7.3 Safety Switches

Neutral Safety Switch

Vehicles equipped with automatic transmissions require the use of a neutral safety switch. The neutral safety switch prevents the engine from being started unless the shift selector of the transmission is in neutral or park. It disables the starting circuit when the transmission is in gear.

The neutral safety switch is wired into the circuit going to the starter solenoid. When the transmission is in forward or reverse gear, the switch is in the open position (disconnected). This action prevents current from activating the solenoid and starter when the ignition switch is turned to the start position. When the transmission is in neutral or park, the switch is closed (connected), allowing current to flow to the starter when the ignition is turned.

A misadjusted or bad neutral safety switch can keep the engine from cranking. If the vehicle does not start, you should check the action of the neutral safety switch by moving the shift lever into various positions while trying to start the vehicle. If the starter begins to work, the switch needs to be readjusted.

To readjust a neutral safety switch, loosen the fasteners that hold the switch. With the switch loosened, place the shift lever into park (P). Then, while holding the ignition switch in the start position, slide the neutral switch on its mount until the engine cranks. Without moving the switch, tighten the fasteners. The engine should now start with the shift lever in park or neutral. Check for proper operation after the adjustment.

If after adjusting the switch to normal, operation is not resumed, you may need to test the switch. All that is required to test the switch is a 12-volt test light. To test the switch, touch the test light to the switch output wire connection while moving the shift lever. The light should glow as the shift lever is slid into park or neutral. The light should not work in any other position. If the light is not working properly, check the mechanism that operates the switch. If the problem is in the switch, replace it.

Starter Safety Switch

Some late model vehicles have the brake light switch wired into the same control circuit as the neutral safety switch. In order to operate the starter, you must press and hold the brake pedal. This is in addition to ensuring that the vehicle is in neutral or park and in the case of a manual transmission; the clutch pedal is pressed down as well.

Clutch Safety Switch

Vehicles equipped with manual transmissions require the use of a clutch safety switch to prevent engine cranking. The switch is closed only when the operator presses the clutch pedal down. This prevents the vehicle from moving while the engine is cranking.

Starting Circuit Maintenance

The condition of the starting motor should be carefully checked at each PM service. This permits you to take appropriate action, where needed, so equipment failures caused by a faulty starter can be reduced, if not eliminated.

A visual inspection for clean, tight electrical connections and secure mounting at the flywheel housing is the extent of the maintenance check. Then operate the starter and observe the speed of rotation and the steadiness of operation.



Do NOT crank the engine for more than 30 seconds or starter damage can result. If the starter is cranked too long, it will overheat. Allow the starter to cool for a few minutes if more cranking time is needed.

If the starter is not operating properly, remove the starter, disassemble it, and check the commutator and brushes. If the commutator is dirty, you may clean it with a piece of No. 00 sandpaper. However, if the commutator is rough, pitted, or out-of-round or if the insulation between the commutator bars is high, it must be reconditioned using an armature lathe.

Brushes should be at least half of their original size. If not, replace them. The brushes should have free movement in the brush holders and make good, clean contact with the commutator.

Once you have checked the starter and repaired it as needed, you should reassemble it, making sure that the starter brushes are seated. Align the housings and install the bolts securely. Install the starter in the opening in the flywheel housing and tighten the attaching bolts to the specified torque. Connect the cable and wire lead firmly to clean terminals.

Starting Motor Circuit Tests

There are many ways of testing a starting motor circuit to determine its operating condition. The most common tests are as follows:

Starter Current Draw Test

The starter current draw test measures the amount of amperage used by the starting circuit. It quickly tells you about the condition of the starting motor and other circuit components. If the current draw is lower or higher than the manufacturer’s specifications, there is a problem in the circuit.

To perform a starter current draw test, you may use either a voltmeter or inductive ammeter or a battery load tester. These meters are connected to the battery to measure battery voltage and current flow out of the battery. For setup procedures, use the manufacturer’s manual for the type of meter you intend to use.

To keep a gasoline engine from starting during testing, disconnect the coil supply wire or ground the coil wire. With a diesel engine, disable the fuel injection system or unhook the fuel shutoff solenoid. Check the manufacturer’s service manual for details.

With the engine ready for testing, crank the engine and note the voltage and current readings. Check the manufacturer’s service manual. If they are not within specifications, there is something wrong with the starting circuit.

Starting Circuit Voltage Drop Tests

A voltage drop test will quickly locate a component with higher than normal resistance. This test provides an easy way of checking circuit condition. You do NOT have to disconnect any wires or components to check for voltage drops. The two types of voltage drop tests are the insulated circuit resistance test and the starter ground circuit test.

The insulated circuit resistance test checks all components between the positive terminal of the battery and the starting motor for excess resistance.

Using a voltmeter, connect the leads to the positive terminal of the battery and the starting motor output terminal.

With the ignition or injection system disabled, crank the engine. Note the voltmeter reading. It should not be over 0.5 volts. If voltage drop is greater, something within the circuit has excessive resistance. There may be a burned or pitted solenoid contact, loose electrical connections, or other malfunctions. Each component must then be tested individually.

The starter ground circuit test checks the circuit between the starting motor and the negative terminal of the battery.

Using a voltmeter, connect the leads to the negative terminal of the battery and to the end frame of the starting motor. Crank the engine and note the voltmeter reading. If it is higher than 0.5 volts, check the voltage drop across the negative battery cable. The engine may not be properly grounded. Clean, tighten, or replace the battery cable if needed. A battery cable problem can produce symptoms similar to a dead battery, bad solenoid, or weak starting motor. If the cables do NOT allow enough current to flow, the starter will turn slowly or not at all.

Test your Knowledge

7. What safety switch prevents a vehicle equipped with a manual transmission from starting in an unsafe situation?

A. Neutral
B. Clutch
C. Brake
D. Automatic

8. What is the maximum amount of time, in seconds, a starter may be cranked before damage can occur?

A. 60
B. 45
C. 30
D. 20