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DIY Auto Service: Starter Diagnosis and Repair

Updated on September 17, 2016
A Cummins 15L ISX starter and magnetic switch (relay).
A Cummins 15L ISX starter and magnetic switch (relay).

Starter Introduction

The starting system is responsible for cranking the engine over. The starting system consists of the batteries, cables, starter relay, starter control circuit, starter solenoid, starter motor, starter drive and flywheel ring gear. Battery construction and testing is covered in a previous Hub (DIY Auto Service: Batteries Diagnosis and Repair). See my Hubs "DIY Auto Service: Basic Electrical and Electronic Testing" for basic electrical theory and "DIY Auto Service: Basic Digital Volt Ohm Meter (DVOM) Electrical and Electronics Testing" for meter usage.

The starter motor is an electromagnetic device that turns electricity into motion that will be used to turn the engine over. The forces of like and unlike poles in a magnet are used to rotate the armature inside the starter motor. The armature of the starter turns the drive gear, when engaged, turns the ring gear on the flywheel and turns the engine over.

Starter Components

The armature is connected to the starter drive either directly or through a gear drive.
The armature is connected to the starter drive either directly or through a gear drive.
On heavy duty starters the end frame may be able to be "Clocked".
On heavy duty starters the end frame may be able to be "Clocked".
The contacts in the starter solenoid connect the battery to the starter motor when "energized". Bad contacts could cause a no crank condition.
The contacts in the starter solenoid connect the battery to the starter motor when "energized". Bad contacts could cause a no crank condition.

Starter Components

The starter motor is an electromagnetic device that turns electricity into torque. The main parts of starter motors are:

  • The housing, which is made of steel to help contain the magnetic field, provides a grounding path and a strong mounting component.
  • The field coils will provide a strong magnetic field due to a large amperage flow from the batteries. They are bolted to the internal surface of the starter housing. The magnetic field could also be provided by permanent magnets and they will replace the field coils on some late model starters.
  • The armature is the output of the starter. The armature has loops of a conductor that connect on opposite sides of the commutator end of the armature. The commutator end has copper bars or segments which connect to the armature loops. As power travels thru the armature loops a magnetic field is formed. This magnetic field reacts against the magnetic field generated by the field coils and tries to move out of that field. Due to the fact the armature shaft is supported at each end by bushings the armature rotates.
  • Brushes are used to make the electrical connection to the spinning armature. There are 2 positive brushes connected to power and 2 negative brushes connected to the starter housing, which is ground, to complete the circuit.
  • On gear reduction starters, the armature does not drive the starter drive directly. The armature drives a sun gear in a planetary gear set. The planetary gear set consists of a sun gear, ring gear and planetary carrier with pinion gears. The output to the starter drive is from the carrier while the ring gear is held. The gear set allows the starter to multiply its torque by about 3.5 to one. The starter is much smaller and lighter than a comparable direct drive starter.
  • The starter drive or Bendix drive slips over splines on the armature. When engaged it will mesh with the flywheel ring gear teeth and turn the engine over. It also has a one-way clutch built into it so when the engine starts the one-way clutch releases and does not over-rev the starter.
  • The starter nose or drive housing provides the mounting surface for the starter to the flywheel housing. It also contains a bushing to support the armature and a mounting for the solenoid. On a heavy duty starter is designed to fit a number of engines, the starter drive housing may have a bolted on mounting ring that can be rotated or “clocked” for different mounting arrangements.
  • The end cap or housing contains a bushing to support the other end of the armature, provides a ground stud for negative battery connections and may have a normally closed temperature switch that will open at a given temperature, to protect the starter from overheating from excessive cranking.

The starter is controlled by a starter relay or magnetic switch. This controls the power (B+) to the "S" terminal. Some starters have an overheat switch to protect the starter from excessive cranking.
The starter is controlled by a starter relay or magnetic switch. This controls the power (B+) to the "S" terminal. Some starters have an overheat switch to protect the starter from excessive cranking.

Starting System Controls

The control circuit for the starter starts at the batteries and continues into the passenger compartment of the vehicle then back down to the starter.

The main components are:

  • The key switch and possibly a starter button on older heavy duty trucks. When the key switch is rotated to the spring loaded start position, battery voltage is sent to the starter relay coil or magnetic switch coil. When the starter relay is energized it will send power to the “S” terminal on the starter by closing the contacts of the relay. This reduces the amperage load on the key switch circuit because the high amperage circuit is separate from the key switch. The key switch is only powering the relay coil. Most vehicles today use the key switch as an input into the powertrain computer. The computer controls the starter relay.
  • The starter relay or magnetic switch is used to cut down the amperage used to control the starter. The relay coil usually has power coming from the key switch and grounds thru the thermal switch on the starter. The switch contacts have one connected to the battery and the other contact to the “S” terminal on the starter solenoid. So when the coil is energized it will send battery power to the “S” terminal to engage the starter.
  • The neutral safety switch is used with automatic transmissions. It will only let the starter crank in Neutral and Park (if equipped). If in reverse or a forward drive position, when the key is turned to start, nothing should happen.
  • The clutch switch is used with manual transmissions and only allows the engine to crank if the clutch is pushed in. The switch is normally open and closes as the clutch pedal is pushed close to the floor.
  • If the vehicle has a computer controlled starting system; the key switch, neutral safety switch or clutch switch are inputs to the computer. The computer controls the starter relay to control the starter. When you turn the key to start, you are requesting the computer to crank the engine. If the computer senses something wrong, it may not crank the engine.

Starter Voltage Drops

Good voltage drops are below .5Volts when added together.
Good voltage drops are below .5Volts when added together.
High (bad) voltage drops are above .5Volts when added together. Each side should be less than .25 Volts. Identify the problem area.
High (bad) voltage drops are above .5Volts when added together. Each side should be less than .25 Volts. Identify the problem area.

Starting System Testing and Diagnosis

The starting system starts and ends at the batteries. The batteries should be the first things checked along with battery cables. If the starter is tested without knowing the condition of the other areas of the system, it could lead to a starter being replaced that is good or a problem still exists that caused the starter to go bad. To find a problem in any system a logical path must be followed to eliminate what is good and find what is bad. If the proper testing procedures are not followed inaccurate diagnosis will be the likely results.

Battery Tests

Perform the following battery tests that are available for the batteries you are working with. Outlined in a previous article (DIY Auto Service: Batteries Diagnosis and Repair) are test procedures that will give you a clear picture of the battery conditions.

Repair any battery problems before going on to the starter.

  • Specific Gravity Test

  • Static or Open Circuit Voltage Tests

  • Battery Load Tests

  • Capacitance or Conductance Tests

Starter Voltage Drop Tests

Voltage drop tests are used to check for bad connections and excessive resistance in the circuit. The starter has three separate circuits that must work together to turn over the engine. A problem in one of the three could cause a no crank or slow crank problem. To do a starter voltage drop test the engine must be disabled, so it will be able to crank but not start. The maximum voltage drop is .5 Volts or 1/2 Volt when the positive and negative voltage drops are added together. This means either the positive or negative side should not be .25Volts or higher.

Starter Motor Voltage Drop Tests:

  1. Connect a voltmeter from the battery positive post to the starter “M” terminal and crank the engine. Record the reading. _______V

  2. Connect a voltmeter from the battery Negative post to the starter ground stud or housing and crank the engine. Record the reading. ______V

  3. Add the two readings together. Do they exceed .5 Volts?

  4. Does either reading exceed .25 Volts?

  5. If the answer to #3 or #4 is yes, perform more voltage drops with the meter leads closer together to find the major source or sources of the voltage drops.

  6. If the answer to #3 or #4 is no, there is not a problem with the connections, cables or corrosion in the starter motor circuit.

Key Switch Voltage Drop Tests:

  1. Connect a voltmeter from the battery positive post to the starter relay coil positive terminal and crank the engine. Record the reading. _______V

  2. Connect a voltmeter from the battery negative post to the starter relay coil negative terminal and crank the engine. Record the reading. ______V

  3. Add the two readings together. Do they exceed .5Volts?

  4. Does either reading exceed .25Volts?

  5. If the answer to #3 or #4 is yes, perform more voltage drops with the meter leads closer together to find the major source or sources of the voltage drops in the key switch circuit.

  6. If the answer to #3 or #4 is no, there is not a problem with the connections, cables or corrosion in the key switch circuit.

Starter Solenoid Voltage Drop Tests:

  1. Connect a voltmeter from the battery positive post to the starter solenoid “S” terminal and crank the engine. Record the reading. _______V

  2. Connect a voltmeter from the battery negative post to the starter solenoid “G” terminal and crank the engine. Record the reading. ______V

  3. Add the two readings together. Do they exceed .5 Volts?

  4. Does either reading exceed .25 Volts?

  5. If the answer to #3 or #4 is yes, perform more voltage drops with the meter leads closer together to find the major source or sources of the voltage drops in the solenoid circuit.

  6. If the answer to #3 or #4 is no, there is not a problem with the connections, cables or corrosion in the key switch circuit.

Starter Amp Draw Tests

The amp clamp must be over all positive or negative battery cables for a good reading.
The amp clamp must be over all positive or negative battery cables for a good reading.
This starter was specified to have and amperage draw of 450Amps maximum. 575 Amps would indicated too high a reading.
This starter was specified to have and amperage draw of 450Amps maximum. 575 Amps would indicated too high a reading.

Starter Amperage Draw Tests

The starter draws high amperage while cranking the engine. A gasoline engine typically draws between 90 and 150 Amps depending on the engine size. On a 14L or 15L Diesel engine, they will draw around 450 Amps. Check the service information for the engine/starter to be tested to be sure. The batteries may be connected to the starter with one or two positive cables and one or two negative cables. For accurate results the Inductive Amp Clamp must be on all the positive cables or on all the negative cables. Since amperage is the same anywhere in the series starter circuit, it can be measured on either the positive or negative cables providing all the positive or negative cables are in the amp clamp. The set up is the same as the voltage drop tests. The engine must be disabled so it cranks but doesn’t start. The VAT is clamped to the battery and the inductive lead around the negative or positive cable. The typical VAT amp clamp may only fit over 1 cable so if there is more than one cable you may need to use a different amp clamp or a meter that has a larger amp clamp.

To perform the test:

  1. Disable the engine so it cranks but won’t start.

  2. Hook the VAT up.

  3. Crank the engine and observe the amperage reading. _________A

  4. Compare the reading to the specification _______________A (Specification)

  5. A reading above the specification indicates the starter is drawing to much amperage.

  6. If all the other tests on the batteries and voltage drops pass, this will usually indicated the starter is bad. An engine mechanical problem could also cause a high amperage draw.

Starter Replacement

Before removing the starter disconnect all the battery negative cables. Then remove the battery cables and connections at the starter. To make this job easier, always tie all the negative cables together and all the positive B+ leads together after removal. The “S” terminal wire is by itself. Older GM vehicles may have an "R" terminal for the ignition system.

On diesel engine starters the “G” terminal wire can be left on. Disconnect the temperature switch, if equipped.

Gas and small diesel engines today use a permanent magnet starter that is not very heavy. Two to three bolts hold the starter to the engine. Some starters may use shims between the starter and the engine block. Do not lose these shims.

Medium and heavy duty diesel engine starters are heavy, so be prepared for the weight when you remove it. Typically 3 bolts hold the starter in place in a triangular pattern. Depending on the engine and chassis model, the starter may be at frame height (Cummins and Caterpillar) or a high mount above the frame (Detroit). Remove two of the three bolts. As the third bolt is removed, support the starter. Then remove it from the vehicle.

Before installing the starter, check the flywheel ring gear for any damage. If the ring gear is damaged the transmission, clutch and flywheel will have to be removed to replace it.

To install the starter, check against the old starter that the “clocking” of the drive housing is correct. If the old starter has a temperature switch on the end housing, make sure the replacement starter also has the temperature switch. Install the starter into the flywheel housing and start the bolts. Tighten all 3 bolts to the proper torque. Hook up the “S” terminal, B+, ground wires and cables. Reconnect the battery negative cables.

Confirm the repair by cranking the engine and see if the problem has been corrected.

Service Tip

Most major replacement starters and alternators have a core charge, so return the bad starter in the original box to get the core charge refunded.

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