GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C-Series Medium Duty Truck B7 🢒 Engine 🢒 Engine Controls - 6.0L and 7.0L 🢒 Neutral Switch Diagnosis - Automatic Transmission Only

Object Number: 339424 Size: SF
Engine Controls Components
Cell 20: EGR, EOP Gauge and Sensor, Indicators, PCM, HI IDLE and Neutral Switch
Handling ESD Sensitive Parts Notice

Circuit Description

The neutral switch contacts are closed to ground in neutral and open in drive ranges.

The ECM the supplies ignition voltage, through a current limiting resistor, to the neutral signal circuit. The ECM senses a closed switch when the voltage on the neutral signal circuit drops to less than one volt.

The ECM uses this signal as one of the inputs to control the Idle Air Control (IAC) and fuel delivery.

Diagnostic Aids

If the neutral signal circuit indicates neutral (grounded), while in a drive range, the EGR would be inoperative, resulting in possible detonation.

If the neutral signal circuit always indicates drive (open), a drop in the idle may exist when the gear selector is moved into the drive range.

Test Description

The numbers below refer to the step numbers on the Diagnostic Table.

  1. This step checks for a closed switch to ground in neutral position.

  2. Be sure the scan indicates drive, even while wiggling shifter to test for an intermittent switch in drive range.

    3.

  3. This step checks for an open switch in drive or reverse range.

Neutral Switch Diagnosis (Automatic Transmission Only)

Step

Action

Value(s)

Yes

No

1

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

--

Go to Step 2

Go to Powertrain On Board Diagnostic (OBD) System Check

2

  1. Install a scan tool.
  2. Turn the ignition ON leaving the engine OFF.
  3. Shift the transmission into neutral.

Does the scan tool indicate the transmission is in neutral?

--

Go to Step 3

Go to Step 6

3

Shift the transmission into drive.

Does the scan tool indicate the transmission is in drive?

--

Go to Step 5

Go to Step 4

4

Disconnect the neutral switch at the shift console.

Does the scan tool display the drive range?

--

Go to Step 7

Go to Step

5

No trouble found. System OK.

--

Go to Step 15

--

6

  1. Disconnect the neutral switch harness connector located in the shift console.
  2. Jumper the neutral signal circuit to the ground circuit at the harness connector terminals.

Does the scan tool indicate the transmission is in neutral?

--

Go to Step 7

Go to Step 9

7
  1. Check for a faulty neutral switch connection.
  2. If you find a problem, repair as necessary.

Did you a problem?

--

Go to Step 15

Go to Step 8

8

Replace the neutral switch.

Is the action complete?

--

Go to Step 15

--

9

Jumper the neutral signal circuit at the harness connector terminal to a ground.

Does the scan tool indicate the transmission is in neutral?

--

Go to Step 10

Go to Step 11

10

Repair the open ground circuit.

Is the action complete?

--

Go to Step 15

--

11
  1. Check for an open neutral signal circuit.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 15

Go to Step 12

12
  1. Check for a faulty ECM connection.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 15

Go to Step 14

13
  1. Check for a short to ground in the neutral signal circuit.
  2. If you find a problem, repair as necessary.

Did you find a problem?

--

Go to Step 15

Go to Step 14

14

Replace the ECM. Refer to Engine Control Module Replacement .

Is the action complete?

--

Go to Step 15

--

15

Operate the vehicle within the conditions of the original symptom.

Does the system now operate properly?

--

System OK

Refer to Diagnostic Aids