram on page 29-3 shows the wiring connections that are engaged in each discrete gear position, and page 29-4 shows the wiring connections to each of the internal shift solenoids and sensors. Finally, page 29-5 presents a pinout diagram of the two harness connectors attached to the transmission, including wire color codes, pin numbers, and circuit functions.

By focusing on the connections specific to Neutral position, and determining which solenoids are actually called for in Neutral, I designed a jumper box in a waterproof case. Several wires are simply connected, whereas a few others are joined by resistors with specific OHM values. The large resistors imitate the presence of the internal shift solenoids, so the ECU self-check doesn't detect any faults and trigger a code.