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For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 Regal 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Powertrain Control Module Controlled Tachometer Diagnosis

Object Number: 240273 Size: MF
Indicators and Gauges
OBD II Symbol Description Notice
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

Circuit Description

The powertrain control module (PCM) supplies the ground to energize the Instrument Cluster tachometer. When the ignition 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 engine RPM. The tachometer is controlled at a rate of two pulses per crankshaft revolution.

Diagnostic Aids

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
       - Poor terminal to wire connection
    • Damaged harness -- Inspect the wiring harness for damage. If the harness appears to be OK, disconnect the PCM, turn ON the ignition and observe a DMM 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

The numbers below refer to the step numbers on the diagnostic table.

  1. Normally, ignition feed voltage should be present on the output driver circuit with the PCM disconnected and the ignition turned ON.

    2.

  2. This step tests for a shorted component or a short to B+ on the output driver circuit. Either condition would result in a measured current of over 0.5 amps.

    3.

  3. This step tests for a faulty instrument cluster.

    4.

  4. 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.

PCM Controlled Tachometer Diagnosis

Step

Action

Values

Yes

No

1

Did you perform the Instrument Cluster System Check in Instrument Panel, Gauges and Console?

--

Go to Step 2

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

2

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

--

Go to Step 3

Go to Powertrain On Board Diagnostic (OBD) System Check

3

  1. Turn OFF the ignition, leaving the PCM disconnected.
  2. Turn ON the ignition, leaving the engine OFF.
  3. Use a DMM in order to measure voltage between the tachometer control circuit at the PCM harness connector and ground.

Does the voltage measure near the specified value?

B+

Go to Step 4

Go to Step 7

4

  1. Set the DMM to measure 10 amp range.
  2. Measure the current between the tachometer control circuit and ground.
  3. Monitor the current reading for at least 2 minutes.

Does the current measurement remain less than the specified value?

0.5 A

Go to Step 12

Go to Step 5

5

  1. Disconnect the instrument cluster. Leave the PCM disconnected.
  2. Use a DMM in order to measure voltage between the tachometer control circuit and ground.

Did the voltage measure at the specified value?

0 V

Go to Step 15

Go to Step 6

6

Locate and repair the short to voltage in the tachometer control circuit. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 17

--

7

Inspect the ignition feed fuse for the instrument cluster.

Is the fuse open?

--

Go to Step 8

Go to Step 9

8
  1. Locate and repair the following short condition:
    2. • Short to ground in ignition feed circuit.
    • Shorted component. Refer to Wiring Repairs in Wiring Systems.
  2. Replace the fuse.

Is the action complete?

--

Go to Step 17

--

9
  1. Disconnect the instrument cluster.
  2. With the ignition ON, and the engine OFF, measure voltage between the ignition feed circuit for the instrument cluster and ground.

Does the voltage measure near the specified value?

B+

Go to Step 10

Go to Step 14

10
  1. Test the tachometer control circuit for an open or a short to ground.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 11

11
  1. Inspect the tachometer control circuit and the ignition feed circuit for a poor connection at the instrument cluster and at the PCM.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 15

12
  1. Inspect for improper connections at the PCM.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 16

13
  1. Inspect the tachometer control circuit for a poor connection at the PCM.
  2. If a poor connection is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 16

14

Locate and repair the open in the ignition feed circuit to the instrument cluster. Refer to Wiring Repairs in Wiring Systems.

Is the action complete?

--

Go to Step 17

--

15

Replace the instrument cluster.

Is the action complete?

--

Go to Step 17

--

16

Important: The replacement PCM must be programmed.

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

Is the action complete?

--

Go to Step 17

--

17

Operate the vehicle while observing the I/P tachometer.

Does the I/P tachometer operate properly?

--

System OK

Go to Powertrain On Board Diagnostic (OBD) System Check