Usage:
3406B (PEEC) And 3176 Truck Engines
The new 3E0020 Vehicle Speed Buffer is used. It is a direct replacement for the former 7T6398 and 9X4863 Vehicle Speed Buffers which are canceled. The new buffer has improved ability to tolerate electrical noise in the vehicle wiring.
The new vehicle speed buffer has no effect on Caterpillar's recommendations for vehicle speed circuit wiring. There are still three acceptable options for wiring of the vehicle speed circuit. Those options follow.
NOTE: For all three options, the ground for the vehicle speed buffer should be connected to the same point as the ground for the Electronic Control Module (ECM), using a separate, dedicated ground wire. This is true for either the new 3E0020 or the former 7T6398 and 9X4863 Vehicle Speed Buffers. On 3176 Truck Engines, the ECM ground point is the cylinder head ground stud. The digital return may also be used for the vehicle speed buffer ground on 3176 Engines if the installation has been reviewed by Caterpillar. On 3406B (PEEC) Truck Engines, the ECM ground point is determined by the OEM or vehicle manufacturer (typically the starter negative post, a cab ground post, or the engine block).
Separate Speed Sensors
Use separate speed sensors, with the second sensor supplying the signal to the speedometer. This option completely isolates the two circuits. This is the option preferred by Caterpillar. See Illustration 1.
Illustration 1. Recommended vehicle speed circuit wiring when using two separate sensors. (For 3E0020 or 7T6398 Vehicle Speed Buffer)
Single Speed Sensor
Use a single speed sensor, with the vehicle speed buffer supplying the signal to the speedometer. This option provides good results when correctly wired. When using the former 7T6398 Vehicle Speed Buffer, the circuit should be wired as shown in Illustration 2a. The ground connection for the speedometer should be connected to the ECM ground point, and SHOULD NOT be connected to the cab ground. Note that Illustration 2a is electrically identical to the schematic in Service Manual Module, Electrical Schematics for 3406B (PEEC) Diesel Truck Engine, SENR3486-01 and Service Manual Module, Electrical Schematic for 3176 Diesel Truck Engine, SENR3912-03. But, Illustration 2a shows the required grounding for the speedometer more clearly.
Illustration 2a. Recommended vehicle speed circuit wiring when using a single speed sensor with the former 7T6398 Vehicle Speed Buffer. See Illustration 2b for the 3E0020 Vehicle Speed Buffer.
When using a single speed sensor with the new 3E0020 Vehicle Speed Buffer, the circuit should be wired as shown in Illustration 2b. Improvements included in the new buffer allow the speedometer to be grounded as shown. Note that the new buffer also provides a second output line for speedometers requiring two signal lines.
Illustration 2b. Recommended vehicle speed circuit wiring when using a single speed sensor with the new 3E0020 Vehicle Speed Buffer. See Illustration 2a for the former 7T6398 Vehicle Speed Buffer.
Dual-Winding Speed Sensor
Use a dual-winding speed sensor, with the second winding supplying the signal to the speedometer. This option is not preferred by Caterpillar, but can provide acceptable results if correctly installed. Dual-winding sensors may be used in some new OEM installations which have been specifically reviewed by Caterpillar. See Illustration 3.
Illustration 3. Recommended vehicle speed circuit wiring when using a dual-winding speed sensor. (For 3E0020 or 7T6398 Vehicle Speed Buffer)