TesTech™ Technical Lessons Lesson #3: Electrical Fundamentals

To TRNW Group Member Technicians: Ladies and gentlemen,
Our Technical Department strongly urges you to review this technical information carefully. The subsequent TesTech™ Technical Lessons will be discussing intermediate and advanced technical information concerning Electronic Control Systems some of which will contain Diagnostic Routines and Procedures directly related to this Electrical Fundamentals lesson. Our best wishes for your continued achievements and success.

Fix It Right The First Time, Every Time!

Applying Basic Automotive Electricity To Your Work

1. Although we are in an era of highly advanced computer electronics, some very basic automotive fundamentals have not changed in over forty years. General Motors, for example, introduced a 12 volt battery in 1955 and an alternator in 1963. In 1996, the domestic and import Auto Makers are still using 12 volt batteries and alternators. Indeed, alternators have progressed to be more powerful to keep pace with all the factory and after market add on accessories and the 12 volt batteries have made some progress with increased cranking and reserve power. The point is that the BASIC automotive electricity is still BASICALLY the same as it was almost forty years ago.

2. In 1980, the automotive industry was introduced to an On-Board computer system. A bright, silver cased box was located under the dash panel. The vehicle was equipped with a feed back carburetor that featured a throttle position sensor and mixture control solenoid. The exhaust system featured an oxygen sensor. This set the stage for a full burst of electronic controls and a lot of misunderstandings that surrounded these systems. Around 1989, the term Powertrain was being widely used throughout the transmission industry. Automatic transmissions were becoming not only fully electronically controlled but also integrated with the engine electronic systems. The throttle position sensor not only controlled fuel mixture and spark timing, the same TPS signal was and still is being used for the transmission to determine shift scheduling, TCC, and pressure rise control.

3. Service technicians have been struggling to keep up with the new technology. It seems that once they learn a system, the system changes during the next model year. A good example of new technology is the Chrysler CCD Multiplex System. Today, service technicians feel overwhelmed with the modern vehicles. Indeed, today's modern computer controlled vehicle is very advanced however, understanding and applying basic automotive electricity principles to your work will eliminate a lot of unnecessary work, wasted time, and lost profits not to mention customer dissatisfaction.

Remember one thing about electronic problems. The actual root cause of the concern or malfunction is usually located at a very basic level. With this in mind, don't go looking on the tenth floor for the cause. Instead, start in the lobby with basic tests.

Understanding And Applying Ohms Law Of Electricity

4. Ohms law of electricity has been used throughout the automotive industry for at least forty years. When the Auto Makers are designing their vehicles, they must take into account the electrical loads such as lights, blower motors, heated glass etc., and also the associated wiring to operate these loads. Therefore, Ohms Law, which is actually a formula, is used to determine the load factors to avoid premature failure of the load or device and also avoid a potential fire from overheated wiring circuits. The professional transmission service technician can use Ohms Law to help determine the root cause of a failed device thus avoiding a repeat failure and dissatisfied customer.

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Understanding And Applying Ohms Law (cont'd)

1. Ohms law consists of three factors which are voltage, resistance, and current. Resistance is measured in ohms and current is measured in amperage or amps. Ohms law states that if any two of these three factors are known, the third factor can be determined with simple basic math. The Ohms chart is shown below. Take note of the large letters E, I, and R. The "E" is the voltage factor, the "I" is the amperage factor, and the "R" is the resistance factor. To make it easier for you to understand and apply it to your diagnostic work, we have also inserted the letters V, A, and R to represent Voltage, Amperage, and Resistance. You will need to either multiply or divide. Here's an easy way to remember. Examples: To determine the amperage, divide the voltage (E) by the resistance (R). Simply use the horizontal line to remember to divide. Again using the horizontal line, divide the voltage (E) by the amperage (I) to determine the resistance. To determine the voltage (E) multiply the amperage (I) by the resistance (R) using the vertical line.

2. We will apply Ohms law to a specific transmission circuit, the Torque Converter Clutch. It might help you to understand why a particular vehicle is returning to you time and again with a failed PCM. Below is an illustration of a TCC electrical circuit in a General Motors vehicle equipped with a THM 440-T4 (4T60) transaxle.

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Example Of An Electronic Component Failure Using Ohms Law To Find The Root Cause

1. Here's a specific vehicle complaint. After you have overhauled a 440-T4, you discover that there is no TCC engagement. Your diagnostic routines confirm that if you use a transmission test box, there is no PCM ground signal being sent to the transaxle. Further testing shows the PCM itself is at fault. A replacement PCM is installed, TCC returns to normal operation, and the vehicle is sent on its way. A short time later, the vehicle returns with an inoperative TCC system. Your tests show ANOTHER failure of the PCM. What do you do now? Of all the various circuits that the PCM controls, why did the TCC circuit in particular fail twice? Defective PCM's? Not likely. Overload on the PCM's TCC ground driver circuit? Very likely. To diagnose this vehicle so it doesn't experience a THIRD PCM failure, begin by establishing the three factors of Ohms law.

2. For voltages, establish this factor by measuring it twice on two separate occasions. First with KOEO, second with the vehicle engine running at 1500 rpm. The vehicle might be overcharging in which case, the readings would change. Second, establish the resistance of the load (device) in the TCC circuit which is the TCC solenoid. Third, the amperage can be checked at the edgeboard connector of the PCM by back probing the ground driver circuit. At this point, let's establish the voltage and amperage. The voltage with KOEO is 12.5 volts. The resistance of the TCC solenoid is 15 ohms. Start the engine and take the voltage reading again. Let's be sure it isn't overcharging. The voltage reading steadies at 13.5 volts. We now have two of the factors established. We will place them in Chart 1. At this point, let's do the math to find what the amperage will be before we check it with the meter. 13.5 volts divided by 15 ohms= 0.9 amps. Here's the problem! There is excessive amperage in the TCC circuit! The cause of the excessive amperage is the TCC solenoid and / or the wiring. The TCC solenoid circuit at 70 degrees ambient temperature should be 20-30 ohms. As the resistance goes down, the amperage goes up. A typical vehicle computer circuit cannot withstand more than 0.7 amps. That circuit will overheat and fail. In this example case, the TCC solenoid itself was faulty. A new solenoid has been checked for the resistance. It measures 25 ohms. Let's do the math again and insert it into Chart 2.

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Basic Diagnostic Testing & Electrical System Diagnostics

A component malfunction detected by the On-Board computer can be caused by a basic ground fault, poor connection, or other fundamental fault within the electrical system. Vehicles operated in cold climate regions may be subjected to snow and salt whereas vehicles operated in high humidity regions can be subjected to dampness. Both climates can lead to electrical faults particularly the electrical systems most deadliest enemy which is corrosion. The service technician should begin a systematic diagnostic routine even when and if Diagnostic Trouble Codes are present. Where do you begin? Start at the source which is the battery. To begin the test sequence, the engine must be off and should also be cooled down.

Battery Post Voltage Test For State Of Charge

Place the positive lead of the multimeter on the positive post of the battery and the negative lead of the meter on the negative post of the battery. A fully charged battery will read between 12.4 and 12.6 volts. A reading of 12.2 indicates a 50% state of charge. A reading below 12.0 volts indicates a 25% state of charge. There may be a parasitic draw bringing the battery down to discharge levels or perhaps it may be time for a new battery. Don't begin further testing until the battery voltage state is at least 12.4 volts.

Battery Test Under Load

Place the positive multimeter lead to the battery terminal and negative lead to the negative terminal. Set the MAX-AVG-MIN function of the meter. Disable the ignition system so that the engine will crank but not start. Crank the engine for 15-20 seconds. Check the MIN display. At 70 degrees, the reading should not fall below 9.6 volts. This could indicate a weak battery.

Voltage Drop Testing Procedures (VERY IMPORTANT !!!)

Voltage drop is the difference in voltage potential when measured across a circuit or component creating resistance. The resistance decreases the amount of voltage available. Remember, resistance is an opposing force created by a circuit or component. There is a small amount of natural resistance when voltage flows through wires, switches, grounds or connections. The resistance increases beyond acceptable limits if corrosion develops, connectors become loose, or wire strands start to fray. Resistance also increases each time a wire, a switch, and a connection are added into the circuit. Each wire, ground, connection, switch, solenoid, and the complete circuit should be checked as a potential source of increased resistance.

Voltage Drop Testing At The Engine: This will check the engine ground efficiency. Place the meter negative lead to the negative terminal on the battery. Place the meter positive lead close to the ground cable connection on the engine block. Disable the ignition and crank the engine for about five seconds. The maximum allowable voltage drop that is acceptable is 0.3 volts. Above 0.3 volts indicates that a problem exists.

Voltage Drops Are Very Common Concerns !!!
Take The Time To Fix It Right The First Time!

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Basic Diagnostic Testing & Electrical System (cont'd)

The problem could simply be a worn, dirty, or corroded ground cable. The fastening bolt of the ground cable to the engine block could be loose from years of vibration especially if this is a diesel application. Does the cable fastening bolt appear to have been replaced? If so, is the bolt the correct length or, is it slightly longer? Is there a locking washer on the bolt or is the bolt head marching its way through the terminal? Look at the battery side of the ground cable. Is there an "emergency" battery terminal end present? These type of terminals are for "spot" or "emergency" repairs on the road. These terminals DO NOT provide good solid contact for everyday use.

Measure The Accessory Ground For Voltage Drop: You do this by placing the positive meter lead on the accessory ground point which oftentimes is the fender or radiator support. Place the negative meter lead on the negative battery terminal at the battery. With KOEO, turn on all accessories such as headlights, blower motor on high, windshield wipers, heated glass, etc. Work quickly so you don't run the battery level down. The maximum allowable accessory ground voltage drop should not exceed 0.3 volts. If the reading exceeds 0.3 volts, check the ground wire contact point for rusted sheet metal, stripped screw hole, wires spliced in from previous repairs. Question: Does the accessory ground exist at all? Has the battery cable been replaced? Has the wire end connector been left intact without an accessory ground wire tied in?

For The Professional Powertrain Service Technician

You can also conduct voltage drop testing at the transmission case bell housing, pan, pump, etc. All too many times a transmission may fail for unknown reasons. Internally, the bushings look like they have been subjected to mini-arcing. Has the front seal blown out of the pump for no apparent reason? Look at the pump bushing. Does it look gray and pitted? Have you replaced torque converters in diesel THM 400's one after another for bearing failure? Do the bearing races have arcing lines across them? How about a floor shifter cable that gets stiff, jams, and then breaks every two weeks?

Stop worrying and Fix It Right The First Time. Begin by cleaning the battery posts and terminal ends along with the ground distribution points. If the cables have been spliced with spot or emergency terminal ends, now would be the time to replace the cables. We suggest running two independent grounds on certain applications prone to ground faults. One such application is the Ford E4OD diesel applications. Run a heavy battery cable with round eye terminal ends from the drivers side bell housing ear to the left chassis rail near the left suspension strut. The bolt hole is already there in the chassis. Next, run a long cable or heavy duty strap from any bell housing fastening bolt point to the negative post on the battery.

Testing Summary Up To This Point

Intermittent driveability malfunctions, individual circuit failures, repeated failure of computer controlled devices etc., are often caused by erratic electrical behavior. Some of the basic testing we have discussed so far can lead you to the root cause of these problems. Work quickly but work in an orderly manner. Follow systematic testing. This is the best way to correct the problem on the first go around.

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Individual Circuit Checks For Fault Diagnosis

Checking multiple or individual circuits begins with having a wiring schematic for the vehicle application you are working on. This is like having a road map asking the technician: Where do you want to go?

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