WASHER 2K0420 - Caterpillar

System Operation Description:Use this procedure when the Engine Control Module (ECM) provides a signal to the tachometer and there is a problem with the indication on the tachometer.Background InformationThe vehicle's tachometer is connected to terminal P1-38 and/or terminal P1-39 of the ECM connector. Some tachometers require only one of the signal lines to operate while other tachometers require both signal lines to operate. The ECM can also send the signal to the tachometer via the SAE J1587 data link.The ECM uses the signal from the engine speed/timing sensor in order to determine engine speed. The ECM converts the signal from the engine speed/timing sensor into engine rpm. The ECM creates an equivalent signal in pulses per revolution that is output to the tachometer.The ECM must have the tachometer's conversion factor in order to develop an accurate signal for the tachometer. The conversion factor is usually marked on the tachometer. The conversion factor is programmed into the "Tachometer Calibration" parameter. The parameter is programmable from 12.0 to 500.0 pulses per revolution in 0.1 increments.The most likely source of a tachometer circuit problem is the vehicle wiring. The second most likely source of a tachometer circuit problem is the tachometer. The least likely source of a tachometer circuit problem is the ECM.Note: A problem with the tachometer circuit does not indicate that there is a problem with the engine speed/timing sensors. Any problem that is associated with the engine speed/timing sensors will also affect the operation of the engine. An electrical problem with the engine speed/timing sensors will generate a diagnostic code. If a code that is related to the engine speed/timing sensors is present, refer to the diagnostic functional test Troubleshooting, "Engine Speed/Timing Sensor Circuit - Test".
Illustration 1 g01272867
Schematic for the tachometer circuit
Illustration 2 g01272878
Schematic for the tachometer circuit
Test Step 1. Inspect the Electrical Connectors and the Wiring
Illustration 3 g01152498
Terminal locations on the P1 connector
(P1-38) Tachometer+
(P1-39) Tachometer-
Thoroughly inspect ECM connector J1/P1 and the terminals for the tachometer. Refer to the diagnostic functional test Troubleshooting, "Electrical Connectors - Inspect".
Refer to Illustration 3. Perform a 45 N (10 lb) pull test on each of the wires in the ECM connector that are associated with the terminals for the tachometer:
Check the allen head screw on each of the ECM connectors for the proper torque. Refer to the diagnostic functional test Troubleshooting, "Electrical Connectors - Inspect".
Check the harness and wiring for abrasion and for pinch points from the tachometer to the ECM. Expected Result:All connectors, pins and sockets are completely coupled and/or inserted and the harness and wiring are free of corrosion, of abrasion and of pinch points.Results:
Yes - The wiring is OK. Proceed to Test Step 2.
Not OK - The wiring is not OK. Repair: Repair the connectors or wiring and/or replace the connectors or wiring. Ensure that all of the seals are properly in place and ensure that the connectors are completely coupled. Verify that the repair eliminates the problem.STOPTest Step 2. Check the ECM Connection to the TachometerNote: Some tachometers require only one