Heavy Duty Truck Electrical Starters

This topic has prerequisites of Basic Electricity-1, Basic Electricity-2, and Electricity & Magnets. Without the knowledge of these prerequisites, you will be seriously handicapped in understanding the electrical troubleshooting concepts of this topic.

Truck starters are very heavy duty electrical motor devices. They are also physically heavy because of their massive copper and steel components. The starter must convert electrical energy to physical work energy. Spinning a diesel engine requires a lot of work effort. The truck starter generates heat from the work being done, and this heat can destroy the starter if you run the starter to long without providing ample recovery cooling off time. Never run the starter for more than 15 seconds without allowing a 2-3 minute cooling off time. This allows the starter to dissipate the accumulated heat generated from the battery power expended in cranking the engine.

Starter Frame and Major Components

The major component parts of the starter consist of the frame, the nose gear housing, the lever housing, and the solenoid; as shown in the picture at the top of this page. The frame is the actual motor part of the starter unit. It contains the field windings, the armature, and the brushes as shown below. The motor converts the electrical energy into cranking torque which spins the engine.

Starter Bendix

The nose housing is where the starter bolts directly to the bell housing (the bell housing mates the engine to the transmission). The nose housing also contains the starter bendix. The bendix is the spring loaded gear assembly that engages the flywheel teeth. The bendix is designed so that the starter motor rotation forces the bendix pinion gear teeth to move into the flywheel teeth. If the pinion gear teeth clash with the flywheel ring gear teeth when the solenoid is first engaged, then rotation of the starter motor will align the teeth and the spring (inside the bendix) will help to force the teeth fully into mesh. When the engine starts, the flywheel teeth acceleration kicks the pinion teeth away from the flywheel automatically, even though the solenoid may not have retracted the bendix lever. In other words, the starter pinion spins out beyond the levered action of the starter bendix when the starter is engaged, and sucks back into the bendix assembly when the engine starter ring gear teeth speed exceeds the starter motor pinion gear speed.

Starter Solenoid

In the nose housing drawing above, the solenoid pulls the top of the lever to the left, which jams the bottom of the lever and the bendix gear assembly to the right. Shoving the bendix to the right enables engagement of the pinion teeth into the flywheel ring gear teeth when the motor starts spinning. This drawing also shows the proper clearance adjustment for the bendix pinion gear adjustment while it is fully engaged.

The other purpose of the solenoid is to apply electrical power to the starter motor. A detailed electrical diagram of the starter solenoid is shown below. Notice that there is a hold-in winding and a pull-in winding within the solenoid unit. The pull-in winding has one side connected to the M terminal which is in series with the motor windings. The other side of the pull-in winding is connected to the S terminal which goes back to the starter switch.

Notice that just above terminals M and BAT there is a horizontal dark line. This line represents a copper flat washer which is slammed against the M and BAT terminals when the pull-in winding activates. This shorting together of M and BAT terminals, connects the battery power to the starter motor windings, and makes the starter run.

Closing of the start switch places a surge of current through the pull-in winding until the solenoid copper washer shorts terminals M and Bat. This ensures a rapid and positive engagement of the solenoid action. Once the solenoid copper washer has shorted terminals M and Bat, then the pull-in winding is shorted by the copper washer, and the pull-in magnetic field disappears. The hold-in winding continues to hold the solenoid engaged until the start switch is released. It takes more power to engage a solenoid than to hold it engaged. The pull-in winding helps force a rapid solenoid engagement action which is needed by the bendix lever. Some solenoids do not have the G terminal. Instead, that wire goes to the solenoid case which is grounded to the truck chassis.

Once the start switch is released, the springs inside the solenoid pushes the copper washer away, and the starter motor stops. The solenoid springs also retract the bendix assembly by moving the lever described above.

Starter Electrical Circuit Components

Typical starter electrical circuit components are shown below. The key switch applies power to the pushbutton starter switch, which when pressed, applies power to the magnetic switch. The magnetic switch is simply a relay. Light duty wires can be used from the switch to the relay, and heavy duty wires apply power from the relay to the starter solenoid. Notice the thermostatic connector in the drawing. This is a temperature sensor built into the starter, which inhibits the magnetic switch during starter overtemp, which prevents energizing of the starter solenoid once the starter gets to hot.

12V Starter Troubleshooting Procedures

Use of a voltmeter will identify specifically where the starter circuit problem is located. If you need voltmeter user instructions, refer to our using meters topic for details. These voltage checks will help you to determine exactly what is wrong with the starter circuit.

If an engine will not turn over, then the problem could be with the engine and not with the starter circuit. Broken parts inside the engine could be jammed and preventing the engine from turning over. Engine coolant could have flooded one or more cylinders resulting in a hydralic lock which prevents the engine from turning over. A quick test to eliminate these problems is to slowly turn the engine damper pulley through 360 degrees of rotation using a socket and breaker bar on the damper pulley bolt.

Starter problems come in 2 flavors. Either the engine will not turn over (most common problem) or the engine turns over slower than normal. There are 2 quick checks that you can perform to break the problem into half; either a battery problem, or a starter or high resistance path problem. Sometimes the starter will develop internal shorts and these shorts will give the symptoms of a high resistance problem. Following the procedures described in the engine turns over slowly discussion below, will determine whether or not the starter has a short.

This page is designed with a seperate window for viewing the schematic diagram while reading the descriptions below. When this new window opens, adjust it's size as required, and then use alt-tab keys to switch back and forth between the windows. Alt-tab means to hold the ALT key and then tap the TAB key once, then let go of the ALT key. By using alt-tab, you can switch which window is on top. If you have already opened the 24V window below, alt-tab to that window and close it now to reduce confusion. To open the 12V starter diagram, click 12-volt starter schematic.


Quick checks - Measuring the battery voltage under load is the first check that should be made. If the battery is bad or discharged, then this could be the starter problem. If the battery produces proper voltage under load, then the problem is in the starter circuits. Never exceed 15 seconds engine cranking time without allowing at least 2 minutes cooling off time for the starter motor. All of the voltage measurements below are taken while the starter is (or should be) engaged. To prevent the engine from starting, disconnect the fuel control solenoid, or tie the stop lever in the stop position. Have an assistant engage the starter circuits while you take the measurements.

Step 1. Please note the resistance shown in the battery drawing which is labeled as Rbatt. This represents the internal resistance of the battery. This resistance is what makes the battery voltage at the battery terminals drop about 2 volts while cranking the engine. Place the voltmeter across +C and -D to measure the battery voltage. These instructions mean to place the meter positive (+ red) lead on point C in this diagram, and to place the meter negative (- black) lead on point D in this diagram. Do not connect the meter leads to the cable terminals, but connect the meter leads to the battery posts themselves. Observe the voltage across the battery. This voltage should always remain greater than 9.5 volts. If it falls below 9.5 volts, then the battery is discharged or defective. A defective starter motor could drag this voltage low, but this is not a typical problem. Charge or replace the battery first, and if the voltage remains low, then continue with the engine turns over slowly steps below.

Step 2. If you don't want to follow the procedures below, then the following rules of thumb should help you conduct your own voltage drop fault isolation:

  1. Starter cables 3 feet or less, voltage drop of 0.1 volts or less
  2. Starter cables 6.5 feet or less, voltage drop of 0.2 volts or less
  3. A mechanical switch voltage drop of 0.1 volts or less
  4. Starter solenoid switch voltage drop of 0.2 volts or less
  5. Starter magnetic switch voltage drop of 0.3 volts or less
  6. Battery voltage should not drop below 10 volts under starter load

Engine will not turn over - Under this circumstance, we will be looking for a problem with the starter engagement circuits. If the starter engagement circuits are normal, then we will be looking for a very high resistance in the starter current circuits. All of the voltage measurements below are taken while the engine starter switch is pressed. Have an assistant press the starter button with the ignition switched on, while you make the following voltage measurements.

Step 1. Place the voltmeter across the magnetic switch terminal +I and -A to measure the output voltage from the magnetic switch. This voltage should approximate cranking battery voltage. If the voltage is good, then go to step 4.

NOTE: If the starter does not have a temperature cut-out switch, then the magnetic switch is not used. In this case, starter switch terminal "J" would go directly to starter solenoid terminal "S". Skip to step 5 if the magnetic switch isn't used.

Step 2. Place the voltmeter across +H and -A to measure the supply voltage to the magnetic switch. If the voltage is approximately the cranking battery voltage, then go to step 3. If the voltage is low or missing, troubleshoot the supply circuit.

Step 3. Place the voltmeter across +K and -A. If you find approximate cranking battery voltage, then go to step 4. If the voltage is low or missing, troubleshoot the circuit back towards the starter switch.

Step 4. Place the voltmeter across +L and -A to measure the voltage across the starter overtemp switch. This voltage should be 0.5 volts or less. If you find battery voltage here, then there is an open in the temperature switch circuit. Find and repair the open circuit. Remember, if the starter is overheated, then this circuit should be open. Allow a few minutes for the starter to cool off before declaring a temperature switch problem.

Step 5. Place the voltmeter across +S and -A to measure the voltage at the "S" terminal of the starter solenoid. It should approximate cranking battery voltage. If the voltage is low or missing, check the wire going back to the magnetic switch, for high resistance or an open. If the voltage at terminal "S" of the solenoid is normal, then we have a high resistance starter motor current path problem. Continue with the next checks to find the high resistance current problem.

Engine turns over slowly or not at all - under this circumstance, we will be looking for a defective starter motor or a high resistance in the starter current path. Never exceed 15 seconds engine cranking time without allowing at least 2 minutes cooling off time. All of the voltage measurements below are while the engine should be cranking. To prevent the engine from starting, disconnect the fuel control solenoid, or tie the stop lever in the stop position. Have an assistant engage the starter circuits while you make the measurements. When you find a bad voltage, repair the problem and then retest. When you have fixed the starter circuit, reconnect the fuel control solenoid or untie the fuel stop lever.

Step 1. Place the voltmeter across +D and -A to measure the voltage drop in the battery negative cable. Once again, make the +D connection with the battery post and not the battery cable. And make the -C connection to the truck frame. Observe that the voltage never exceeds 0.5 volts. If the voltage exceeds 0.5 volts, then the negative battery cable is bad, or it has a high resistance connection at either end.

Step 2. Place the voltmeter across +B post and -C posts to measure the voltage drop of the starter battery cable. Observe that this voltage is less than 0.5 volts. If this voltage is greater, then the positive battery cable is bad or it has a high resistance connection at either end. Remove the negative battery cable (to prevent shorting out of the positive cable to chassis ground), then retighten both ends of the positive battery cable, then reconnect the negative battery cable. If the voltage is still high, then replace the positive battery cable.

Step 3. Place the voltmeter across +B to -M solenoid posts to measure the voltage drop across the solenoid. If this voltage drop exceeds 0.5 volts, you have a bad solenoid. Replace the starter solenoid. Remove the negative battery cable (to prevent shorting out of the positive cable to chassis ground), then replace the solenoid, then reconnect the negative battery cable.

Step 4. Place the voltmeter across +M post to -F posts to measure the voltage drop across this cable. If the voltage exceeds 0.5 volts, then the cable could be bad or a terminal could have a high resistance. Remove the negative battery cable (to prevent shorting out of the positive cable to chassis ground), then retighten both ends of the cable, then reconnect the negative battery cable. If this voltage remains high, replace this cable.

Step 5. Place the voltmeter across +F post and -A to measure the voltage at the starter motor. If the voltage from +F to -A is less than 8 volts, and the voltage from +G to -A is less than 0.5 volts, and all the above voltage checks are normal, then the starter motor is defective and needs to be replaced. If the +G to -A voltage is high, retighten the cable connections and or replace the cable before replacing the starter.

24V Parallel Switch Starter Troubleshooting

Use of a voltmeter will identify specifically where the starter circuit problem is located. If you need voltmeter user instructions, refer to our using meters topic for details. These voltage checks will help you to determine exactly what is wrong with the starter circuit.

If an engine will not turn over, then the problem could be with the engine and not with the starter circuit. Broken parts inside the engine could be jammed and preventing the engine from turning over. Coolant water could have flooded one or more cylinders resulting in a hydyrolic lock which prevents the engine from turning over. A quick test to eliminate these problems is to slowly turn the engine damper pulley through 360 degrees of rotation using a socket and breaker bar on the damper pulley bolt.

Parallel switch starter problems come in 2 flavors. Either the engine will not turn over (most common problem) or the engine turns over slower than normal. There are 3 quick checks that you can perform to break the problem into thirds, either a battery problem, a parallel switch problem, or a starter or high resistance path problem. Sometimes the starter will develop internal shorts and these shorts will give the symptoms of a high resistance problem. Following the procedures described in the engine turns over slowly discussion below, will determine whether or not the starter has a short.

This page is designed with a seperate window for viewing the schematic diagram while reading the descriptions below. When this new window opens, adjust it's size as required, and then use alt-tab keys to switch back and forth between the windows. Alt-tab means to hold the ALT key and then tap the TAB key once, then let go of the ALT key. By using alt-tab, you can switch which window is on top. If you have already opened the 12V window above, alt-tab to that window and close it now to reduce confusion. To open the 24V starter diagram, click 24-volt starter schematic.


Quick checks -
Measuring the battery voltage under load is the first check that should be made. If the batteries are bad or discharged, then this could be the starter problem. If the batteries produce proper voltage under load, then the problem is in the starter circuits. Never exceed 15 seconds engine cranking time without allowing at least 2 minutes cooling off time for the starter motor. All of the voltage measurements are taken while the starter is (or should be) engaged. To prevent the engine from starting, disconnect the fuel control solenoid, or tie the stop lever in the stop position. Have an assistant engage the starter circuits while you make the measurements.

Step 1. Please note the resistance shown in the battery drawing which is labeled as Rbatt. This represents the internal resistance of the battery. This resistance is what makes the battery voltage at the battery terminals drop about 2 volts while cranking the engine. Place the voltmeter across +C and -E to measure the first battery voltage. These instructions mean to place the meter positive (+ red) lead on point C in this diagram, and to place the meter negative (- black) lead on point E in this diagram. Do not connect the meter leads to the cable terminals, but connect the meter leads to the battery posts themselves. Observe the voltage across the battery. This voltage should always remain greater than 9.5 volts. If it falls below 9.5 volts, then the battery is discharged or defective, or you have a dragging starter.

Step 2. Repeat the battery measurement of the second battery. Place the voltmeter across +C2 and -D1 battery posts. Observe the voltage across the battery. This voltage should always remain greater than 9.5 volts. If it falls below 9.5 volts, then the battery is discharged or defective. A defective starter motor could drag this voltage low, but this is not a typical problem. Charge or replace the battery first, and if the voltage remains low, then continue with the engine turns over slowly steps below.

Step 3. Check to see if the parallel switch is working properly. Place the voltmeter across +C5 and -A to see if 24 volts is provided to the starter circuits. If the voltage is at least 20 volts, then the parallel switch appears to be working properly. For more detailed information about troubleshooting the parallel switch, refer to the parallel switch topic.

Step 4. If you don't want to follow the procedures below, then the following rules of thumb should help you conduct your own voltage drop fault isolation:

  1. Starter cables 3 feet or less, voltage drop of 0.1 volts or less
  2. Starter cables 6.5 feet or less, voltage drop of 0.2 volts or less
  3. A mechanical switch voltage drop of 0.1 volts or less
  4. Starter solenoid switch voltage drop of 0.2 volts or less
  5. Starter magnetic switch voltage drop of 0.3 volts or less
  6. Battery voltage should not drop below 10 volts under starter load

Engine will not turn over - under this circumstance, we will be looking for a problem with the starter engagement circuits. If the starter engagement circuits are normal, then we will be looking for a very high resistance in the starter current circuits. All of the voltage measurements below are taken while the engine starter switch is pressed. Have an assistant press the starter button with the ignition switched on, while you make the following voltage measurements.

Step 1. Place the voltmeter across the magnetic switch terminal +I and -A to measure the output voltage from the magnetic switch. This voltage should approximate cranking battery voltage. If the volage is good, then go to step 4.

NOTE: If the starter does not have a temperature cut-out switch, then the magnetic switch is not used. In this case, starter switch terminal "J" would go directly to starter solenoid terminal "S". Skip to step 5 if the magnetic switch isn't used.

Step 2. Place the voltmeter across +H and -A to measure the supply voltage to the magnetic switch. If the voltage is approximately the battery cranking voltage, then go to step 3. If the voltage is low or missing, troubleshoot the supply circuit.

Step 3. Place the voltmeter across +K and -A. If you find approximate battery cranking voltage, then go to step 4. If the voltage is low or missing, troubleshoot the circuit back towards the starter switch.

Step 4. Place the voltmeter across +L and -A to measure the voltage across the starter overtemp switch. This voltage should be 0.5 volts or less. If you find battery cranking voltage here, then there is an open in the temperture switch circuit. Find and repair the open circuit. Remember, if the starter is overheated, then this circuit should be open. Allow a few minutes for the starter to cool off before declaring a temperature switch problem.

Step 5. Place the voltmeter across +S and -A to measure the voltage at the "S" terminal of the starter solenoid. It should approximate battery cranking voltage. If the voltage is low or missing, check the wire going back to the magnetic switch, for high resistance or an open. If the voltage at terminal "S" of the solenoid is normal, then we have a high resistance starter motor current path problem. Continue with the next checks to find the high resistance current problem.

Engine turns over slowly or not at all - Under this circumstance, we will be looking for a defective starter motor, a defective parallel switch, or a high resistance in the starter current path. Never exceed 15 seconds engine cranking time without allowing at least 2 minutes cooling off time. All of the voltage measurements below are while the engine should be cranking. To prevent the engine from starting, disconnect the fuel control solenoid, or tie the stop lever in the stop position. Have an assistant engage the starter circuits while you make the measurements. When you find a bad voltage, repair the problem and then retest. When you have fixed the starter circuit, reconnect the fuel control solenoid or untie the fuel stop lever.

Step 1. Place the voltmeter across +E and -A to measure the voltage drop in the battery negative cable. Once again, make the +E connection with the battery post and not the battery cable. And make the -A connection to the truck frame. Observe that the voltage never exceeds 0.5 volts. If the voltage exceeds 0.5 volts, then the negative battery cable is bad, or it has a high resistance connection at either end.

Step 2. Place the voltmeter across +C5 post and -A to measure the voltage drop out of the parallel switch. If this voltage is less than the battery cranking voltage, then find the high resistance in the parallel switch cabling.

Step 3. Place the voltmeter across +B to -C5 to measure the voltage across the starter battery cable. Observe that this voltage is less 0.5 volts. If this voltage is greater, then the starter battery cable is bad or it has a high resistance connection at either end. Remove the negative battery cables E & D1 (to prevent shorting out of the positive cable to chassis ground), then retighten both ends of the positive battery cable, then reconnect the negative battery cables. If the voltage is still high, then replace the starter battery cable.

Step 4. Place the voltmeter across +B to -M solenoid posts to measure the voltage drop across the solenoid. If this voltage drop exceeds 0.5 volts, you have a bad solenoid. Remove the negative battery cables E & D1 (to prevent shorting out of the positive cable to chassis ground), replace the solenoid, retighten both large terminals on the solenoid, then reconnect the negative battery cables.

Step 5. Place the voltmeter across +M post to -F studs to measure the voltage drop across this cable. If the voltage exceeds 0.5 volts, then the cable could be bad or a terminal could have a high resistance. Remove the negative battery cables E & D1 (to prevent shorting out of the positive cable to chassis ground), then retighten both ends of the cable, then reconnect the negative battery cables. If this voltage remains high, then replace this cable.

Step 6. Place the voltmeter across +F post and -A to measure the voltage at the starter motor. If the voltage +F to -A is less than 16 volts, and the voltage from +G to -A is less than 0.5 volts, and all the above voltage checks are normal, then the starter motor is defective and needs to be replaced. If the +G to -A voltage is high, retighten the cable connections and or replace the cable before replacing the starter.

This concludes the starter circuit description and troubleshooting procedures. If you still have a diagram window open, alt-tab to the window and close the window to prevent confusion.

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