IDLER AS 3331207 - Caterpillar

3406B (PEEC) and 3176 Truck EnginesReports have indicated problems with wiring in the vehicle speed circuit on 3406B (PEEC) and 3176 Truck Engines. Symptoms include erratic operation of the dash mounted speedometer, erratic operation ( "cutting out") in cruise control mode, and possibly Fault Code 31, Vehicle Speed Signal Fault, or Fault Code 36, Vehicle Speed Signal Overrange on both 3406B (PEEC) and 3176 Engines.
A "single" winding magnetic pickup sensor (Caterpillar 6S7772 Sensor or equivalent) is recommended for use as a vehicle speed sensor for both systems. Experience has shown that use of a "dual" winding vehicle speed sensor (dual signal outputs) can contribute to electrical noise problems. Most of these problems can be corrected by rewiring the speedometer and vehicle speed buffer per Caterpillar recommendations, as in Illustrations 3 or 4, using a single winding (single output) vehicle speed sensor. Illustrations 1 and 2 show how this circuit may have been wired, using either a dual winding (Illustration 1) or single winding vehicle speed sensor (Illustration 2) for a 3406B (PEEC) Engine.
Illustration 1. Dual winding vehicle speed sensor for 3406B (PEEC) Engines (not recommended).
Illustration 2. Single winding vehicle speed sensor for 3406B (PEEC) Engines (not recommended).
Illustration 3. Single winding vehicle speed sensor for 3406B (PEEC) Engines (recommended).
Illustration 4. Single winding vehicle speed sensor for 3176 Engines (recommended).For both engines, a single point ground connection at the speedometer is required for the vehicle speed buffer and the speedometer. Wiring the circuit in this manner reduces electrical noise to the PEEC or 3176 System. The 3406B (PEEC) Truck Engine Electrical Schematic, SENR3486, and Special Instruction SEHS8916 "PEEC Related OEM Truck System Response and Troubleshooting Guide, " will be updated to show the recommended wiring and grounding procedures, as shown in Illustration 3. The 3176 Truck Engine Electrical Schematic, SENR3912-02 and 3176 Truck Engine OEM Systems Troubleshooting Guide, SENR4250, contain the recommended wiring and grounding for the vehicle speed circuitry, shown in Illustration 4.
Because electrical noise is hard to observe, it is difficult to diagnose these problems without the use of an oscilloscope. When problems arise with the vehicle speed circuit, follow these steps to reduce diagnosis and troubleshooting time.
1. Remove the vehicle speed sensor and inspect the sensor for damage or wear. Metal chips in the transmission can collect on the vehicle speed sensor and cause intermittent or erratic vehicle speed signals similar to those caused by a faulty connector. If the speed sensor has collected metallic debris, wipe the sensor clean and reinstall. This problem may occur again in the near future if the oil is contaminated.If the problem with the vehicle speed circuit persists, continue with Step 2.
2. If the speedometer is receiving its signal directly from a dual winding vehicle speed sensor (Illustration 1), disconnect the two wires from the sensor that go to the speedometer. If the speedometer is receiving its signal from the vehicle speed buffer (Illustrations 2 and 4), disconnect the two signal wires leading from the vehicle speed buffer to the speedometer.