KIT-SEAL 1799638 - Caterpillar

System Operation Description:If the vehicle tachometer is driven by the Engine Control Module (ECM) the vehicle tachometer will be connected to the ECM through connector P1-38 and/or P1-39. Some tachometers require only one of the ECM tachometer signal lines to operate while other tachometers may require both of the ECM tachometer signal lines to operate.The following background information is related to this procedure:The tachometer circuit consists of the tachometer that is connected to the ECM through connector J1/P1 and the associated vehicle wiring. The ECM engine speed signal is provided by the primary engine speed/timing sensor. The ECM converts the signal from the primary engine speed/timing sensor into engine rpm in pulses per revolution before the ECM sends a signal to the tachometer. The selection of the pulses per revolution for the tachometer is done through the "Tachometer Calibration" parameter.The ECM tachometer signal is programmable from 12.0 to 500.0 pulses per revolution in 0.1 increments. The tachometer must be calibrated identically to this programmable value for proper operation.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 and the least likely source of a tachometer circuit problem is the ECM.Note: A tachometer may also be driven by the SAE J1587 data link or the J1939 data link. If the ECM communicates with Caterpillar Electronic Technician (ET) and the instrument cluster uses the signal from a data link, the problem is in one of two locations. Either the problem is in the wiring from the ECM to the dash, or the problem is in the dash.Note: Do not disturb the secondary engine speed/timing sensor and the primary engine speed/timing sensor in order to troubleshoot a tachometer circuit problem. If there is a problem that is associated with the sensors, refer to Troubleshooting, "Engine Speed/Timing Sensor Circuit - Test". Any problem that is associated with the secondary engine speed/timing sensor and the primary engine speed/timing sensor will be apparent because of engine operation with a diagnostic code that is present. There are no diagnostic codes that are associated with the tachometer circuit.
Illustration 1 g01259212
Schematic No. 1 for the tachometer
Illustration 2 g01259219
Schematic No. 2 for the tachometer
Test Step 1. Use Cat ET to Check the ECM Tachometer Signal
Connect Cat ET at the data link connector.
Turn the keyswitch to the ON position.
Access the "Tachometer Circuit Test". Access the following display screens in order:
"Diagnostics"
"Diagnostic Tests"
"Special Test"
While the dash tachometer is being monitored, begin the test. Expected Result:RESULT 1 The dash tachometer indicates approximately 1500 150 rpm and the dash tachometer is stable.RESULT 2 The tachometer does not indicate an engine rpm or the tachometer is erratic.RESULT 3 The tachometer rpm is stable but the tachometer rpm is