Circuit Description
The powertrain control module (PCM) supplies the ground in order 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 18X reference pulse. Once the 18X reference pulse is received, the PCM controls the pulse frequency in relationship to the engine RPM. The tachometer is controlled at a rate of two pulses per crankshaft revolution.
Diagnostic Aids
Check for the following conditions:
| • | A poor connection at the PCM or at the instrument cluster--Inspect the harness connectors for the following conditions: |
| • | Backed-out terminals |
| • | Improper mating |
| • | Broken locks |
| • | Improperly-formed terminals |
| • | Damaged terminals |
| • | Poor terminal-to-wire connection |
| • | A damaged harness--Inspect the wiring harness for damage. If the harness appears OK, disconnect the PCM, turn the ignition ON, and observe a Digital Multimeter (DMM) that is connected to the tachometer control circuit at the PCM harness connector while moving connectors and the wiring harnesses that are related to the tachometer control circuit. A change in voltage will indicate the location of the fault. |
Test Description
The numbers below refer to the step numbers 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 with the ignition turned ON.
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This step tests for a shorted component, or for a short to B+ on the output driver circuit. Either condition will result in a measured current of more than 0.5 amps.
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This step tests for a faulty instrument cluster.
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This vehicle is equipped with a PCM which utilizes an electrically erasable programmable read-only memory (EEPROM). When you replace the PCM, you must program the new PCM.
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Instrument Cluster diagnosis in Instrument Panel, Gauges, and Console? | -- | Go to Instrument Cluster System Check in Instrument Panel, Gauges and Console | |||||
2 | Did you perform the Powertrain On-Board Diagnostic System Check? | -- | ||||||
Is the voltage near the specified value? | B+ | |||||||
Does the current measurement remain less than the specified value? | 0.5 Amp | |||||||
Is the voltage at the specified value? | 0V | |||||||
6 | Locate and repair the short to voltage in the tachometer control circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |||||
7 | Inspect the ignition feed fuse for the instrument cluster. Is the fuse open? | -- | ||||||
8 |
Did you complete the repair? | -- | -- | |||||
9 |
Is the voltage measure near the specified value? | B+ | ||||||
10 |
Did you find and correct the condition? | -- | ||||||
11 |
Did you find and correct the condition? | -- | ||||||
12 |
Did you find and correct the condition? | -- | ||||||
13 |
Did you find and correct the condition? | -- | ||||||
14 | Locate and repair any open in the ignition feed circuit to the instrument cluster. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | -- | -- | |||||
15 | Replace the instrument cluster. Did you complete the repair? | -- | -- | |||||
|
Important: Replacement PCM must be programmed. Refer to Powertrain Control Module Replacement/Programming Replace the PCM. Did you complete the replacement? | -- | -- | ||||||
17 | Operate the vehicle while observing the instrument panel tachometer. Does the tachometer operate properly? | -- | System OK |
