Computer Controlled Cranking Circuits: Part 3

What Do You Test and When?

With all the new technology that captures a lot of our imagination and increases our desire to learn more advanced electronics, essential electrical troubleshooting techniques often fall by the wayside. That creates a lot of frustration in technicians when they encounter an electrical problem that could be found with the same type of electrical troubleshooting procedures that have proven to be successful through the years. We simply cannot ignore these electrical troubleshooting techniques. They will always be valid.

Below in figure 3-1 there is a cranking problem. For purposes of this training, let’s say the starter motor doesn’t turn the engine over fast enough to allow the engine to start running. Back in Part 2 we discussed how to measure the cranking voltage and the cranking amps. Let’s also say that the cranking voltage did not drop as much as expected and the cranking amps reading was lower than expected. The starter motor is not drawing enough cranking amps to work properly and crank the engine fast enough.

There is a problem with the battery supplying sufficient current to the starter motor. This would be due to a bad connection or a corroded cable between the battery and the starter motor which is best diagnosed by a voltage drop measurement. You should remember that voltage drop tests must be performed when electron current is flowing, that is, the circuit is turned ON and operating to some extent or not operating at all.

Fig. 3-1 Testing the voltage drop of the B+ (voltage) side of the starter motor circuit.

In Figure 3-1 the starter motor is engaged and electron current is flowing. The meter is shown measuring the Voltage drop of the voltage side (Vdvs) of the cranking circuit. The red test lead connects to the battery positive post the black lead touches the connection of the battery cable to the starter motor. This test places the entire voltage side of the cranking circuit between the DMM test leads and the reading obtained is the voltage dropped by all the cable and connections on the voltage side of the cranking circuit.

A normal reading should be around 0.50V. The reading will be higher if the battery positive cable is corroded or if the heavy-duty contacts in the starter solenoid are corroded a significant voltage drop reading will be noted.

The greater the corrosion or deterioration in the starter solenoid heavy-duty contacts the higher the voltage drop reading. If the voltage drop reading is battery voltage (12V) then you have an OPEN (broken) connection between the battery positive post and the positive terminal on the starter motor. Voltage drop testing is the fastest way to find any electrical circuit corrosion on the voltage side of the cranking circuit.

(If you are interested in more training like this in voltage drop (Vd) testing we suggest our book "Vehicle Electrical Troubleshooting SHORTCUTS", 250 pages of electrical training with how-to-do-it exercises and work sheets.)

Fig. 3-2 Testing the voltage drop of the B- (ground) side of the starter motor circuit.

Move the DMM test leads by placing the black test lead on the battery negative post in the red test lead touches the case of the starter motor.

This test places the entire ground side of the cranking circuit between the DMM test leads and the reading obtained is the voltage dropped by all the cable and connections on the ground side of the cranking circuit. A normal reading should be around 0.10V. The reading will be higher if the battery negative cable and connection is corroded or if the starter motor ground connection is corroded a significant voltage drop reading will be noted. The greater the corrosion or deterioration in the battery ground cable or the starter motor ground connection the higher the voltage drop reading. This is the fastest way to find any corrosion or electrical connection on the ground side of the cranking circuit.

If the voltage drop reading is battery voltage (12V) then you have an OPEN (broken) connection between the battery negative post and the ground terminal on the starter motor. Voltage drop testing is the fastest way to find any electrical circuit corrosion on the ground side of the cranking circuit.

These two voltage drop measurements are simple to perform but remember the starter motor has to be engaged for the bad connection or corroded cable to produce a higher than normal voltage drop. The two voltage drop readings given are what you can expect to find on a new vehicle. As vehicles get older, these “perfect” voltage drop readings will begin to rise slightly as cables and connections begin to develop a little corrosion. You will find that even as these two voltage drop readings rise slightly that the vehicle cranks with no problem. With practice you will come to learn how high the voltage drops can be before they affect the cranking circuit.

Problems With The Starter Solenoid

There are two simple voltage tests to perform if you have a situation where the starter solenoid does not engage fully or is not engaged at all.

Fig. 3-3 Testing B+ voltage to the starter solenoid.

In Figure 3-3 the DMM measures the operating voltage (B+) to the starter solenoid which is the cranking voltage, 10.21V. We are performing this test on a good starter solenoid circuit to illustrate that a good DMM B+ reading to the solenoid is the cranking voltage.

If DMM Reading Is Low

The starter solenoid will be sluggish and may not close the heavy-duty contacts. This could be due to a corroded connection tracing back through the START and P/N switches and wiring through the Fuse F1. It could also be corroded contacts in one or both of the START or P/N switches.

If DMM Reading Is 0.00V

The starter solenoid does not operate. There is an OPEN connection (broken) wire going back through the START and P/N switches. Fuse F1 could be blown or is not connected back to B+.

(If you are interested in more training like this to test voltages in a circuit and tracing through the circuit to find the location of an OPEN circuit or a voltage drop we suggest our hands-on electrical training program, starting with The Starter Kit, H-111A. You can watch a You Tube video. Type “H-111A vince” in the You Tube search bar.)

Fig. 3-4 Testing the starter solenoid ground connection.

In Figure 3-4, the starter solenoid ground wire is connected to the outer case of the starter motor. Electron current flowing through the starter solenoid passes through the case of the starter motor to energize the starter solenoid. This ground wire could also be connected directly to sheet metal ground.

The DMM is measuring the ground voltage at the starter solenoid. The DMM reading indicates .012V. The solenoid ground terminal should have a ground voltage drop of 0.10V or less. We are performing this test on a good starter solenoid circuit to illustrate what a good DMM reading should be.

If The Reading Is Higher Than 0.10V

The solenoid ground could be corroded or the wire is damage but not completely broken.

If The Reading Is 12V

The solenoid ground wire is broken or not connected to a good ground connection.

(Interested in more training like this to test voltage in a circuit and tracing through the circuit to find the location of an OPEN circuit or a voltage drop? We suggest our hands-on electrical training program, starting with The Starter Kit, H-111A.

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