GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 1998 🢒 DeVille 🢒 Engine 🢒 Engine Controls - 4.0L and 4.6L 🢒 Powertrain Control Module Controlled Tachometer Diagnosis

Circuit Description

The IPC supplies 12 volts to the TACH circuit. The PCM Pulse Width Modulates (PWM) this circuit. This PWM signal varies with the speed of the engine.

Test Description

  1. Connecting a test lamp in series between battery voltage and the tach circuit to the PCM allows the PCM to create the PWM signal on the tach circuit. With normal circuit operation the test lamp will pulse while running. A higher engine speed will create a faster pulse of the test light.

  2. When checking for shorts to ground, resistance of more than 10K ohms is usually acceptable although most readings should be infinite (OL).

    When checking for circuit continuity (opens) resistance should be 5 ohms or less.

    When checking for shorts to voltage take measurements with the ignition ON and OFF.

    Disconnect the circuit from all components before performing any circuit check.

    Always check or zero the meter before performing resistance checks.

Step

Action

Value(s)

Yes

No

1

Did you perform the Powertrain On-Board Diagnostic (OBD) System Check?

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Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

2

Important: Verify no PCM or IPC DTC's are set before proceeding with this diagnostic procedure.

Operate the vehicle while monitoring Instrument Panel Cluster operation.

Do all other IPC displays function correctly?

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Go to Step 3

Go to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges, and Console

3

Important: Failure to have the engine running during the following step may cause a Vehicle Theft Deterrent DTC to set. Failure of the vehicle to start may also occur.

  1. Start and idle the engine.
  2. Disconnect the IPC connector containing the TACH circuit.
  3. Connect a test lamp between battery positive and the tach circuit at the IPC connector (harness side).
  4. Using the DMM J 39200 measure the frequency on the TACH circuit with the engine idling.

Is the frequency measured on the TACH circuit near the value specified?

22 Hz

Go to Step 4

Go to Step 5

4

While monitoring the DMM, vary the engine speed between the RPM value range specified.

Does the frequency vary with the engine speed?

Idle - 3000 RPM

Go to Step 6

Go to Step 8

5

  1. Record the fluid life indexes.
  2. Turn the key OFF and wait 30 seconds.
  3. Disconnect PCM connector C2.
  4. Using the DMM measure the TACH circuit for a short to ground, open, short to voltage or a short to another circuit.
  5. Repair the TACH circuit if needed.

Was a repair made to the TACH circuit?

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Go to Powertrain Control Module Diagnosis

Go to Step 8

6
  1. Check terminal contact at the IPC connector.
  2. Repair terminal contact if needed.

Was terminal contact repaired?

--

Go to Powertrain Control Module Diagnosis

Go to Step 7

7

Replace the IPC. Refer to the appropriate procedure:

Is the replacement complete?

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Go to Powertrain Control Module Diagnosis

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8
  1. Check terminal contact at the PCM connector.
  2. Repair terminal contact if needed.

Was terminal contact repaired?

--

Go to Powertrain Control Module Diagnosis

Go to Step 9

9

Replace the PCM. Refer to Powertrain Control Module Replacement/Programming .

Is the replacement complete?

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Go to Powertrain Control Module Diagnosis

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