Circuit Description
The PCM supplies the ground to energize the Instrument Cluster tachometer. When the ignition switch is first turned on the PCM grounds the tachometer circuit. The circuit will remain grounded until the PCM receives the first 24X reference pulse. Once the 24X reference pulse is received, the PCM controls the pulse frequency in relationship to engine RPM. The tachometer is controlled at a rate of two pulses per crankshaft revolution.
Diagnostic Aids
Refer to Instrument Cluster in Electrical Diagnosis section 8A for additional diagnostic information.
Check for the following conditions:
| • | Poor connection at the PCM or instrument cluster. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. |
| • | Damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn the ignition ON and observe voltmeter connected to the tachometer control circuit at the PCM harness connector while moving connectors and wiring harnesses related to the tachometer control circuit. A change in voltage will indicate the location of the fault. |
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
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Normally, ignition feed voltage should be present on the output driver circuit with the PCM disconnected and the ignition turned ON.
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Checks for a shorted component or a short to B+ on the output driver circuit. Either conditions would result in a measured current of over 0.5 amps.
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Checks for a faulty instrument cluster.
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A fault in the 24X/Camshaft position sensors feed or ground circuits will result in the loss of both PCM input pulses. Without a 24X reference pulse the PCM can not begin tachometer pulse control causing an inoperative tachometer.
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This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Instrument Cluster diagnosis in Electrical Diagnosis performed? | -- | Refer to Instrument Cluster Diagnosis in Electrical Diagnosis | |
2 | Was the Powertrain On-Board Diagnostic System Check performed? | -- | ||
Turn OFF the ignition switch, disconnect the PCM. Turn ON the ignition switch. Using J 39200 digital multimeter (DVM), measure voltage between the tachometer control circuit at the PCM harness connector and ground. Is voltage near the specified value? | B+ | |||
Set up the DVM to measure 10 amp range. Measure current between the tachometer control circuit and ground. Monitor the current reading on the DVM for at least 2 minutes. Does the current reading remain less than the specified value? | 0.5 Amp | |||
Disconnect the instrument cluster (leave the PCM disconnected). Using the DVM, measure voltage between the tachometer control circuit and ground. Is voltage at the specified value? | 0V | |||
6 | Locate and repair short to voltage in the tachometer control circuit. Refer to Repair Procedures in Electrical Diagnosis. Is action complete? | -- | -- | |
7 | Check the ignition feed fuse for the instrument cluster. Is the fuse blown? | -- | ||
8 | Locate and repair the following short conditions: Short to ground in the ignition feed circuit. Shorted component. Refer to Fuse Block Details in Electrical Diagnosis Replace the fuse. Is action complete? | -- | -- | |
9 | Disconnect the instrument cluster. Turn ON the ignition switch, measure the voltage between the ignition feed circuit for the instrument cluster and ground. Is voltage near the specified value? | B+ | ||
10 | Check the tachometer control circuit for an open or a short to ground. If a problem is found, repair as necessary. Refer to Repair Procedures in Electrical Diagnosis. Was a problem found? | -- | ||
11 | Check the tachometer control circuit and the ignition feed circuit for a poor connection at the instrument cluster and at the PCM. If a problem is found, replace the faulty terminal(s). Refer to Repair Procedures in Electrical Diagnosis. Was a problem found? | -- | ||
12 | Turn OFF the ignition switch, reconnect the PCM and disconnect the instrument cluster. Connect J 35616-200 test light between the tachometer control circuit and the tachometer feed circuit at the instrument cluster harness connector. With the engine idling, observe the test light. Does the test light flash ON and OFF? | -- | Refer to Diagnostic Aids. | |
13 | Is the test light on steady? | -- | ||
Locate and repair the following circuit conditions? Short to ground in the 24X/Camshaft position sensor feed circuit. Open in the 24X/Camshaft position sensor ground circuit Faulty splice connection in the 24X/Camshaft position sensor feed or ground circuits. Is action complete? | -- | -- | ||
15 | Check the tachometer control circuit for a poor connection at the PCM. If a problem is found, replace the faulty terminal. Refer to Repair Procedures in Electrical Diagnosis. Was a problem found? | -- | ||
Locate and repair open in the ignition feed circuit to the instrument cluster. Refer to Repair Procedures in Electrical Diagnosis. Is action complete? | -- | -- | ||
17 | Replace the instrument cluster. Is action complete? | -- | -- | |
18 | Replace the PCM. Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming . Is action complete? | -- | -- | |
19 | Operate the vehicle while observing the I/P tachometer. Does the I/P tachometer operate properly? | -- | System OK | Refer to Instrument Cluster Diagnosis in Electrical Diagnosis |
