How to Test a Starting System

Sept. 28, 2010

A diagnosis of any electrical system, whether it is 12 or 24 volts, must include two key elements. First, you must test the complete circuit. Second, you must understand how that circuit works.

One of the most important circuits in the 24-volt electrical system in any vehicle or piece of construction equipment is the cranking circuit. It typically has the following components:

  • Battery pack. Two 12-volt batteries connected in series to obtain the 24 volts.
  • Cranking motor. A 24-volt unit that converts the electrical energy from the batteries to mechanical energy to crank the engine fast enough to achieve ignition.
  • Magnetic switch. Normally mounted near the cranking motor, this device takes a low-current signal from the ignition key switch and closes large contacts in the magnetic switch that connect the cranking motor's "S"-terminal of the solenoid to battery voltage.
  • Ignition key switch. This can be a push button or a spring-loaded ignition key switch. It allows the operator to either push or turn to signal the cranking circuit that the cranking motor process needs to start.
  • Cables. The most ignored portion of the circuit. None of the other components will work if you cannot provide enough current at the proper voltage. The size of the cable is dependent on the amount of current that this portion of the circuit must handle. The large cables between the battery and cranking motor can see well over 1,000 amps in cold weather at 24 volts. The amount of current needed to drive the magnetic switch coil is four amps, so the wire size might be as low as 18 gauges.

For the cranking system to operate as designed, all of these components must work in conjunction and each component must be able to do its job in the correct sequence. Although electrical issues seem to be the most perplexing items for technicians, they need not be. Most misdiagnosis is not caused by not doing the correct test, but by doing the tests out of sequence. It is as simple as 1, 2, and 3: Check the battery pack, check the main starter cables, and then check the cranking motor control circuit.

Battery pack

The first step is to start at the battery pack. This is the heart of the electrical system. If you don't have electrical problems and just want to confirm the battery pack is charged, the bank test that electronic testers offer will work well. But if you have electrical problems, you must first take all the battery cable off of the batteries. Each battery must be tested individually.

First, check the battery for physical damage. If the case is cracked or broken, or if the post is damaged or loose, the battery needs to be replaced. If a condition exists that caused the damage, make that repair before the new battery is installed.

If it is a filler-type battery, make sure the water level in each cell is correct. If not, add clean water.

Make sure the stud or lead pad area of the terminal is clean and free of corrosion.

The battery should be at least 12.3 volts to conduct a proper test.

Check with your battery supplier for a recommended battery tester, and have him show your technicians how to use it. If you are using a standard carbon pile load tester, you still must use half the CCA for the load for 15 seconds. At 70F the voltage should be 9.6 volts or higher at the end of the 15 seconds. Less than 9.6 volts, the battery failed.

For every 10 degrees below 70F that the battery has been subjected, the minimum voltage should be reduced by 0.1 volts. For example, at 40F the minimum voltage would be 9.3 volts not 9.6. Newer electronic testers will typically adjust for temperature.

Because of its mass, a battery takes a long time to change temperature. If a vehicle sits out all weekend in a zero-degree environment, it might take 30 hours to warm up the battery. Use the outside temperature, not the shop's.

If batteries are connected in series, you have to treat them as a system. You must change both if one is bad. Each battery has its own internal resistance that will change with age, so if you put a bad one with a new one, you would over charge one and under charge one.

You now have good charged batteries in the equipment. Make sure the hold downs are securely holding the batteries in place. Then clean the cable connection and reattach to the batteries.

Battery cables

An electronic tester (designed for 24 volts) or 24-volt carbon pile is required to test the main battery cables. Using a 12-volt carbon pile tester on a 24-volt system will damage the tester.

Connect the large lead of the tester to the cranking motor's battery post on the solenoid and the ground post of the cranking motor. Connect the small leads to the battery pack or a separate voltmeter. On an electronic tester, simply follow the instructions on how to perform the voltage test and the tester will run the test and report the results.

On the carbon pile load tester, you will need to pull 250 amps through the cables and measure the voltage at the batteries (source voltage and the voltage at the starter ending voltage) to determine your voltage drop. You are allowed 0.5 volts at 250 amps. Using Ohms law, divide 0.5 volts by 250 amps to calculate the resistance of the circuit. In this case it would be .002 ohms.

The last method to test cables is to use a clip-on ammeter to measure current while cranking and still measure the voltage at both ends of the circuit. For example, say you measured a 1.0-volt difference between the battery and cranking motor during cranking. If the current draw was 800 amps, the resistance of this circuit is 0.00125, well within the allowable limits. It will take more than one person to conduct this test. It is also next to impossible to make repeatable because of the human factor of observing all of the meters at the same time.

Resistance in the cables (positive and/or negative) will cause voltage drop that could cause the cranking motor to lose speed during cranking. When you hear slow or dragging cranking, before you condemn the cranking motor, check the cables. Lots of cranking motors are replaced because of bad cables, not because the cranking motor is defective. This will result in warranty rejections and unnecessary removals.

If your cables are out of spec, take off both ends and clean them. Reconnect and retest. If this does not fix the problem, you might have to replace the cables. To reduce voltage drop, remember that the shorter the cable and bigger the gauge, the cable will have more ability to deliver energy at the other end. Cables are typically much cheaper than cranking motors or batteries.

Cranking motor control circuit

This critical circuit rarely ever gets tested. When one first turns the ignition key, it sounds like the starter begins to crank right at that moment. Not true. When one turns the ignition switch, current flows from the ignition key switch to the coil of the magnetic switch. The coil could draw as low as 2 to 3 amps, but when the coil is energized, this causes the large contact to move and it connects the two large posts on the magnetic switch. Now power to the cranking motors solenoid coils is provided from the batteries.

This current (up to 180 amps on a 24-volt system) creates a strong magnetic pull within the solenoid that causes the plunger to move toward the cap of the solenoid, thus the drive to move down the armature shaft in the opposite direction and engage the ring gear. When the contacts of the solenoid connect the battery and motor post, the cranking motor's armature and field are provided this energy and the cranking motor begins to turn. As soon as the motor begins to turn, the pull-in coil drops out of the circuit. This is important because the pull-in coil is much higher in its current draw. It is designed to produce the most force, but for very short periods of time.

Although this happens in a fraction of second, if the cables that connect the cranking motor's S-terminal to battery power are worn, pitted or damaged, the power to the S-terminal is reduced and the solenoid cannot do its job as designed. It will not be able to force the pinion in to mash with the ring gear and all the operator will hear is click, but no crank. Most heavy- duty cranking motors are designed so that if the pinion does not makes its full travel no rotation will happen. This is good design that keeps the pinion from milling the ring gear and causing expensive ring gear replacements.

This is the only test where the circuit will have to be disturbed. The S-terminal will have to be removed from the cranking motor. We have to apply a specific load to this circuit rather than using the solenoid coils. The solenoid might not operate and/or the load would be incorrect. In addition, the pull-in drops out as soon as cranking begins. The resolution of the tester's voltmeter will not allow for accurate testing.

When hooking up the tester, be careful as this is not at battery voltage now but it will be during the testing. Use care to make sure these connections do not touch ground.

Have someone turn the ignition key switch to start. You will hear the magnetic switch close and the meter will show battery voltage. Now draw 40 amps. Record the voltage at point A and point B. Run this test quickly, do it three times, and most magnetic switch contacts will rotate each time the magnetic switch is energized. You are allowed 1.0-volt drop in this circuit. If the drop is more, test each part of the circuit and repair as necessary.

If you conduct all of these tests and the cranking motor still does not operate, it is defective and must be replaced. While these seem like complex tests, they are much easier to do as compared to changing a cranking motor.

Because of its mass, a battery takes a long time to change temperature. If a vehicle sits out all weekend in a zero-degree environment, it might take 30 hours to warm up the battery. Use the outside temperature, not the shop's.
The most ignored portion of the circuit is the cables. None of the other components will work if you cannot provide enough current at the proper voltage.
In this story:
  • Battery pack
  • Battery cables
  • Cranking motor control circuit
Temperature Vs. Load Voltage
For every 10 degrees below 70F that the battery has been subjected, the minimum voltage should be reduced by 0.1 volts.
9.670F & Above
Author Information
— Bruce Purkey is vice president of Purkeys' Fleet Electric Inc., a major supplier of electrical products to the transportation industry.