Starter Circuit Diagnosis

The Starter has two circuits , the Control Circuit and the Motor Circuit , to be able to diagnose the starter circuit successfully lets first take a thorough look at their components ,

Control Circuit:

• Ignition Switch.
• Neutral or Clutch safety switch.
• Sometimes a relay.
• (S) terminal on the Starter.
  

Motor Circuit:(also called Load or Battery Circuit)

• Battery positive cable, (Battery Positive to Starter B+).
• Solenoid magnetic switches, (B+ (Battery+) and M (Motor) Terminals).
• Battery Negative cable, (Starter M to Battery Negative).
  

Diagnosing the Control Circuit:
1.Pull the wire connected to the S Terminal on the starter.
2.Connect a Test-Lamp to it.
3.Crank the engine, the light should turn on.

If no electricity was detected (no light), the problem is in the control circuit, you should proceed to test the components backwards until you detect the faulty component.

If the light was detected at the (S) terminal wire the Control Circuit is fine, proceed to the Motor Circuit.

Diagnosing the Motor Circuit:

Diagnosing a Motor Circuit depends on the equipment you have, to do it professionally you should get a Battery And Starter System Tester, that can test the whole electrical system including the charging system. If you don't have access to such equipment a Digital Clamp Ammeter and a Voltmeter should do the trick.

1.Current Draw Test :

Current draw test can be done using a Clamp Ammeter , check the current in either the positive or negative Battery-to-Motor cables while cranking the engine , ignition should be disabled to be able to perform this test.
Different results should be expected from different engine types , below is a rough estimates on different engine types and maximum expected Amperes:

• 4Cylinder , Gas = 120 to 250 Ampere
• 6Cylinder and 8Cylinder , Gas = 250 Ampere
• 4Cylinder , Diesel = 350 Ampere
• 6Cylinder , Diesel = 450 Ampere
• 8Cylinder , Diesel = 650 Ampere
  

Draw Test Results :
High current draw could mean a bad starter-motor , or engine problems.
Low current draw could mean a high resistance issue in Battery-To-Motor cables.


2.Voltage-Drop Test:

If the Current Draw test resulted in low current, your next step would be a Voltage-Drop test, this test will detect high resistance in the Starter System. High resistance is caused by faulty cables, dirty contacts, or loose contacts.

Positive Cable Test:
Connect the Voltmeter's Positive to the Battery Positive and the Voltmeters Negative to Starter's B+ terminal, crank the engine, acceptable voltage drop should not exceed 0.4Volts.

Negative Cable Test:
Connect the Voltmeter's Positive to a clean spot on the Starter Housing and the Voltmeter's Negative to the Negative Battery terminal, crank the engine, acceptable voltage drop should not exceed 0.4Volts.

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